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Purely a theoretical discussion but this is something that has been bugging me for the longest:

 

I’ve long since been curious to learn more about adjusting suspension and chassis geometry. To learn more, I bought Dave Moss’ Intro to Sportbike suspension DVD. At the time I purchased it, I didn’t feel that it was an entry level DVD. I’ve watched it several times over about 1½ year’s time and feel that I could now at least have a decent conversation about the subject.

 

(My review here http://www.cbrforum.com/m_268287/tm.htm)

 

One thing I’ve learned is that setting sag is so important and should be done first. When I had my suspension done last year, I just installed it and rode away. I tweaked rear preload a bit, but only a when going to 2 up and I constantly struggled to get it back.

 

I noticed that I liked the feeling from having the rear set higher than it was on the stock suspension (a healthy 220 fully dressed). Well, I have a proper suspension now. How will I know when I should raise the clevis on my Ohlins shock to give more ride height versus adding preload (we’re talking 1 up riding here)? What are the pros/cons of each?

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If you have the ability to adjust ride height directly, then do so. You don't want to compromise your top out/bottom out head room by adjusting pre-load for ride height. Adjust pre-load to keep yourself centered in the travel and leave it there (except two up).

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If you have the ability to adjust ride height directly, then do so. You don't want to compromise your top out/bottom out head room by adjusting pre-load for ride height. Adjust pre-load to keep yourself centered in the travel and leave it there (except two up).

Thanks for the quick response Racer. I had hoped to bait you into this discussion :-)

 

So you're saying that I should crunch the spring about 1/2 way? What's this business about setting up a bike with enough spring to eliminate the need for preload that I've heard/ read so much about?

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Ride height is about adjusting the overall geometry of the frame and steering head.

 

Pre-load, or more properly SAG, is about your "Q-point" or the place where your suspension lives without being acted upon by outside forces, ie. bumps in the road. Pre-loading your spring is setting the level where the shock (compressed only by your butt) lives between the bump stops (top and bottom) of your shock absorber by extending the shock more or less. Oddly enough, pre-loading the spring has nothing to do with the spring.

 

The spring is the spring and will compress the same amount to the same point when you sit on it no matter how much you pre-load it. It doesn't make your suspension stiffer or harder. So, you can pre-load and compress the spring to the same point as it would when you sit on it and there will be no sag. Or you can use less preload such that when you sit on it, the bike sags a lot. Your butt is still sitting at the same compression point on the spring no matter what you do. In reality, what you are doing is extending and compressing the shock. The bottom line is that you want the shock to be in the middle of its range when you sit on the bike so it doesn't bottom out or top out when you hit bumps in the road because that causes the tire to lose traction or even contact with the road because it can't move any further to absorb the bumps.

 

So, the more you have to pre-load the spring to get the right sag, the closer to binding the spring you are, the less potential movement the spring has. Ultimately, a straight rate spring reacts the same to the same forces no matter how far it is compressed, hence, a spring that is too soft at the top of the range is still too soft at the bottom. The more you have to pre-load the spring to not bottom out, the closer you get to topping out. The more you can stay away from either, the better off you are, the more ptoential your suspension has to absorb bumps, the more adjustability you have to fine tune things, the less you need to use damping to make up for soft spring and use it for what it is intended for and... the less you need to pre-load the spring to get the right sag, the less you alter ride height or the geometry of the bike.

 

So, ultimately, from a parametric point of view, no preload is best.

 

What does Dave Moss say about it?

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Ride height is about adjusting the overall geometry of the frame and steering head.

 

Pre-load, or more properly SAG, is about your "Q-point" or the place where your suspension lives without being acted upon by outside forces, ie. bumps in the road. Pre-loading your spring is setting the level where the shock (compressed only by your butt) lives between the bump stops (top and bottom) of your shock absorber by extending the shock more or less. Oddly enough, pre-loading the spring has nothing to do with the spring.

 

The spring is the spring and will compress the same amount to the same point when you sit on it no matter how much you pre-load it. It doesn't make your suspension stiffer or harder. So, you can pre-load and compress the spring to the same point as it would when you sit on it and there will be no sag. Or you can use less preload such that when you sit on it, the bike sags a lot. Your butt is still sitting at the same compression point on the spring no matter what you do. In reality, what you are doing is extending and compressing the shock. The bottom line is that you want the shock to be in the middle of its range when you sit on the bike so it doesn't bottom out or top out when you hit bumps in the road because that causes the tire to lose traction or even contact with the road because it can't move any further to absorb the bumps.

 

So, the more you have to pre-load the spring to get the right sag, the closer to binding the spring you are, the less potential movement the spring has. Ultimately, a straight rate spring reacts the same to the same forces no matter how far it is compressed, hence, a spring that is too soft at the top of the range is still too soft at the bottom. The more you have to pre-load the spring to not bottom out, the closer you get to topping out. The more you can stay away from either, the better off you are, the more ptoential your suspension has to absorb bumps, the more adjustability you have to fine tune things, the less you need to use damping to make up for soft spring and use it for what it is intended for and... the less you need to pre-load the spring to get the right sag, the less you alter ride height or the geometry of the bike.

 

So, ultimately, from a parametric point of view, no preload is best.

 

What does Dave Moss say about it?

 

From what I understood from your post, it's better to setup geometry with ride-height adjustments. However, the spring is what gives rebound potential. As the spring is compressed it wants to return (rebound) that energy somewhere, and that's where the rebound circuit come in-the attempt to regulate the rate at which that energy is transferred.

 

I'm not sure I agree with NO preload.

 

Not sure I agree with half either and I'll tell you why.

Most of the surface imperfections that will affect the rider are bumps. In theory there should be an equal number of bumps as are dips, but there just aren't and even if there were, they still don't have the same impact on the rider's feel of the road surface.

 

I agree with you that we need the potential of the travel available to us. In my humble opinion (grain of salt) in a ideal setup suspension we should use 99.9% of travel at the hardest braking/bumps/acceleration points that we will ever encounter on that track on that day during an event. The softest spring that will give us this, and still give geometry range appropriate to maintain vehicle control will be best. The damping circuits would then only be used to control the movement artifacts that present themselves.

 

Now onto your 1/2:

Wouldn't about 2/3s work better? Considering that we see more bumps and that we wouldn't need as much top out potential.

Throw me a bone here, I'm trying to get all this straight in my head.

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yes, Jay. You are right.

 

I didn't mean a measured half, simply the point that allows enough head room in each direction so as to not top out or bottom out when riding.

 

As for choosing a spring, it can be something of a personal preference which can vary with riding type, style and/or conditions. Choosing one that allows the minimum amount of pre-load necesary to capture the spring is one philosophy that I've found to be common. I'm sure there are others.

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yes, Jay. You are right.

 

I didn't mean a measured half, simply the point that allows enough head room in each direction so as to not top out or bottom out when riding.

 

As for choosing a spring, it can be something of a personal preference which can vary with riding type, style and/or conditions. Choosing one that allows the minimum amount of pre-load necesary to capture the spring is one philosophy that I've found to be common. I'm sure there are others.

 

So I should be able to reduce preload a both ends, thereby giving me more overall height while maintaining geometry?

What's this business with swingarm angle?

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yes, Jay. You are right.

 

I didn't mean a measured half, simply the point that allows enough head room in each direction so as to not top out or bottom out when riding.

 

As for choosing a spring, it can be something of a personal preference which can vary with riding type, style and/or conditions. Choosing one that allows the minimum amount of pre-load necesary to capture the spring is one philosophy that I've found to be common. I'm sure there are others.

 

So I should be able to reduce preload a both ends, thereby giving me more overall height while maintaining geometry?

 

I'm not sure I quite follow what you mean here, Jay. You should set your pre-load according to your shock travel, typically between 3/4"-1.25" of sag depending on what kind of riding you are doing. However, as I understand it, the more you pre-load the spring, the more ride height you will gain as you are basically extending the shock to compress the spring. So, when you sit on the bike, the spring compresses to the same place it always does, but the cshock is now less compressed and you are further from bottoming out. But closer to topping out. As I understand it, the proper length spring with no preload at 1" sag should give the maximum amount of headroom in either direction.

 

It does seem logical to me to choose a spring that is soft enough to absorb the bumps with your body weight and that that spring rate will vary from one machine to another as the machine itself varies in weight. And, at the end of the day, the amount the spring needs to be pre-loaded will be a secondary function of the proper spring rate. Of course other details like the geometry will also affect how the suspension reacts. That is why I said "from a parametric point of view". In other words, merely considering the parameters or measurements we were discussing, namely the range of travel and the topping out/bottom out.

 

What's this business with swingarm angle?

 

I don't konw, Jay. You're the one with the Dave Moss book. And I'm certainly no expert on the subject. What is this business with the swingarm angle?

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I don't konw, Jay. You're the one with the Dave Moss book. And I'm certainly no expert on the subject. What is this business with the swingarm angle?

He said something about 11-12.5 degrees for a racebike. All I know is that he said you 'don't want the swingarm to go flat when you get on the gas' because "the geometry stops working for you and begins to work against you". He's a lot more technical than I am, but I enjoy trying to understand it. I just assumed that you could help me out a bit with your knowledge of the technical side of things.

 

I suppose one of these days I'll take a suspension seminar. I read about one that Dan Kyle did one awhile back, Paul Thede does them periodically, and Dave Moss does them once in awhile too.

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Taking a seminar sounds like a good idea. I could certainly stand to learn more about the subject myself.

 

Unfortunately, suspension is one area where I know just enough to make adjustments for riding conditions to a suspension that is, in theory, already set-up to a proper baseline, ie. I rely on an expert to tell me the correct spring for my weight, machine and type of riding.

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Interesting topic. Very nicely explained Racer.

I remember reading some time ago that riders were unwinding the preload on the rear spring to attempt to get a "softer" ride.

Now this combined with rising rate linkages in fact made the suspension "harder" because the swingarm was now in the region where a lot more force was required to compress the spring.

In other words, more preload can = softer ride (softer meaning compliant).

Always best to get the right springs for the job and set the sag properly. (insert smiley here)

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I don't konw, Jay. You're the one with the Dave Moss book. And I'm certainly no expert on the subject. What is this business with the swingarm angle?

He said something about 11-12.5 degrees for a racebike. All I know is that he said you 'don't want the swingarm to go flat when you get on the gas' because "the geometry stops working for you and begins to work against you". He's a lot more technical than I am, but I enjoy trying to understand it. I just assumed that you could help me out a bit with your knowledge of the technical side of things.

 

I suppose one of these days I'll take a suspension seminar. I read about one that Dan Kyle did one awhile back, Paul Thede does them periodically, and Dave Moss does them once in awhile too.

Ah, that's gotta be the old bike rising under acceleration thing. If the swingarm is pointing upwards at the pivot point, then it's gonna make the bike rise ...unless the spring is too soft or too much race/static sag.

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Interesting topic. Very nicely explained Racer.

Thanks.

 

I remember reading some time ago that riders were unwinding the preload on the rear spring to attempt to get a "softer" ride.

Now this combined with rising rate linkages in fact made the suspension "harder" because the swingarm was now in the region where a lot more force was required to compress the spring.

In other words, more preload can = softer ride (softer meaning compliant).

Interesting. Very interesting.

 

I hadn't considered the relationship of rr-linkage orientation to pre-load.

 

Thank you for that insight, Big Willy.

 

racer

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  • 1 month later...
Interesting topic. Very nicely explained Racer.

I remember reading some time ago that riders were unwinding the preload on the rear spring to attempt to get a "softer" ride.

Now this combined with rising rate linkages in fact made the suspension "harder" because the swingarm was now in the region where a lot more force was required to compress the spring.

In other words, more preload can = softer ride (softer meaning compliant).

Always best to get the right springs for the job and set the sag properly. (insert smiley here)

 

 

Hello. Yes, very interesting topic. Nice your explanations racer. I am french spoken, and not a specialist in english but I will try to explain my point of vue.

 

What you are explaining here Big Willy, is ok for a bike not in movement or with little throttle. Effectively, with more preload, you have more rear ride height and your rear shock is working in a softer region because higher linkage ratio and then more rear end movement for a given load. But, if you imagine all the forces who are there when on the throttle, that's not the same results... If with more preload (or more rear ride height) the linkage gives static softer suspension working, the swingarm angle with regard to the ground becomes more important. Then the driving forces at the ground and especially the chain pull are trying to extend the suspension when you accelerate the bike. You have to keep in mind that this 2 forward forces vectors are applied (with their direction, sense and intensity), at the rear wheel axel and they have, with more swingarm angle, more torque with the swingarm pivot. The weight transfert to the rear tries to compress the suspension but the 2 anti squat forces "help" the spring and make the rear suspension stiffer. With a lot of rear ride height, the rear suspension can even rise under acceleration. In the opposite, with little ride heigt, the swingarm is more flat, the 2 anti-squat forces apply then a lower torque with the swingarm pivot, and then the rear height squat a lot under acceleration. This is why the linkage has lower ratio near bottoming out (then more vertical wheel rate) to offset a little the lower action of the 2 anti squat forces. Then with less preload, you get a softer reaction of the resultant of all the forces who interact (weight transfer, chain pull, driving force and spring via his linkage)

 

Note that the chain pull is the more important anti squat force for 2 reasons :

1) the chain pull as +/- 4 time the intensity of the ground driving force (the ratio between the diameters of the rear wheel and the rear sproket)

2) the chain pull has more angle in the opposite direction (forward and downward) of the swingarm (forward and upward), and then more torque with the swingarm pivot (the ground driving force vector is always horizontal forward).

 

PS : my swingarm angle (depending the track) is +/- 10 degree (CBR600RR with Ohlins) because I like to have more grip on the throttle out the corner, then I have to setup with a little more pro-squat

 

That's not easy to explain but I hope that will be comprehensible. I play with this when I am racing my sportbike, that's fun ;)

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