stangmx13
not Stan
hoping to get a bit of discussion on this one. im not sure theres a definitive answer to my "why" question, but we'll c. at worst, we all learn something.
ive been dealing with an annoying problem on my race bike... spin and pump over corner exit bumps. smooth corner exits are no big deal. the tire spins and just drives. but any time the pavement has even minor ripples, the rear suspension pumps. the harder the drive and the larger the bumps, the worse it is. T6 at Miller, T3 & Riverside at Buttonwillow, T9 at Fontana are good examples. Riverside hasnt been bad and is almost cured since i put a new piston in my shock, but that corner was always minor compared to the others. the other corners are so bad i def have to slow my roll-on and work hard to not get bucked off.
so yesterday at Fontana, i changed something every session and was able to accomplish a lot. here are my findings:
1. increasing rebound damping helped the issue some
- too much rebound made the bike feel wooden and reduced grip.
- even w/ the setting at "too much", the pump was still bad enough to affect my laptime, ie rebound is not the solution
2. compression damping adjustment had almost no effect
- less comp made other bumps feel better, suggesting im near the limit of max comp damping
- more comp did almost nothing
3. less preload reduced the issue dramatically
- an equal amount of shock length was added to compensate, to make the change only about preload
- with the issue smaller due to preload change, i could make small changes to damping to get the chassis feel and grip where i wanted it
- more preload def made the problem worse
the results of #1 & #2 are mostly expected. i made those adjustments first to be sure that my damping was in a good range for this track and for the swingarm length. for #3, what im surprised about is how dramatic the preload change affected the issue. so now, my question is WHY?!?! i cant nail down the physical reasoning behind it all and am looking to pick some brains.
setup info:
- modern Yamaha R6
- 90 N/mm shock spring, which most would consider soft.
- total preload started at 11mm, changed to 9mm. tried 8mm and the diff was hard to notice. ran out of adjustment and couldnt try 7mm.
- i dont measure sag. its a useless measurement because i know that this spring and this much preload has me within range. i bet my sag is >30mm anyways
i talked to some racers at the track about this and they all kinda said "well less preload means theres less force"... but thats mostly wrong. preload does not change springrate nor how much force is applied by your riding. it only changes where in the stroke the suspension is working. the input force is the same, the output position is different because of the preload. so here are my initial thoughts as to why this small preload change produced such a large effect and some responses:
1. the spin and pump was mostly caused by the rear suspension not having enough travel to extend into the dips between bumps.
- I find this hard to believe because of the magnitude of the change. 2mm of preload is ~4mm of extra sag at the wheel. could that really be enough to keep the tire on the ground?
2. lowering the suspension in its travel affects its wheel rate, ie the effective springrate at the wheel. most OEM linkages arent linear, so using a diff part of the stroke changes everything
- i swear that linkages were regressive until u go past horizontal, meaning the suspension should be a tiny bit softer, not harder. tiny bit is also a key word there too.
3. something else involving preload’s effect on the spring, maybe the velocity or acceleration of the system is different
- Hooke’s law w/ preload is F = -k(x+p) where p is a distance. but that doesnt really matter since F is the input here. x is smaller when p is added.
- F=ma is really what we care about. since F & m stay the same, a must as well.
4. top out spring craziness! no idea
#1 is def the best candidate, the only one that makes sense really. im just very surprised the effect could be this large. im also surprised because my preload, springrate, and sag are not out of range. if i was cruising around with 20mm sag, i could understand the effect. but thats not whats going on. hell, i will go measure sag anyways. i bet its so large that most trackday suspension tuners would recommend MORE preload.
TL;DR… spin and pump on corner exit sucks. why is the best solution less preload even though preload/springrate/sag are already within range?
ive been dealing with an annoying problem on my race bike... spin and pump over corner exit bumps. smooth corner exits are no big deal. the tire spins and just drives. but any time the pavement has even minor ripples, the rear suspension pumps. the harder the drive and the larger the bumps, the worse it is. T6 at Miller, T3 & Riverside at Buttonwillow, T9 at Fontana are good examples. Riverside hasnt been bad and is almost cured since i put a new piston in my shock, but that corner was always minor compared to the others. the other corners are so bad i def have to slow my roll-on and work hard to not get bucked off.
so yesterday at Fontana, i changed something every session and was able to accomplish a lot. here are my findings:
1. increasing rebound damping helped the issue some
- too much rebound made the bike feel wooden and reduced grip.
- even w/ the setting at "too much", the pump was still bad enough to affect my laptime, ie rebound is not the solution
2. compression damping adjustment had almost no effect
- less comp made other bumps feel better, suggesting im near the limit of max comp damping
- more comp did almost nothing
3. less preload reduced the issue dramatically
- an equal amount of shock length was added to compensate, to make the change only about preload
- with the issue smaller due to preload change, i could make small changes to damping to get the chassis feel and grip where i wanted it
- more preload def made the problem worse
the results of #1 & #2 are mostly expected. i made those adjustments first to be sure that my damping was in a good range for this track and for the swingarm length. for #3, what im surprised about is how dramatic the preload change affected the issue. so now, my question is WHY?!?! i cant nail down the physical reasoning behind it all and am looking to pick some brains.
setup info:
- modern Yamaha R6
- 90 N/mm shock spring, which most would consider soft.
- total preload started at 11mm, changed to 9mm. tried 8mm and the diff was hard to notice. ran out of adjustment and couldnt try 7mm.
- i dont measure sag. its a useless measurement because i know that this spring and this much preload has me within range. i bet my sag is >30mm anyways
i talked to some racers at the track about this and they all kinda said "well less preload means theres less force"... but thats mostly wrong. preload does not change springrate nor how much force is applied by your riding. it only changes where in the stroke the suspension is working. the input force is the same, the output position is different because of the preload. so here are my initial thoughts as to why this small preload change produced such a large effect and some responses:
1. the spin and pump was mostly caused by the rear suspension not having enough travel to extend into the dips between bumps.
- I find this hard to believe because of the magnitude of the change. 2mm of preload is ~4mm of extra sag at the wheel. could that really be enough to keep the tire on the ground?
2. lowering the suspension in its travel affects its wheel rate, ie the effective springrate at the wheel. most OEM linkages arent linear, so using a diff part of the stroke changes everything
- i swear that linkages were regressive until u go past horizontal, meaning the suspension should be a tiny bit softer, not harder. tiny bit is also a key word there too.
3. something else involving preload’s effect on the spring, maybe the velocity or acceleration of the system is different
- Hooke’s law w/ preload is F = -k(x+p) where p is a distance. but that doesnt really matter since F is the input here. x is smaller when p is added.
- F=ma is really what we care about. since F & m stay the same, a must as well.
4. top out spring craziness! no idea
#1 is def the best candidate, the only one that makes sense really. im just very surprised the effect could be this large. im also surprised because my preload, springrate, and sag are not out of range. if i was cruising around with 20mm sag, i could understand the effect. but thats not whats going on. hell, i will go measure sag anyways. i bet its so large that most trackday suspension tuners would recommend MORE preload.
TL;DR… spin and pump on corner exit sucks. why is the best solution less preload even though preload/springrate/sag are already within range?
