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Old 07-24-2017, 10:57 AM   #1
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spin and pump on corner exit

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?
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Old 07-26-2017, 04:57 PM   #2
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I often see that as a rebound issue. I agree that the area of the stroke has marked impact. Have you tried a bit less spring? That worked wonders for me on the 750 when it was pumping.
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Old 07-27-2017, 08:14 AM   #3
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Originally Posted by afm199 View Post
I often see that as a rebound issue. I agree that the area of the stroke has marked impact. Have you tried a bit less spring? That worked wonders for me on the 750 when it was pumping.
in this case, i know it wasnt a rebound issue because no amount of rebound would fix the problem.

less spring, using an 85, would put me out of range for what racers run on R6's. i wouldnt be surprised if i was the only one using a 90 at the MA events, let alone an 85. it might be worth some experimentation.

do u know why area of stroke has a marked impact? even any educated guesses?
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Old 07-27-2017, 07:36 PM   #4
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in this case, i know it wasnt a rebound issue because no amount of rebound would fix the problem.

less spring, using an 85, would put me out of range for what racers run on R6's. i wouldnt be surprised if i was the only one using a 90 at the MA events, let alone an 85. it might be worth some experimentation.

do u know why area of stroke has a marked impact? even any educated guesses?
Yup, if adding a bit doesn't help, it's not rebound or it's that with another issue.

Ok, then on the spring.

I don't have a clue about the area of stroke. There, hope that helped.
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Old 07-28-2017, 02:00 PM   #5
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My knowledge of suspension is almost non-existent, so I may be completely off base here, but my guess at this (mostly just because I'm interested in seeing where this goes) is thus:

In a variable rate/progressive spring, preload is going to change the energy and mass of the spring, thus it requires more force for the spring to compress from a static setting than it does with less preload. Reducing preload means less force is required for the spring to compress. More preload would then make this far worse, since you would require even more force to cause the spring to compress. For linear rate springs this all goes out the window, so meh.

Since your rebound and compression otherwise seem to be on point, that's my mostly uneducated thoughts (some education related to springs but NOT in relation to suspension, mostly physics).
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Old 07-28-2017, 02:07 PM   #6
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We'll be actively monitoring this forum to answer your questions as well. Thanks!
I guess not so much?
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Old 07-28-2017, 03:43 PM   #7
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Originally Posted by Honey Badger View Post
My knowledge of suspension is almost non-existent, so I may be completely off base here, but my guess at this (mostly just because I'm interested in seeing where this goes) is thus:

In a variable rate/progressive spring, preload is going to change the energy and mass of the spring, thus it requires more force for the spring to compress from a static setting than it does with less preload. Reducing preload means less force is required for the spring to compress. More preload would then make this far worse, since you would require even more force to cause the spring to compress. For linear rate springs this all goes out the window, so meh.

Since your rebound and compression otherwise seem to be on point, that's my mostly uneducated thoughts (some education related to springs but NOT in relation to suspension, mostly physics).
u already said it. the setup is linear so theres no change in energy or force when u change preload. more preload puts some extra "force" in the beginning, so u get less total displacement when riding. less preload doesnt, so u get more total displacement when riding.

this does change slightly when a shock has a topout spring, as most aftermarket ones do. im still checking over the math on that one.
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Old 08-04-2017, 04:12 PM   #8
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u already said it. the setup is linear so theres no change in energy or force when u change preload. more preload puts some extra "force" in the beginning, so u get less total displacement when riding. less preload doesnt, so u get more total displacement when riding.

this does change slightly when a shock has a topout spring, as most aftermarket ones do. im still checking over the math on that one.
My take on spin and pump is that the spin breaks the compressed contact patch and lets the rear end extend, then when it hooks up again, it compresses again and repeats the cycle. What I might try is either a lighter spring to keep the spin from happening or a heavier one to minimize the compression and keep the pumping to a minimum. The lighter spring will weight the rear more and reduce spin, the heavier one will lighten the rear more and increase spin but reduce movement.
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Old 08-04-2017, 08:00 PM   #9
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I don't see any mention of an aftermarket rear shock on there... I would start there if you are serious about your track riding / performance! ... or often you can get your stock one rebuilt / revalved with better damping!
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Old 08-04-2017, 11:01 PM   #10
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I don't see any mention of an aftermarket rear shock on there... I would start there if you are serious about your track riding / performance! ... or often you can get your stock one rebuilt / revalved with better damping!
lol that would be a hilarious burn if I didn't think you were serious.
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Old 08-05-2017, 07:20 PM   #11
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I don't see any mention of an aftermarket rear shock on there... I would start there if you are serious about your track riding / performance! ... or often you can get your stock one rebuilt / revalved with better damping!
I don't think stangmx13 is quite advanced enough to get serious about track riding!
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Old 08-05-2017, 09:39 PM   #12
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Vroom, I've got a Ktech DDS Pro with their upgraded piston. The bike baseline was setup by a somewhat well-known MotoAmerica crew chief that has contributed to a good number of pro wins and countless club racing championships. The whole bike is built to the limit of MotoAmerica STK600 rules. A better rider could prob win pro races on it, but I've still managed to score 14pts in 4 rounds.

When I can afford to pay that crew chief for his services, he does amazing work and generally "fixes" the bike. With limited time at races, he also tells me "just ride the damn bike", which I should. This thread is more about understanding what's going on now that I have the time.
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Old 08-06-2017, 08:49 AM   #13
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Vroom, I've got a Ktech DDS Pro with their upgraded piston. The bike baseline was setup by a somewhat well-known MotoAmerica crew chief that has contributed to a good number of pro wins and countless club racing championships. The whole bike is built to the limit of MotoAmerica STK600 rules. A better rider could prob win pro races on it, but I've still managed to score 14pts in 4 rounds.

When I can afford to pay that crew chief for his services, he does amazing work and generally "fixes" the bike. With limited time at races, he also tells me "just ride the damn bike", which I should. This thread is more about understanding what's going on now that I have the time.
MFMC! The minute you said that statement.


You might enjoy this clip of a pro mech and rider working together on suspension. https://www.facebook.com/Michael.J.C...1832504974164/
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Old 08-06-2017, 10:13 AM   #14
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What about chain slack/tension? Seems that even among regular track riders there's a debate about how much slack is appropriate. Could definitely affect behavior over large bumps.
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Old 08-06-2017, 11:02 AM   #15
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What about chain slack/tension? Seems that even among regular track riders there's a debate about how much slack is appropriate. Could definitely affect behavior over large bumps.
True, but in this case we're talking about spin and pump on corner exit, not bumps.
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