Rear End Wiggling Around While Braking

[2bigalow]

LOL! Racer, you been hangin' out with Stu lately?

[racer]

LOL! Racer, you been hangin' out with Stu lately?

 

LOL :chagrin:

 

 

I've been biting my tongue working doubleshifts six days a week for almost a month.

 

I definitely need a time out!

[harnois]

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[racer]

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[harnois]

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[Cobie Fair]

I dunno about EVERY time, but yes it was quite consistent relative to the corner. I mostly remember it happening in turns 1 and turn 14 (top of the hill right hander) at VIR North Course. I don't recall it happening when braking for other turns but it was kinda a long time ago at this point.

 

If something doesn't happen every time, makes me think of the time Keith was working with a top factory rider, and he was going to re-build his front end (his mechanics were) because the front was coming off the ground in a turn--120 mph turn. Keith figured out it wasn't happening every lap, so it was not the bike, it was the rider.

[harnois]

But, FYI, "zero pre-load" is THE RIGHT way to set up a suspension. But don't take my word for it, please consult any professional suspension technician you like (probably not the local dealer/shop mechanic). Then report back to us what advice they give you.

 

A couple years ago I bought a penske shock for my YZF600R from Traxxion Dynamics. When I first got the shock it had an 800-pound spring on it. When I installed that on the bike and went for a ride, it thought it was way way too stiff. It felt like I had a metal bar bolted in where the shock was supposed to be. I know there is some preference involved, but it was so stiff I could not imagine how anyone would possibly consider it to be optimal. Traxxion has a good reputation for customer service and I contacted them about the problem. What they had me do was set the sag to around 30mm (I had done this before I rode it the first time) then measure the length of the spring while the shock was topped out. It was a 6-inch spring and when I measured it, it was about 3mm shorter than that, so it had 3mm of preload on it. They told me that definitely indicated that the spring was too stiff, that it should have around 8mm of preload when the sag was properly set. So I thought, cool, the math was matching up with what I was feeling when riding.

 

It was a while ago so I could be getting some of these numbers slightly wrong but I'm being accurate enough here to express the idea.

 

They sent me a 700-pound spring and I sent the 800 back. I repeated the sag setup and then measured preload with this spring and it was about 6mm. When I rode it, it was ok, felt reasonable some of the time, but in bigger bumps felt like a rock.

 

I ended up getting a 650 pound spring, and that one rode pretty well, and had the recommended amount of preload with the sag properly set. So once again the math was matching up with my riding observation.

 

Based on this experience and the math involved, I would deduce that if you had a spring that you could run at zero preload and still have the recommended level of sag, it would be absurdly stiff, stiffer even than the 800 pound "metal bar."

 

They also had some rough recommendations for how much the bike should sag under its own weight only, vs, with the rider on board. Those numbers came into range also when I moved toward the softer springs.

 

I did some googling but have so far not been able to find any more information about the zero-preload concept. Do you know of any setup articles out there on the web that I could read that talk about the zero-preload concept?

 

I did find this article which says the preload should be no more than 10 to 12mm, and it does this in the context of how to determine the correct spring for your weight.

http://www.dynobike.com/motorcycle_suspension.htm.

 

I get the idea that with the spring at zero preload or "negative preload" the suspension would be able to float most freely when topped out allowing it to hold the road better under such circumstances as hard braking, rather than the suspension being clamped firmly between the spring and the top out stop. Aren't some suspensions these days using top out springs to acheive this affect, so that when the suspension is topped out, the top out spring and main spring will balance against each other and the suspension can float between them? I remember reading about that somewhere but I don't know how common it is. I did some quick googling on top out spring and found these articles:

 

http://www.sportrider.com/bikes/2003/146_0...aper/index.html - This one mentions that the 2003 kawasaki ninja has a top-out spring on the shock and the forks.

 

http://pantablo500.tripod.com/id43.html - this one mentions the top-out spring on ohlins shocks. It also talks about the 0 sag for a 125 and loss of momentum, I think somebody was discussing that earlier in this thread.

[racer]

Traxxion Dynamics...

 

Mad Max and I spent several years racing Nationals together before he quit racing to found Traxxion Dynamics. If I wanted a Penske shock, he'd be the man I'd call.

 

 

What was the stock YZF spring rated? And how much pre-load did you need to dial in to set the correct sag for the stock shock and spring? I'll bet 8mm is a lot less than the pre-load that was needed to set sag on that stock shock/spring ... like we have been discussing.

 

So ... it's a bit relative. If a stock machine needs say 40 mm of pre-load for a rider to acheive the correct sag ... 8 mm is practically nothing. Especially when compared to an overall spring length of 6" (175 mm) ... 8 mm ~ 4%.

 

But let's get to the heart of the matter...

 

Why do they recommend 8 mm (1/3") and NO MORE THAN 10 or 12 mm (less than 1/2") of pre-load?

 

Perhaps creating enough tension to capture the spring well without using up too much of the stroke or range? Like, ideally, zero pre-load might be optimum ... but, having the spring flopping around might not be a good idea?

 

In any case, my gut feeling is that your personal experience probably had a lot to do with what kind of riding you were doing. Why did TD install an 800 lb spring in the first place?

 

 

---------------------------------

 

 

Cool links and info on "top-out" springs.

 

From the last link:

 

"Note: Ohlins racing shocks features a "top-out" spring ... The top-out spring also effects the negative sag, making it difficult to adjust the sag with the shock on the bike."

 

Thanks for that.

 

Good luck sorting out your issues with the wiggling rear end.

[harnois]

I am very glad I did business with TD. Max at TD has the best customer service ethic I have ever experienced in any industry.

 

Although the spring is 6 inches long we know it can not compress that far. Seems like the shock itself had roughly 2 inches of travel so that plus the preload applied to it would be the practical compression range of the spring. So that means the 8mm of preload is something more like 14% of it's range. Plus since the shock appears to have about 2 inches of travel and the suspension has like 4 or 5 inches of travel there is a roughly 2-to-1 ratio between how much the shock moves and how much the suspension moves. 8mm of spring compression translates into 16mm suspension movement at the axle. The recommended sag range I hear most often is 25 to 35mm - a range of 10mm. Hence that 16mm is more than the whole sag adjustment range. All this is probably pointlessly over-analytical but hopefully you can see how I would not perceive 8mm of preload to be practically zero.

 

I never measured the preload on the stock spring but no doubt like you said it was a lot more than 8mm. I get your point that compared to the stock that 8mm is not much at all. The concept I'm getting from you here is to have "enough tension to capture the spring" but not take it any further than you have to, and that's making sense to me and something I hadn't thought of before. This is why I come on this forum and open threads - to learn stuff like this.

 

Funny as soon as I read "enough tension to capture the spring" I realized that I apply that very same concept to my remote control car suspension setup, even though I'd never put it into words. It's funny to me to see such correlations.

 

In any case, my gut feeling is that your personal experience probably had a lot to do with what kind of riding you were doing. Why did TD install an 800 lb spring in the first place?

 

I have noticed that suspension that seems ridiculously hard when riding easy can be annoyingly soft when riding hard. I wonder if, since the pros can ride a heck of a lot harder than I can, if they use much harder springs. But man that 800 pound spring just felt wrong. I asked them why they sent it. What I understood was that since the YZF600R is not that popular I think they hadn't ever sold a shock for one of those before, and in that scenario they start out with the penske recommendation, which can sometimes be a bit off.

[harnois]

...

 

Pre-load will effectively increase the spring rate (essentially at the expense of geometry and suspension travel).

 

What do you mean by "at the expense of geometry and suspension travel?" "At the expense of" to me means it will have a negative impact. I understand it will change but do you mean to say that it will have a negative impact? Seems to me it could be negative or positive depending on what you are trying to do. Just wondering if there's something else for me to learn here.

[harnois]

Earlier in this thread somebody responded and said after reading the thread he was able to solve his same problem. Cool!

[racer]

I am not a suspension expert or 'guru'. What I 'know' is mostly what others with more experience have imparted over the years combined with my own attempts to really understand the 'why' (as opposed to merely going by rote); and, then applying that in practical "real world" circumstances and conditions. So, while, at the end of the day, what I 'know' is generally based on empirical evidence and experience, by the same coin, I listen to the experts and I am always open to more data.

 

That said, I do not know a scientific reason, based on the physics of springs, that would dictate a flat 8 or 10 mm of pre-load across the board for all springs as "the right number" to set pre-load to acheive proper suspension characteristic, much less using minor differences in a relatively small range of pre-load at the bottom of the scale to ascertain the proper spring rate. Frankly, if your memory is correct, I am a little mystifiied as to why you were told that a spring was "clearly too hard" based on the fact that you were only using 3 mm of pre-load, except possibly that they were concerned that 3 mm of compression might not create enough pressure or stress to hold the spring well. (I believe 'stress' is a more proper term than 'tension' (tensile), ie stress and strain in the engineering usage. )

 

Logically, to my mind, the more of the spring that is available for action, the better. Perhaps there is a "sweet spot" or more consistent linear range of response in the characteristic of springs which makes the first 10% (or whatever) of compression significantly different than the middle 80% or so. Like, if the response curve was exponential for the first 10% and then subsequently linear for the rest of its compression until it approached binding, that might make some sense to my mind in that you would have more consistency in the middle, eg. it might be easier to tune damping for a broad range of loads and inputs with a more consistent or predictable effect. It should be a simple matter to find some graphs of spring characteristic response curves ... but, for our application and purposes, we then need to think about things like "speed" of stroke and specific impulse or big bumps vs undulations in road and how that might affect any "harmonics", ie. if you hit a bump while cresting or compressing in a dip, etc, etc ... and that is way beyond my expertise ... lol.

 

In any case, I still don't really see the logic of 3 mm vs 8 mm vs 12 mm or ease of tuning dampening as the way to determine the proper spring rate and I am not getting any intuitive hints from the back of my mind which, while hardly conclusive, leads me to consider the possiblity of more 'practical' reasons for TD's recommendation that may have been either mis-communicated or perhaps merely interpreted within the context of your understanding at that time. I just don't see a direct connection between 3 mm vs 8 mm and the best rate for your weight and riding style. That doesn't mean there isn't one, I just don't know what it might be. Perhaps it is simply a rule of thumb that TD have discovered through practice that what rate works best for them, or what rate their customers say works best for them, seems to fall at or around 8 mm of pre-load. But less than 1/4" seems like a really small difference. And, if 12 mm is OK, why not 4 mm? On the flipside, capturing the spring well seems like a practical issue (and is, in fact, one that those who originally described the 'absolute zero pre-load with room to spare' set up to me also were concerned with, hence, the qualifying codicile to my original remarks to the effect that "I don't know what modifications might be necessary...").

 

So, my speculating about your memory of a conversation with TD some years ago is hardly going to get us anywhere (lol). The bottom line is that, for clarification on the 'why' of their recommendation(s), I think contacting TD is probably the best course of action. Of course, there are other, less 'brand oriented' and far more experienced suspension experts than Traxxion Dynamics out there. I don't mean to take anything away from them; but, like your experience is a singular event of limited scope, perhaps a wider sampling of a broader, more general, data set would yield some additional insights...?

[racer]

...

 

Pre-load will effectively increase the spring rate (essentially at the expense of geometry and suspension travel).

 

Since either adding pre-load or switching to a stiffer spring would reduce fork dive when braking, I realize that, especially within the context of this thread, your comment makes perfect sense. Sorry for my negative comments about it before, and for being rude and full-of-myself. We all have our moments.

 

What do you mean by "at the expense of geometry and suspension travel?" "At the expense of" to me means it will have a negative impact. I understand it will change but do you mean to say that it will have a negative impact? Seems to me it could be negative or positive depending on what you are trying to do. Just wondering if there's something else for me to learn here.

 

 

Specifically, I meant "at the expense of" in the most direct sense that it reduces travel and 'alters' geometry. I suppose that might not always be a bad thing under all circumstances; but, I am of the opinion that the geometry as designed by the engineers probably has some value as a starting point ... from time to time ... (lol). And that, for me, the broadest range of adjustment yields greater ease with which to acheive accuracy or better resolution from which to "fine tune", as it were. For example, if your analog radio tuner/dial has a really small knob, it will be more difficult to adjust precisely than if it has a really big knob which makes fine adjustments easier. I don't know if that analogy really gets it, but ...

 

Anyway ... I believe the primary geometry issue is that, pre-loading the spring adds ride height. Logically, you might think that the overall height will simply sag to the same point relative to ground because you are merely pre-compressing the spring and the spring will only sag or compress as far as it would anyway with you on it. However, as you yourself pointed out before, there is more to the suspension than merely the spring. And I think it is precisely that combined with the fact that you are not on the bike when compressing the spring that is the critical factor.

 

Ordinarily, the spring compresses under your weight, the suspension moves and the bike sags. So, what happens if you compress the spring without you on the bike to make it sag and sort of "equal out" the suspension?

 

 

(Edited for clarity. Will re-post info in better terms later.)

[harnois]

I am not a suspension expert or 'guru'. What I 'know' is mostly what others with more experience have imparted over the years combined with my own attempts to really understand the 'why' (as opposed to merely going by rote); and, then applying that in practical "real world" circumstances and conditions. So, while, at the end of the day, what I 'know' is generally based on empirical evidence and experience, by the same coin, I listen to the experts and I am always open to more data.

 

That said, I do not know a scientific reason, based on the physics of springs, that would dictate a flat 8 or 10 mm of pre-load across the board for all springs as "the right number" to set pre-load to acheive proper suspension characteristic, much less using minor differences in a relatively small range of pre-load at the bottom of the scale to ascertain the proper spring rate. Frankly, if your memory is correct, I am a little mystifiied as to why you were told that a spring was "clearly too hard" based on the fact that you were only using 3 mm of pre-load, except possibly that they were concerned that 3 mm of compression might not create enough pressure or stress to hold the spring well. (I believe 'stress' is a more proper term than 'tension' (tensile), ie stress and strain in the engineering usage. )

 

Logically, to my mind, the more of the spring that is available for action, the better. Perhaps there is a "sweet spot" or more consistent linear range of response in the characteristic of springs which makes the first 10% (or whatever) of compression significantly different than the middle 80% or so. Like, if the response curve was exponential for the first 10% and then subsequently linear for the rest of its compression until it approached binding, that might make some sense to my mind in that you would have more consistency in the middle, eg. it might be easier to tune damping for a broad range of loads and inputs with a more consistent or predictable effect. It should be a simple matter to find some graphs of spring characteristic response curves ... but, for our application and purposes, we then need to think about things like "speed" of stroke and specific impulse or big bumps vs undulations in road and how that might affect any "harmonics", ie. if you hit a bump while cresting or compressing in a dip, etc, etc ... and that is way beyond my expertise ... lol.

 

In any case, I still don't really see the logic of 3 mm vs 8 mm vs 12 mm or ease of tuning dampening as the way to determine the proper spring rate and I am not getting any intuitive hints from the back of my mind which, while hardly conclusive, leads me to consider the possiblity of more 'practical' reasons for TD's recommendation that may have been either mis-communicated or perhaps merely interpreted within the context of your understanding at that time. I just don't see a direct connection between 3 mm vs 8 mm and the best rate for your weight and riding style. That doesn't mean there isn't one, I just don't know what it might be. Perhaps it is simply a rule of thumb that TD have discovered through practice that what rate works best for them, or what rate their customers say works best for them, seems to fall at or around 8 mm of pre-load. But less than 1/4" seems like a really small difference. And, if 12 mm is OK, why not 4 mm? On the flipside, capturing the spring well seems like a practical issue (and is, in fact, one that those who originally described the 'absolute zero pre-load with room to spare' set up to me also were concerned with, hence, the qualifying codicile to my original remarks to the effect that "I don't know what modifications might be necessary...").

 

So, my speculating about your memory of a conversation with TD some years ago is hardly going to get us anywhere (lol). The bottom line is that, for clarification on the 'why' of their recommendation(s), I think contacting TD is probably the best course of action. Of course, there are other, less 'brand oriented' and far more experienced suspension experts than Traxxion Dynamics out there. I don't mean to take anything away from them; but, like your experience is a singular event of limited scope, perhaps a wider sampling of a broader, more general, data set would yield some additional insights...?

 

 

Getting Max involved would be interesting. Since you raced with him for years I'll leave that up to you. I don't want to be the one to drag him into it.

 

If we have 3mm of preload, and 30mm of suspension sag with the rider on the bike... The 30mm of sag measured at the axle translates roughly to 15mm of spring compression (due to that roughly 2-to-1 ratio previously mentioned). The 15 mm of sag plus the 3mm of preload, we can deduce that the weight of the rider and bike on the rear wheel is compressing the spring 18mm. We can increase preload to reduce sag, but in the end the spring itself will still compress 18mm with the rider on the bike - same spring same amount of weight on that spring means same amount of spring compression.

 

If we go to the softer spring, we can get the 8mm of preload and 30mm of sag at the axle, makes 8mm plus 15mm of spring compression means the spring is compressing 23mm under the weight of that rider and that bike. 18mm of spring compression with the heavier spring, now up to 23mm of compression with the lighter spring, under the same amount of weight, makes a 28% difference. So from that we can say that a going from 3mm of prelaod to 8mm of prelaod while sag remains at 30mm is a 28% difference in how far the spring is compressing! In my specific example of going from the 800 pound spring to the 650 pound spring that's a 23% difference in spring rate so the math comes pretty close to matching up.

 

If ya know how much a spring is compressing under the weight of the rider and bike it makes sense they could deduce from experience roughly how well that spring will work out as far as absorbing bumps for that rider on that bike. When I talked to the dude at Traxxion (someone other than Max don't remember who) and gave him that 3mm of preload number and was asking all the "why" and "how" I was under the impression that it not only helped him deduce that the spring was too stiff, but also deduce roughly how much to change it. When they are helping people through the mail I guess all this math is all they have to go on.

[racer]

If ya know how much a spring is compressing under the weight of the rider and bike it makes sense they could deduce from experience roughly how well that spring will work out as far as absorbing bumps for that rider on that bike. When I talked to the dude at Traxxion (someone other than Max don't remember who) and gave him that 3mm of preload number and was asking all the "why" and "how" I was under the impression that it not only helped him deduce that the spring was too stiff, but also deduce roughly how much to change it. When they are helping people through the mail I guess all this math is all they have to go on.

 

Right. So they could refer to past experience to deduce a range or good starting point ... for you and your type of riding. That was my point.

 

As for "dragging Max into it", my point was that, for your benefit, you might want to address your questions to TD personally. As YOU have a pre-existing business relationship with TD, they probably would be inclined to discuss the matter directly with you. Customer service and sales and all. In any case, I was not suggesting that you ask Max to "join the fray" or "settle" anything here. And, I would imagine that since it is his profession, he probably wouldn't be interested in holding forth on an internet forum ... for free.

[harnois]

...

 

Pre-load will effectively increase the spring rate (essentially at the expense of geometry and suspension travel).

 

Since either adding pre-load or switching to a stiffer spring would reduce fork dive when braking, I realize that, especially within the context of this thread, your comment makes perfect sense. Sorry for my negative comments about it before, and for being rude and full-of-myself. We all have our moments.

 

What do you mean by "at the expense of geometry and suspension travel?" "At the expense of" to me means it will have a negative impact. I understand it will change but do you mean to say that it will have a negative impact? Seems to me it could be negative or positive depending on what you are trying to do. Just wondering if there's something else for me to learn here.

 

 

Specifically, I meant "at the expense of" in the most direct sense that it reduces travel and 'alters' geometry. I suppose that might not always be a bad thing under all circumstances; but, I am of the opinion that the geometry as designed by the engineers probably has some value as a starting point ... from time to time ... (lol). And that, for me, the broadest range of adjustment yields greater ease with which to acheive accuracy or better resolution from which to "fine tune", as it were. For example, if your analog radio tuner/dial has a really small knob, it will be more difficult to adjust precisely than if it has a really big knob which makes fine adjustments easier. I don't know if that analogy really gets it, but ...

 

Anyway ... I believe the primary geometry issue is that, pre-loading the spring adds ride height. Logically, you might think that the overall height will simply sag to the same point relative to ground because you are merely pre-compressing the spring and the spring will only sag or compress as far as it would anyway with you on it. However, as you yourself pointed out before, there is more to the suspension than merely the spring. And I think it is precisely that combined with the fact that you are not on the bike when compressing the spring that is the critical factor.

 

Ordinarily, the spring compresses under your weight, the suspension moves and the bike sags. But what happens if you compress the spring without you on the bike to make it sag and sort of "equal out" the suspension? That extra pressure now in the spring is being applied to the same set of levers, no? Won't that force the suspension upward to a higher starting point from which the now limited travel sags? And, like you pointed out, changes at the shock body/spring translate into a bigger value at the axle. So, along with reducing travel, I believe that compressing the shock spring without weight on the bike will raise the rear ride height. And the same for the front.

 

Overall, with the suspension rising and falling through a greater range due to a too soft spring and/or that spring range being altered and limited in its travel due to excessive pre-loading, it seems an obvious thing to me that the geometry will be closer to what was intended by the designers with the most proper spring installed.

 

You are saying that adding preload reduces travel? I don't get how. "Travel" to me is a measure of how far the suspension can travel from top out to bottom out, fully extended no weight on it to fully compressed. I don't think this measurement changes when preload is added or removed, unless the preload is cranked down so far that the spring bottoms out on itself, but that'd require settings so grossly out-of-whack it isn't worth talking about. Or are you just saying that the suspension will move less in response to any given bump as a result of increased preload?

[harnois]

If ya know how much a spring is compressing under the weight of the rider and bike it makes sense they could deduce from experience roughly how well that spring will work out as far as absorbing bumps for that rider on that bike. When I talked to the dude at Traxxion (someone other than Max don't remember who) and gave him that 3mm of preload number and was asking all the "why" and "how" I was under the impression that it not only helped him deduce that the spring was too stiff, but also deduce roughly how much to change it. When they are helping people through the mail I guess all this math is all they have to go on.

 

Right. So they could refer to past experience to deduce a range or good starting point ... for you and your type of riding. That was my point.

 

As for "dragging Max into it", my point was that, for your benefit, you might want to address your questions to TD personally. As YOU have a pre-existing business relationship with TD, they probably would be inclined to discuss the matter directly with you. Customer service and sales and all. In any case, I was not suggesting that you ask Max to "join the fray" or "settle" anything here. And, I would imagine that since it is his profession, he probably wouldn't be interested in holding forth on an internet forum ... for free.

 

I already addressed my questions to TD personally back when I got that shock from them. So far, I have no reason to think that I misunderstood any of the answers.

 

"I was not suggesting that you ask Max to "join the fray" or "settle" anything here."

Neither was I. I don't care who is right or wrong about anything. I also don't want to assume that my current understanding of anything is 100% complete and correct, because then I would stop learning.

[racer]

You said that you were not sure you remembered what TD said correctly. And you said that you thought it would be interesting to have Max posting here. In any case, I made my suggestion as an alternative to continued speculation about what TD meant or why they said what they said and it was aimed at helping you resolve the questions that you are still posing in qualified or speculatively phrased interpretations of your memory of TD's words to your satisfaction. You've expressed your positive impressions of TD's CS Dept. and you have expressed a clear desire to communicate with Max. So, I just figured why not go ahead and do that? Seems logical. I didn't mean it to be any sort of characterization.

 

And, besides, maybe he will give you a free primer on motorcycle suspension technology and tuning.

[racer]

...

 

Pre-load will effectively increase the spring rate (essentially at the expense of geometry and suspension travel).

 

The only way to change the spring rate is to change out the spring. Preload simply changes the amount of sag. This is true as long as you are working anywhere within the safe and relevant preload adjustment range. When you adjust preload you are just changing the position of the spring relative to the top-out and bottom-out points, which changes how much the suspension will sag under your weight and the bikes weight. It should be called a sag adjustment so people would stop being so confused about it.

 

 

Your understanding or explanation of what pre-load is or does seems flawed here.

 

When you adjust or increase pre-load you are compressing the spring and making it shorter by "loading" the spring prior (or pre-) to putting your butt on the bike. You are not "just changing the position of the spring relative to the top-out and bottom-out points".

 

 

 

PS: I am surprised that you have left this post intact considering your apologies and, forgive me, how confused you seem to be (about a great many things....lol).

[racer]

You are saying that adding preload reduces travel? I don't get how. "Travel" to me is a measure of how far the suspension can travel from top out to bottom out, fully extended no weight on it to fully compressed. I don't think this measurement changes when preload is added or removed, unless the preload is cranked down so far that the spring bottoms out on itself, but that'd require settings so grossly out-of-whack it isn't worth talking about. Or are you just saying that the suspension will move less in response to any given bump as a result of increased preload?

 

Well, there is potential for travel and then there is how much the suspension actually does travel under "normal" riding conditions. I was thinking the latter; but, the former is also true. And you seem to be confusing the two.

 

The motorcycle is suspended on springs. When you compress the springs, the potential range of motion is decreased. And the actual amount of travel the suspension experiences is also reduced.

 

Try this: Dial out all the pre-load on your stock suspension and go for a ride. Then crank it back up and go for another ride. That should help clarify things a bit.

 

Cheers.

[racer]

http://www.motorcycle.com/products/dvd-rev...ikes-69245.html

[racer]

http://forums.superbikeschool.com/index.ph...dpost&p=250

[harnois]

You are saying that adding preload reduces travel? I don't get how. "Travel" to me is a measure of how far the suspension can travel from top out to bottom out, fully extended no weight on it to fully compressed. I don't think this measurement changes when preload is added or removed, unless the preload is cranked down so far that the spring bottoms out on itself, but that'd require settings so grossly out-of-whack it isn't worth talking about. Or are you just saying that the suspension will move less in response to any given bump as a result of increased preload?

 

Well, there is potential for travel and then there is how much the suspension actually does travel under "normal" riding conditions. I was thinking the latter; but, the former is also true. And you seem to be confusing the two.

 

The motorcycle is suspended on springs. When you compress the springs, the potential range of motion is decreased. And the actual amount of travel the suspension experiences is also reduced.

[...]

Cheers.

 

I'm not confusing anything except perhaps your terminology. Travel to me means the number, for example 4.7 inches listed on this page:

http://www.kawasaki.com/Products/detail.as...=specifications

That is the distance from top out to bottom out. It won't change due to adding or removing preload, not unless the preload has been adjusted way outside the intended adjustment range which is not even possible on any stock shock I've ever seen.

 

If we have 5 inches from top out to bottom out, and 1.5 inches of sag, then we are 1.5 inches from top out and 3.5 inches from bottom out. In other words we have 3.5 inches of upwards travel available and 1.5 inches of downwards travel available. If we increase preload, reduce sag to 1 inch, then we have 1 inch of downward travel and 4 inches of upwards travel available. Upwards travel available increased as a result of adding prelaod, which is why adding preload might prevent bottoming out on large bumps. So I still don't get what you mean by travel is reduced?

[harnois]

...

 

Pre-load will effectively increase the spring rate (essentially at the expense of geometry and suspension travel).

 

The only way to change the spring rate is to change out the spring. Preload simply changes the amount of sag. This is true as long as you are working anywhere within the safe and relevant preload adjustment range. When you adjust preload you are just changing the position of the spring relative to the top-out and bottom-out points, which changes how much the suspension will sag under your weight and the bikes weight. It should be called a sag adjustment so people would stop being so confused about it.

 

 

Your understanding or explanation of what pre-load is or does seems flawed here.

 

When you adjust or increase pre-load you are compressing the spring and making it shorter by "loading" the spring prior (or pre-) to putting your butt on the bike. You are not "just changing the position of the spring relative to the top-out and bottom-out points".

 

The comment that "preload simply changes the amount of sag" is oversimplified. So now, in order to try and explain it, I'll write a bunch of nonsense that probably nobody cares about and only makes sense to me.

 

If we adjust the preload while the suspension/shock is topped out, then yes we are compressing the spring against the top out stop. But, we don't ride around with our suspension topped out all the time, most of the time it is compressed some amount. And perhaps the most important moment for suspension is in the middle of a turn to keep the tires on the ground at maximum lean angle, and in this moment both front and rear suspension are compressed.

 

So lets evaluate how a preload adjustment would affect the bike while the suspension is sagging or compressed and not topped out. Let's say we get on our bike and while sitting on causing the suspension to sag, a friend cranks down on the preload, the spring stays compressed under the weight of the rider and bike, the amount of weight on the spring is not changing, thus, the amount of compression of the spring itself is not changing. Thus, in this scenario, we are NOT actually compressing the spring, it is just staying compressed under the weight of bike and rider, and we are just moving it down the shock, or in other words moving it closer to the top out point and further away from the bottom out point.

 

As we crank the preload down while the suspension is compressed, the shock will extend by exactly the same amount that we move the preload adjuster, while the compressed spring stays the same length. We are extending the shock and a fully extended shock is topped out, thus we are moving the spring closer to shock top out. As we make the adjustment, the back end of the bike will rise up because it is sagging less. If our adjustment takes us from 1.5 inches to 1 inch of sag, then we just moved the rear suspension .5 inches closer to the top out point and .5 inches further away from the bottom out point. Amount of overall available travel did not change, we just have, from the new sag point, more upwards wheel travel available and less downwards wheel travel available.

 

Let's say we set the sag at 1.5 inches, then we ride down the road and hit a bump that causes the suspension to compress 1 inch from the sag point and then return to the sag point. Then we crank down on the preload so that we get only 1 inch of sag, then we go out and hit the same bump at the same speed. Once again, the suspension will compress 1 inch FROM THE SAG POINT. Same bump, same speed, same spring, same starting weight on the spring thus same amount of starting spring compression, it will react the same UNLESS we reach top out or bottom out. Preload is just going to make the whole 1 inch of suspension movement resulting from that bump happen at a different portion of the available travel, or in other words, the bike will sit higher or lower as it rolls over that bump. In the context of this scenario it's perhaps not even accurate to call it "stiffer" or "softer" as is commonly said. So far no one has given me any actual mechanical explanation as to how this description is incorrect.

 

In the case of front end dive under braking, relevant to this thread, if we increase front preload to reduce sag by .5 inches then repeat the hard braking in exactly the same way, then for all practical purposes the front end will have .5 inches less dive. The front end will sit .5 inches higher during the braking and all other times except when bottom out or top out is reached. Changing out the spring could make a much bigger difference, and would allow me to have less brake dive without reducing sag, which is why like you said, it is a much better option - as long as we don't decrease the fork's mid-corner performance.

 

PS: I am surprised that you have left this post intact considering your apologies and, forgive me, how confused you seem to be (about a great many things....lol).

 

I'm not really confused about anything except for some of your statements and explanations. It's obvious to me from other threads you've participated in that you have a good understanding of how suspension works, and I believe my understanding is quite clear as well, but we have a different way of perceiving it and/or explaining it and I think we're mostly just going in circles over that sort of thing. Perhaps this is a peculiarity in how I communicate, but just because I ask a question doesn't mean that I don't already think I know the answer. It just means I'm open to whatever new subtle understandings may come of it. The "enough preload to capture the spring" concept came out of that. Another example would be me opening the thread in the first place. Just cos I asked the question about the rear end wiggling under braking doesn't mean that I had no idea at all how to solve it. It just means, I'm open to more ideas, in case there's something I hadn't thought of or heard of before. I don't ever assume that I know everything about anything.

 

Edit: I have now removed that post. It still remains here in quotes which is good because this is where we finally get around to talking about it in more depth.

[racer]

harnois,

 

My "confused about a great many things" remark was too much. My bad.

 

racer