Big Willy

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.

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)

I forgot, here's a link for you. Your spring rate would seem correct. http://www.racetech.com/evalving/SpringRat...SpringType=Fork Hope this works for ya.

G'day Jaybird, Racetech have a site where you can check if the springs are correct for your weight. Did the chap mean that he thought your 'forks' were twisted? I have n't heard of 'springs' being twisted but it's possible I suppose. Even though you may have the correct weight of spring, are they the correct spring for your bike? i.e. diameter and length? If they are, bottoming out is usually an indication of insufficient oil level in the forks or not enough compression damping. What can sometimes be mistaken for stiction I have found is too much rebound damping. This keeps the forks in the "harder" part of the stroke and doesn't give them a chance to rebound to soak up new stuff. As for forks twisted, this is possible too and here's the fix.. Loosen off the top triple clamp bolts that hold the fork. Place the front wheel between your legs facing the bike and give a yank (no pun intended Racer) on the bars to align them. If they were out, you might hear a "crack". Tighten the bolts to manufactures specifications. This is very important on the lower triple clamps if you have upside down forks as too tight can squeeze them causing some binding on the staunchion. Some prefer to leave them looser than manufacturers' specs. Don't bother with the following steps unless your axle has grease on it. This not only protects it from rust, but it going to help set up the forks. Loosen the lower fork bolts that hold the front axle on the side that DOESN'T have the axle nut. Wedge a little flatbladed screwdriver a little way in to hold the 'clamp' part of the fork open. Apply the front brake and bounce like mad a few times on the forks. Tighten the lower fork leg (after removing screwdriver d'oh). Check all other nuts and bolts are tight. Some consider that getting the length of the forks exactly the same is critical i.e. lifting or lowering a fork leg a couple of mm so that they are even. But there are forks today that have the compression damping in one and rebound in the other. You can imagine the push/pull effect they have against each other so I don't think 1 or 2 mm is going to affect things too much. That said, it's easy enough to get them perfect if you want by checking how easily the front axle slides in and out and making an adjustment. What I also do is spray a lanolin based lubricant under the dust seal and also onto the fork leg. Many do not realise that when they wash their bikes, they wash the oil film off the staunchions. A zipty around a fork leg is also handy to guage fork travel. Just tight enough to slide on the leg and stay in any one spot. Longwinded I know but I'd try some of these first if you have only recently had your forks done.

Why? Acceleration (or lack of it) is the single defining parameter for weight bias (and the most common condition that any motorcyclist might find oneself in when seeking more traction from the rear wheel). More acceleration equals more weight on the rear wheel. Any brake equals less acceleration. Less acceleration equals less weight on the rear wheel. Period. (Have you read A Twist of the Wrist?) Or do you think you can pop a wheelie by applying the rear brake? (I think a world full of stunt riders would disagree.) Here's my rear brake test: Go pop a wheelie and apply the rear brake. What happens? I see your point. But acceleration is only possible should the rear wheel have traction. A rear wheel spinning faster but not gripping does not equal acceleration and no further weight transfer is happening. Let's try to apply your "theory" to a real world situation then. If I was stationary stuck in the mud or snow, applying the rear brake would do nothing as I am not moving. However, I could slide my fat butt back and (compressing the rear suspension in the process) transfer a bit of weight to gain some traction. But that is due to my butt moving, not the suspension compressing. And, in that scenario, dragging the rear brake if the rear wheel was spinning, might compress the rear and cause the bike to slope rearward which might alter the static weight balance toward the rear to help the spinning wheel bite (but not very much). Once I start moving out of the mudhole, continuing to drag the rear brake might help to the degree that the rear wheel continues to spin, but, the more traction I get, the more it will transfer potential weight forward, ie. without the brake, I could accelerate harder and transfer more weight rearward than applying the rear brake might, if it could. Remember the wheelie example. It's primary effect would be to slow me down now that I am making progress out of the mudhole. Once in motion (accelerating), the only practical use I can think of would be to slow or stop the spinning wheel (to help gain traction)... like it was designed to do. But, that is about slowing the spinning wheel more than weight transfer. Which is why advanced roadracers (and I imagine dirtbikers) on especially powerful machinery use it when exiting corners to control rear wheelspin under acceleration, not to transfer weight to the rear for more traction. They are already transfering exponentially more weight under acceleration than dragging the rear brake ever would if it could. No. Loading of the rear spring in that scenario is not due to weight transfer, it is due to force from the motor being transmitted to the spring via the rear suspension geometry. Blah blah blah. PUSSY. You're really quite a whiney little sod aren't you? Typical bully that runs away when he can't win or gets his nose bloodied a bit. (ETA: This last was meant as an in kind jestful poke for continued and unneccesary name calling by you here in this thread, Willy. We've been able to avoid the flaming and other sorts of personal attacks found on other forums here and I think it would be nice if we could continue to do so. Thanks.) That said... dirtbikes have never been my forte. In any case, any acceleration will do more to transfer weight to the rear than the rear brake ever could... if it could at all once you begin to accelerate. Once you are moving, the "weight transfer" created by the brake goes forward no matter the slope of the bike rearward. Look, I see the picture of mechanical forces you are trying to paint. But, just like the effect of chain pull compressing the rear suspension is far outweighed by the effect of the accelerating rear wheel which overhwelms it, the potential for weight transfer you imagine is far outweighed by the weight transfer of acceleration. I fail to see any practical value beyond controlling wheelspin... like it was designed to do. Which is more about the wheel than the weight. G'day Racer, "argumentative sod" is something my dad always used to say to me and there was no malice intended. That's always been the trouble with forums, facial expressions/body language don't exist and well...you know what I mean. If I could sort out how to post those icons below, I'd use one and there'd be no misunderstanding. Anyway, I'm sorry if I upset anyone, wasn't my intention. Onto other things... What you said about using the rear brake is complete bullshit (joking). It's true, it is used to control wheelspin. Dirt riding, I find it much easier to control what the rear wheel is doing coming out of a corner using the rear brake. Strange though isn't it? You'd think you would have more sensitive control through the use of your hand in backing off the throttle rather than using a foot in a sock in a boot to control things. Maybe there is more to the rear wheel braking than this. It may be about controlling the suspension. I'm just putting it out there. Nothing wrong with thinkin' (insert smiley icon).

Blah blah blah. PUSSY. You're really quite a whiney little sod aren't you? Typical bully that runs away when he can't win or gets his nose bloodied a bit. That's not nice.

No I am not from BITOG Common Sense is not a Common Virtue Position of caliper does more than just COG and debris protection. Motorcycles have evolved and placement of componentry is not as arbitrary as "let's put it there because it fits" anymore. Anywho, I think I'm getting "stupider" by the minute. Peace. Apologies. I didn't mean to infer that you lacked commonsense. Peace

Why? Acceleration (or lack of it) is the single defining parameter for weight bias (and the most common condition that any motorcyclist might find oneself in when seeking more traction from the rear wheel). More acceleration equals more weight on the rear wheel. Any brake equals less acceleration. Less acceleration equals less weight on the rear wheel. Period. (Have you read A Twist of the Wrist?) Or do you think you can pop a wheelie by applying the rear brake? (I think a world full of stunt riders would disagree.) Here's my rear brake test: Go pop a wheelie and apply the rear brake. What happens? I see your point. But acceleration is only possible should the rear wheel have traction. A rear wheel spinning faster but not gripping does not equal acceleration and no further weight transfer is happening. My point once again is that you can accelerate while applying rear brake creating drag which causes the rear spring to load, transferring weight to the rear wheel creating more traction. I don't know if I can put it any clearer than that. It's probably more an issue with dirt bikes so let's forget it. Wheelies have never been my forte unfortunately.

I agree we may be missing each other but now I'm missing you as well. Let's stop thinking about the bike decelerating and think about constant drive or acceleration while applying the rear brake. It stands to reason that as the back of the bike comes down (due to chain pull, as I have proved to myself on my own bike), the front must become lighter as the motor is effectively now trying to revolve the rest of the bike around the back wheel. More weight = more traction. I don't think the position of the caliper has any bearing on this but I will stand corrected It's one of the reasons why rear brake calipers have been moved over recent years. Now regarding that chain pull thing: I'd have to reread it, however one thing that often gets overlooked is Newton's 1st law. F1=F2, for every action there's an equal and opposite reaction. Don't overlook opposing forces which often is not what we observe (ex. centripetal vs centrifugal force) Sorry, but that doesn't sound right to me. I will abstain from asking you for proof as I hope common sense will prevail. The position of the caliper is only relevant to weight distribution i.e. if it is on the top of the swingarm, then this obviously alters the COG. If it is on top of the swingarm, then it is less likely to pick up debris from the road. If it is on top of the swingarm, then a pushing force is applied to the stay as opposed to a pulling force on the stay should it be under the rear axle. The way the rear caliper is set up on my bike i.e.with the stay an integral part of the swingarm where the caliper holder slots into lugs on the swingarm, wouldn't really matter much if the caliper was above or below the disc. Are you the same Jaybird from BITOG btw?

I agree we may be missing each other but now I'm missing you as well. Let's stop thinking about the bike decelerating and think about constant drive or acceleration while applying the rear brake. It stands to reason that as the back of the bike comes down (due to chain pull, as I have proved to myself on my own bike), the front must become lighter as the motor is effectively now trying to revolve the rest of the bike around the back wheel. More weight = more traction. I don't think the position of the caliper has any bearing on this but I will stand corrected

Right. Only the mass of the machine and rolling friction to resist forward acceleration...which you don't have with the rear brake on. Not sure how good of a test this is either. Only one cuppa so far and I gotta leave for work. I'll think more about it. What kind of bike do you have? G'day Racer The bike's rear wheel offers the most resistance to forward movement as this is the one that has to do all the driving. What I'm saying is that most resistance comes from the rear wheel whereas the front wheel is "happy" to roll. Evidence?...Which one would wear the most in a 1/4 mile drag? To understand better why the rear rises and forgetting all about chain pull, I thought this might help... Picture the chain as merely the drive, it is rotating the rear wheel. The rear wheel transfers that drive through the axle alone. The axle transfers that drive via the swingarm to the swingarm pivot shaft which transfers it to the bike frame That rear axle is below the swingarm pivot shaft. Provided the suspension can keep the rear axle below the swingarm pivot shaft, then it will continue to try to push "under" the swingarm pivot point. I have an 02 ZX636. BTW, thanks for the interesting discussion.

G'day Racer I've given this some thought and decided that this is what happens... By applying the rear brake with the throttle on means something has to give. If the motor is unable to turn the rear wheel or has resistance, then it would want to spin itself around the rear wheel thus biasing weight towards the rear which in turn creates more friction between the the tyre and surface which enables more traction. Does this sound plausible? Willy Applying the rear brake adds more weight to the front, not the rear. And, in any case, adding more weight and braking force together to the same wheel, front or rear, increases the load and stress on the contact patch which typically reduces the amount of available traction. If the front wheel is light to begin with, for instance when you are cresting a hill, dragging the rear brake can add weight to the front and help create more traction at the front. But, even if you are accelerating, dragging the rear brake counter-acts the acceleration and weight transfer to the rear, hence, why you can control a wheelie with the rear brake or bring the front down with it. You're an argumentative sod Racer Of course, applying rear brake in a deceleration mode will add more weight to the front as evidenced by nose dive, but we're talking applying rear brake under power. To simplify, if you had a piece of rubber in your hand or sandpaper or timber and your ubbed them across a concrete surface, would friction be greater if you applied more weight to these? Same applies to tyres. Would you have any grip on the rear tyre when it is skipping across the tarmac because you are heavy on the front brakes? I agree that braking into a corner can place extra stress on available front wheel traction as evidenced many times watching GP's. That is not to say that there is not more traction available due to weight transfer, more a case of simply overbraking/locking front wheel or just going beyond the limit of available traction. You are correct when you say that the rear brake will bring the front down cresting a hill, as would deceleration. The point I'm making is to apply the rear brake WHILE accelerating. Just finished work, had a few beers, will get back asap

So, contracting chain pull under acceleration tries to lengthen the wheelbase? Can you post a video of you conducting your experiment that demonstrates the rear compressing? Wheelbase lengthening really doesn't come into it but yes, wheelbase will lengthen. So what? It's not as though there is any resistance to lengthening the wheelbase after all there is no wall in front of you same as there isn't when you are racing/riding. As for video, I don't think so. How 'bout you sit on your bike in your garage, start it up, put it in first, apply the back brake, let out the clutch and you tell me what happens. It'll take about a minute. I've just done it.

G'day Racer I've given this some thought and decided that this is what happens... By applying the rear brake with the throttle on means something has to give. If the motor is unable to turn the rear wheel or has resistance, then it would want to spin itself around the rear wheel thus biasing weight towards the rear which in turn creates more friction between the the tyre and surface which enables more traction. Does this sound plausible? Willy

After playing with preloads, compression and rebounds, the thing that alleviated much of the problem was more compression on the forks. From what I can gather, the bike was diving so forcibly when applying the front brake that the steering angle would alter considerably and all at once. Am now happy once again. Cheers for your input fellas