johncw

I'm was talking about anytime the suspension is in a state other than fully extended (99.9% of the time when riding). But yes you would be correct, increasing preload will make the front suspension a tiny bit 'stiffer' at the point of landing after the front wheel has completely left the road, and the suspension was fully extended (a wheelie). That's the only time it will make any difference. So by all means if you want to soften the landing a tiny fraction when pulling wheelies consider reducing the preload. I would have though that better throttle and rear brake control would have been way more effective. But I dunno, I like to keep the front wheel planted firmly on the road (and I'm probably in serious trouble if the rear wheel has completely left the road). I set preload for what its meant to do, and that is position the suspension correctly in its travel range to prevent it from either bottoming or topping out.

No, the example you give makes zero difference. As long as the weight of rider and motorcycle compress the spring, the initial preload on the spring, or the weight of the spring used makes no difference. If the spring has to support X lbs, it will ALWAYS be compress to a length where it supports X lbs (regardless of whether it is a strong or weak spring). It can do nothing else but this. The only difference between a strong and weak spring is how much it compresses to support X lbs. Partially compressing each spring (preload) prior to applying X pounds won't alter how much each compress, as long as X pounds exceeds the preload tension. The only time preload can alter how much a spring compresses if is it is greater than the load applied, and in that case the spring doesn't move at all. In practice you can achieve this with just the weight of the bike (zero free sag) where the suspension is fully topped out with no rider on board (usually a sign of underweight springs for the bike/rider weight). But if you were able to achieve zero sag with a rider on the motorcycle, you would have no suspension. A 250 lb guy will need to screw up the preload with underweight springs to get an acceptable 'rider sag'. But this doesn't increase the spring rate one zot. All it's doing is pushing on one end of the spring to physically lift the motorcycle. That's all its doing. In older damper rod forks with no external adjuster you changed the sag by simply cutting a different length internal spacer (a piece of water pipe). If you wanted 10mm less sag you cut your piece of water pipe 10mm longer than the spacer already in the fork. Stick it in and it just jacked up the fork to sit 10mm higher. Spring remained the exact same length once it was supporting the weight of the bike, or rider and bike.

Your purely theoretical example is correct. If you could (you can't for practical reasons) put so much preload on the springs that the bike doesn't sink even the smallest amount into the suspension travel under the combined weight of bike and rider .......... the ride would be harsher, because now you have no suspension. The amount of preload in the spring would need to exceed the combined weight of motorcycle and rider it is supporting. You would be riding around on a motorcycle with the suspension fully topped out, so not only would it be harsher it would handle like totally crap. Now back to the real world. Firstly you can't put that amount of preload into a typical motorcycle suspension even if you deliberately wanted to. As long as the suspension sinks even a tiny amount when the combined weight of rider and motorcycle is added, the spring length will be IDENTICAL regardless of the amount of initial preload. The only difference will be the amount the spring had to compress before achieving its equilibrium length. If say the combined weight of rider and motorcycle is 100kg, the spring will ALWAYS compress to an identical length where it exerts an opposing 100KG force. Whether the spring initially sits compressed to 25Kg, 50kg, or 75kg is totally irrelevant to the length it will be when 100Kg of force is applied. All that will change is the amount of distance it needs to compress to get to its 100Kg length. P.S. The only reason I'm laboring this point is that so many beginning riders falsely believe that altering preload will affect ride quality. It may have some very minor effect in the final 1/3 of travel due to its impact on oil height, and correctly set prevent the suspension topping or bottoming out. But for all practical purposes ride quality is determined by spring weight, rebound and compression damping, and fork oil height. Preload only affects the spring length when the suspension is fully extended and we don't ride around much like that. Just to reinforce one more time, all it does do for practical purposes is alter the static ride height of the motorcycle. If your not going to accept the word of highly regarded experts e.g. [url=]http://www.promecha.com.au/sag_preload.htm[/url], and author with a PhD, I suppose its silly to think you'd believe me. Perhaps you didn't even bother to read the article.

I disagree with nearly everything you said. Ps - preload does not change the ride height. It simply preloads the spring (which most are progressive springs these days, this affecting how stiff it will be in its initial stroke). That's where we probably differ, I disagree totally with everything you said. The ONLY thing altering preload does is alter the height of the motorcycle (with the exception of suspensions with long rebound springs). "Preload is simply the amount the springs are compressed while the suspension is fully extended" - [url=]http://www.promecha.com.au/sag_preload.htm[/url] That is the only time preload has any effect on the spring length, "when the suspension is fully extended". And that is because this is the only time the spring is 'trapped' between two fixed distance ends. All other times the spring is free to move the inner fork tube or shock absorber shaft up or down the exact same distance the other end of the spring is shifted. As soon as you drop the weight of the bike (or rider and bike) onto the spring it will compress to EXACTLY the same length regardless of the initial preload. It must, simple physics. It will always compress to a length where it exerts a force equal to the weight of bike and rider. The bike and rider weight do not change when preload is adjusted, so the compressed length of the spring to support that weight will always be the same, it MUST. The only difference will be the height of the bike (because the position of one end of the spring has been shifted up or down). Another worthy quote from the above link (the authors of that article are one of the most highly regarded motorcycle suspension specialists in Australia)....... "Preload makes the bike sit higher, or lower. It does not make the spring stiffer. So if someone tells you that you should reduce your preload to make the bike feel less harsh, they probably don’t have a clue."

Couple comments, purely intended to help ...... Forget looking at tire wear to set the suspension. Tire wear WILL give you some guide about tire pressures, but not sag, rebound or compression damping. Adjusting preload has ZERO affect on stiffening or softening the spring. All it does is raise or lower that end of the motorcycle. It's important to get preload/sag correct as having the bike ride in the correct part of the suspension (roughly 1/3 of available travel), as well as having the correct front to rear height relationship is very important to handling. However it doesn't make the springs 'stiffer', not one little bit. Set the front and rear sag correctly, by static measurement in the workshop. How to do it is well documented in numerous articles you'll find on the internet. Once measured and set, don't touch it again. So to your question "how does rebound and pre-load interact? Simple answer, they don't interact at all. As previously explained the sole purpose of preload is to set the static position of the motorcycle in the suspension travel. Rebound is to control the rate of return of the compressed spring after it encounters a bump (or anything else that causes the spring to compress e.g. rear squat, front braking, cornering forces). So in practical terms, set sag by measuring and adjusting it in the workshop. Set rebound damping on the road to get an appropriate balance between firming it up to remove any wallow in the corners on smooth roads, yet at the same time be fast enough to give an acceptable and compliant ride over rough roads. It will always be a balance between these two competing objectives that only the individual rider can determine. Good luck, and if anything needs clarification don't hesitate. P.S. and if your're scratching your head trying to figure why screwing up the preload doesn't stiffen the spring, consider your humble bathroom scale. Does sticking a block of wood under the scale increase what the scale says you weigh (determined by how much the spring inside the scales compresses) or just increase your height when you're standing on the scales??? The preload adjuster is in principal no more than that block of wood. Screwing up the preload adjuster just pushes against one end of the spring, bike just goes up, amount spring compressed remains unaffected (solely determined by the weight of motorcycle and rider it is supporting).

If your trying to remedy the sides of groves running across the tire standing up on one side, and scrubbing out on the other (so you end up with a saw-tooth effect), just be aware that is completely normal and all tires do it. To understand why, get a pencil eraser (we call then rubbers in Australia but I appreciate that term has a different meaning in other countries), and holding it at say a 45 degree angle drag one end of it across the table. Now push that same end of the eraser across the table. When you dragged it, the edge was trailing, flexible and folded up. When you pushed it the same edge was more rigid, dug in, and scrubbed away more. That's what's happening with grooves running across the tire. Just like the eraser the leading and trailing edges of grooves running across the tire respond differently as they pass over the abrasive road surface, so wear differently.