Rear End Wiggling Around While Braking

[harnois]

During my track days at VIR last week, I noticed that the rear end of my bike was wiggling back and forth as I was braking. It was subtle but it was enough that riders behind me could see it happening. It was mostly approaching Turn 1 on VIR North Course where the speed goes from around 150mph down to 50 or 60. I just got a new GSX-R600 and this is the first time I've had this bike on the track. I've exerienced this subtle wiggle before on my previous bike but only when I was really on the brakes hard. This time it seemed to happen even when I really wasn't braking all THAT hard. I was having a good fun spirited ride no doubt but I wasn't really trying to push the braking points that much or anything. Could it be the way I'm holding onto the bike during braking? Suspension settings? I'd like to have the option to brake a lot harder without that sort of instability.

[bellevuetlr]

During my track days at VIR last week, I noticed that the rear end of my bike was wiggling back and forth as I was braking. It was subtle but it was enough that riders behind me could see it happening. It was mostly approaching Turn 1 on VIR North Course where the speed goes from around 150mph down to 50 or 60. I just got a new GSX-R600 and this is the first time I've had this bike on the track. I've exerienced this subtle wiggle before on my previous bike but only when I was really on the brakes hard. This time it seemed to happen even when I really wasn't braking all THAT hard. I was having a good fun spirited ride no doubt but I wasn't really trying to push the braking points that much or anything. Could it be the way I'm holding onto the bike during braking? Suspension settings? I'd like to have the option to brake a lot harder without that sort of instability.

 

 

too much weight on the front. is your susp set up to your weight? might want to stiffen it up abit more.

[nevillejnr]

Check two things:

 

Firstly as above, your front forks could be bottoming out and the rear wheel lifting as a result. Check your travel with a cable tie/zip tie to see how much travel you have left. If there is none, preload the front more.

 

If you are not bottoming the front then the rear rebound damping is too hard. Dial off rebound to allow the wheel to return to the ground and track in a straight line.

[cowpoos]

could be suspension as others have mentioned...but also..your rear tyre is obviously skimming across the ground a lil bit....move you weight back in the seat some if you can without locking your elbows...if not just lightly hold the rear brake on a bit...it will help keep the rear straighter as it skims...and if you already use rear brake...ummm...use it as a scare tactic to keep other racers from passing you into corners

[Cobie Fair]

Have you experienced this every time? In other words, does it always wiggle around with the same amoutn of braking?

 

 

CF

[racer]

Check two things:

 

Firstly as above, your front forks could be bottoming out and the rear wheel lifting as a result. Check your travel with a cable tie/zip tie to see how much travel you have left. If there is none, preload the front more.

 

If you are not bottoming the front then the rear rebound damping is too hard. Dial off rebound to allow the wheel to return to the ground and track in a straight line.

 

 

If you are going to get serious and acquire the perfect spring, you can forget (rear) preload altogether and even leave the spring loose when topped out (like negative preload) allowing the rear of the bike to lift completely off the spring without lifting the rear wheel. This was a common trick used by some superbike racers back in "my day".

[GregGorman]

Check two things:

 

Firstly as above, your front forks could be bottoming out and the rear wheel lifting as a result. Check your travel with a cable tie/zip tie to see how much travel you have left. If there is none, preload the front more.

 

If you are not bottoming the front then the rear rebound damping is too hard. Dial off rebound to allow the wheel to return to the ground and track in a straight line.

 

 

If you are going to get serious and acquire the perfect spring, you can forget (rear) preload altogether and even leave the spring loose when topped out (like negative preload) allowing the rear of the bike to lift completely off the spring without lifting the rear wheel. This was a common trick used by some superbike racers back in "my day".

 

I don't understand this. Can you explain it a bit more? Specifically, I'm not getting the rear spring being loose when topped out, nor the concept of negative preload.

 

Thanks.

[racer]

The truly proper spring rate requires no pre-load. Under normal circumstances, the weight of the bike alone would hold the spring in some compression. As I understand it, under negative G-loading, the rear of the bike would lift off of the spring. The basic idea is that the pre-load adjuster would be backed off such that the spring is left loose on the shock when the rear is on the stand (negative pre-load). I've never done this myself, I've only been told about it by others who said they have. (They were national champions and I took their word for it.) I'm not sure what modifications (if any) might have been needed.

[2bigalow]

The truly proper spring rate requires no pre-load. Under normal circumstances, the weight of the bike alone would hold the spring in some compression. As I understand it, under negative G-loading, the rear of the bike would lift off of the spring. The basic idea is that the pre-load adjuster would be backed off such that the spring is left loose on the shock when the rear is on the stand (negative pre-load). I've never done this myself, I've only been told about it by others who said they have. (They were national champions and I took their word for it.) I'm not sure what modifications (if any) might have been needed.

I've heard this too, but I think this applies to 125's and 250's since the bikes are so light. If memory serving me correctly Keith rode 250's in AMA. He could answer the question for sure.

[2bigalow]

Front and rear suspension could definitly play a role in the rear fishtailing, but what are you doing on the bars? Are you bracing yourself on them? Are you making a steering imput? What are you doing with your legs?

[racer]

The truly proper spring rate requires no pre-load. Under normal circumstances, the weight of the bike alone would hold the spring in some compression. As I understand it, under negative G-loading, the rear of the bike would lift off of the spring. The basic idea is that the pre-load adjuster would be backed off such that the spring is left loose on the shock when the rear is on the stand (negative pre-load). I've never done this myself, I've only been told about it by others who said they have. (They were national champions and I took their word for it.) I'm not sure what modifications (if any) might have been needed.

I've heard this too, but I think this applies to 125's and 250's since the bikes are so light. If memory serving me correctly Keith rode 250's in AMA. He could answer the question for sure.

 

 

Actually, although one of them was a 250 national champion, both the guys who told me about it were superbike racers at the time and were discussing superbikes from the late 80's and early 90's.

 

In any case, I'm not sure why a GP bike being lighter would have any bearing on it. And being that the technique is all about keeping the rear wheel in contact with the pavement under *heavy braking*, in my opinion, a GP bike probably wouldn't benefit much from it.

[GregGorman]

The truly proper spring rate requires no pre-load. Under normal circumstances, the weight of the bike alone would hold the spring in some compression. As I understand it, under negative G-loading, the rear of the bike would lift off of the spring. The basic idea is that the pre-load adjuster would be backed off such that the spring is left loose on the shock when the rear is on the stand (negative pre-load). I've never done this myself, I've only been told about it by others who said they have. (They were national champions and I took their word for it.) I'm not sure what modifications (if any) might have been needed.

 

Hmm, that's just not jiving with my data. I thought the purpose of preload was to force the wheel to the ground when the bike goes over dips. If there's no preload, the wheel wouldn't react fast enough to the bumps in the road.

 

Harnois, regarding the wiggle; since you're on a new bike, and in turn 1 at VIR which doesn't have a straight approach, my first thought is you're not locked into the bike as securely as you were on your old bike and therefore putting more weight and input into the bars when braking. The other thing to check would be your downshifting technique. If you're not downshifting correctly, that can cause the engine to act as a rear brake and cause slides, chatter, and wiggles.

[2bigalow]

The truly proper spring rate requires no pre-load. Under normal circumstances, the weight of the bike alone would hold the spring in some compression. As I understand it, under negative G-loading, the rear of the bike would lift off of the spring. The basic idea is that the pre-load adjuster would be backed off such that the spring is left loose on the shock when the rear is on the stand (negative pre-load). I've never done this myself, I've only been told about it by others who said they have. (They were national champions and I took their word for it.) I'm not sure what modifications (if any) might have been needed.

I've heard this too, but I think this applies to 125's and 250's since the bikes are so light. If memory serving me correctly Keith rode 250's in AMA. He could answer the question for sure.

 

 

Actually, although one of them was a 250 national champion, both the guys who told me about it were superbike racers at the time and were discussing superbikes from the late 80's and early 90's.

 

In any case, I'm not sure why a GP bike being lighter would have any bearing on it. And being that the technique is all about keeping the rear wheel in contact with the pavement under *heavy braking*, in my opinion, a GP bike probably wouldn't benefit much from it.

 

 

I believe the lack of preload on the rear spring (125 and 250) was to maintain momentum and not lose it through suspension. ???

[racer]

How so?

[2bigalow]

I should say zero static preload. How so? um dont know. I remember over hearing momentum can be lost trough suspension on little bikes, but I've never ridden one myself (unless you count an ex500). I then saw a preload graph for gps bikes that recommended zero static preload at the rear. ??? I thought that was odd and have alway wondered why. My post was more of a question than a statement. Maybe it has to do with being able to change directions quickly without the suspension grabing all the energy. Yes / No?

[racer]

I would think that the salient point is the *correct spring rate* combined with zero pre-load.

 

Momentum could be lost due to a too soft spring rate and the excessive damping rates required to control it. Lots of energy being dissipated and "lost" there. The more energy that is loaded into the spring itself (as opposed to the damping system), the more it will be recovered, translated and maintained as momentum.

 

Another reason for using the correct spring rate is that adding pre-load effectively alters ride height and geometry.

[racer]

The truly proper spring rate requires no pre-load. Under normal circumstances, the weight of the bike alone would hold the spring in some compression. As I understand it, under negative G-loading, the rear of the bike would lift off of the spring. The basic idea is that the pre-load adjuster would be backed off such that the spring is left loose on the shock when the rear is on the stand (negative pre-load). I've never done this myself, I've only been told about it by others who said they have. (They were national champions and I took their word for it.) I'm not sure what modifications (if any) might have been needed.

 

Hmm, that's just not jiving with my data. I thought the purpose of preload was to force the wheel to the ground when the bike goes over dips. If there's no preload, the wheel wouldn't react fast enough to the bumps in the road.

 

 

 

What data are you referring to, Greg?

 

The purpose of pre-load is to compensate for a too soft spring rate.

 

Due to gravity, the weight of a bike and rider will compress any spring to "force the wheel to the ground when the bike goes over dips". The critical issue of "fast enough" is a matter of spring rate. Pre-load will effectively increase the spring rate (essentially at the expense of geometry and suspension travel).

[cschluti]

Awesome!

 

After doing willow springs Oct 3/4th, I started wondering why the Ninjas felt so much more stable than my R6. It couldn't just be that my R6 has more power

 

I weight ~200, and was using the factory settings on my front suspension. After reading this thread, I went and added some preload, and all of a sudden, no more rear-wheel wiggle when braking hard! I should probably have EDR in Portland adjust it properly for me....

 

Thanks

 

-Charlie

[racer]

The truly proper spring rate requires no pre-load. Under normal circumstances, the weight of the bike alone would hold the spring in some compression. As I understand it, under negative G-loading, the rear of the bike would lift off of the spring. The basic idea is that the pre-load adjuster would be backed off such that the spring is left loose on the shock when the rear is on the stand (negative pre-load). I've never done this myself, I've only been told about it by others who said they have. (They were national champions and I took their word for it.) I'm not sure what modifications (if any) might have been needed.

 

 

After re-reading this post, I think I can make it easier to understand.

 

In the last part of the sentence: "The basic idea is that the pre-load adjuster would be backed off such that the spring is left loose on the shock when the rear is on the stand (negative pre-load)" ... the phrase "on the stand" could be misinterpreted to mean 'side stand'. And a rear race stand generally holds a bike at the swingarm or rear axle, hence, the swingarm and suspension are not "hanging free" (like under heavy braking). And even if a street bike has a center stand, unless the bike is weighted onto the front wheel, it may still rest on the rear.

 

The main point is that the rear suspension would be free hanging when considering a lack of pre-load or "loose spring" setting. I generally accomplish this by jacking/lifting and resting the frame bottom on blocks. Dirt bikers frequently use a milk crate. I don't recommend the milk crate technique for your Ninja...lol.

[harnois]

Have you experienced this every time? In other words, does it always wiggle around with the same amount of braking?

 

 

CF

 

I dunno about EVERY time, but yes it was quite consistent relative to the corner. I mostly remember it happening in turns 1 and turn 14 (top of the hill right hander) at VIR North Course. I don't recall it happening when braking for other turns but it was kinda a long time ago at this point.

[harnois]

.

[harnois]

could be suspension as others have mentioned...but also..your rear tyre is obviously skimming across the ground a lil bit....move you weight back in the seat some if you can without locking your elbows...if not just lightly hold the rear brake on a bit...it will help keep the rear straighter as it skims...and if you already use rear brake...ummm...use it as a scare tactic to keep other racers from passing you into corners

 

I was not using the rear brake at all. I was positioned at back of the seat during braking.

[harnois]

Check two things:

 

Firstly as above, your front forks could be bottoming out and the rear wheel lifting as a result. Check your travel with a cable tie/zip tie to see how much travel you have left. If there is none, preload the front more.

 

If you are not bottoming the front then the rear rebound damping is too hard. Dial off rebound to allow the wheel to return to the ground and track in a straight line.

 

I was lazy and ran the bike with stock suspension settings. I don't think I even checked the sag. So I'll do that and also do the zip tie thing to see how far it's compressing while braking. I don't think it is bottoming but it could be compressing more than desired, which would steepen the steering head angle and maybe contribute to the wiggling?

 

The rear rebound damping idea makes sense to me I'll try that as well.

[harnois]

Harnois, regarding the wiggle; since you're on a new bike, and in turn 1 at VIR which doesn't have a straight approach, my first thought is you're not locked into the bike as securely as you were on your old bike and therefore putting more weight and input into the bars when braking.

 

Good idea but I tihnk I'm actually more locked on to the new bike. The old bike was a yzf600r which has a fat 5-gallon rounded tank that was hard to hold onto. The gsxr is really shaped well to make it easy to hold onto. I was using the stomp pads on the yzf and found they were unnecessary on the gsxr because it is already so easy to hold onto. I was aware that holding myself back with the bars might cause inadvertent steering inputs and could cause the problem so I was making special effort to pinch the tank with my knees and stay loose on the bars.

 

The other thing to check would be your downshifting technique. If you're not downshifting correctly, that can cause the engine to act as a rear brake and cause slides, chatter, and wiggles.

 

Yeah I was thinking about that only after the track days so didn't get a chance to try changing it. My downshifting is very smooth, so I'm not lurching it into slides or anything nutty like that, but I was downshifting at high rpms. If I were to let the engine speed come down more before downshifting then there would be less engine braking and that may settle down the situation quite a lot. Could be I got away with the aggressive downshifting on the other bike because it just didn't have as much engine braking in general and was a heavier bike.

[racer]

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