Bikes' Behaviour On The Brakes - Suspension Question

[rapid van cleef]

Hey, Ive got a track day in a week or so. I Have just had the suspension serviced. zx10. When braking at the end of the straights I have noticed that the back wheel moves from side to side at maximum braking. The front dips, feels solid, the back end goes light but moves side to side just before i start to release the pressure. I have read on a suspension forum that this can be casued by the front dipping too much.......well heres the quote from the site below

 

I have a zip tie around the fork legs and it goes to around 10mm from bottoming out

 

I have noticed this happening to other riders at the same point on the track when I am watching from the sides. Its not a violent movement from side to side but enought to make the back of the bike feel a little unstable.

 

At this point im bracing my weight with my legs against the tank, rather the outside leg and but is off to the inside.

 

The cause here is way too much front end weight transfer under braking. The front end is compressing so low that the bike's weight tries to pivot around the steering head, causing the side-to-side movement. The quickest solutions here are to increase the front fork spring preload and/or raise the front ride height by dropping the fork tubes in the triple clamps, or decrease the rear ride height by shortening the shock (if possible). try increasing the fork spring preload first, and progressing in small increments until the handling begins to be negatively affected (remember to watch the rebound damping when increasing the spring preload). If that doesn't work, try the ride height modifications; watch for adverse handling reactions in other areas when doing this as ride height changes drastically affect how the bike corners. Other solutions to try--although less effective--are to increase the compression damping in the forks (if possible), or to decrease rebound damping in the rear (to allow the rear tire to follow the pavement quicker). Again, watch for adverse handling reactions in other riding situations when test riding.

 

Any opinions? I would like to see if I can rectify this at the next track day. However, it may be a normal result of hard braking. In which case, all good!!!!! I'm running bridgesone R10s on warmers. Would gentle back brake settle this? My front fork height has around 1 mm left before its as far out as it will go in the triple clamp.

 

thanks

[ktk_ace]

you tried grabbing the tank with your legs and upper body being loose while braking straight up?

 

A sharp rear profile tire might make the bike fidgitty too when braking while loading up the front to its limit (which i heard that the R10's are)

 

I lower the rear tire pressure when it is abit colder but im not sure if its a good idea on your setup.

[YellowDuck]

You'll get more informed opinions than mine for sure, but if you are ending up with 10 cm of unused fork travel I would say you probably have the front preload about spot on. I'd be tempted to try reducing rear ride height or rear peload a *tiny* amount to see if it improves things.

 

I definitely would not try using rear brake in this scenario!

 

You said that it happens right at the end of your braking, just before you start to release pressure. I think that is pretty normal, that the rear would be the lightest and therefore the least stable at the end of your braking zone. If it was happening earlier I would suspect excessive engine braking as you gear down, and suggest you delay your downshifts a bit to keep the rpms down. Does the bike have a slipper clutch?

[csmith12]

The quick and dirty....

 

If the suspension has been serviced & sprung, your next step is to have it set to your weight & preferences (sag and so on) by a suspension tech. They are somewhat common at the larger track events and normally cost under $50 in my area. After that, you just have to put some laps on it to tweak it out. I had a similar issue last year and for my riding style on my r6, I added a click or so of rebound to my rear until it feels "floaty" under max braking but stay inline with the front. A click or two of compression in the front may be in order as well. Your rider/bike combo may be different.

 

Assuming if everything is set up as close as possible, how is your corner setup under very heavy braking? That would be a common place for a SR and the BP setup is not as smooth as it could be maybe? Does your bum slide into the tank under hard braking? You setting up your BP before getting off the gas? Basically what I am getting at is, your shift in body position could be causing the bike to become unstable and slide out a bit. So maybe a bit of technique could be a simple answer.

[csmith12]

Oh... full disclosure on my comment and for your safety. I am still learning to tweak suspension. I have got a year under my belt setting up/tweaking track bike suspensions.

[Hotfoot]

Hey, Ive got a track day in a week or so. I Have just had the suspension serviced. zx10. When braking at the end of the straights I have noticed that the back wheel moves from side to side at maximum braking. The front dips, feels solid, the back end goes light but moves side to side just before i start to release the pressure. I have read on a suspension forum that this can be casued by the front dipping too much.......well heres the quote from the site below

 

I have a zip tie around the fork legs and it goes to around 10mm from bottoming out

 

I have noticed this happening to other riders at the same point on the track when I am watching from the sides. Its not a violent movement from side to side but enought to make the back of the bike feel a little unstable.

 

At this point im bracing my weight with my legs against the tank, rather the outside leg and but is off to the inside.

 

The cause here is way too much front end weight transfer under braking. The front end is compressing so low that the bike's weight tries to pivot around the steering head, causing the side-to-side movement. The quickest solutions here are to increase the front fork spring preload and/or raise the front ride height by dropping the fork tubes in the triple clamps, or decrease the rear ride height by shortening the shock (if possible). try increasing the fork spring preload first, and progressing in small increments until the handling begins to be negatively affected (remember to watch the rebound damping when increasing the spring preload). If that doesn't work, try the ride height modifications; watch for adverse handling reactions in other areas when doing this as ride height changes drastically affect how the bike corners. Other solutions to try--although less effective--are to increase the compression damping in the forks (if possible), or to decrease rebound damping in the rear (to allow the rear tire to follow the pavement quicker). Again, watch for adverse handling reactions in other riding situations when test riding.

 

Any opinions? I would like to see if I can rectify this at the next track day. However, it may be a normal result of hard braking. In which case, all good!!!!! I'm running bridgesone R10s on warmers. Would gentle back brake settle this? My front fork height has around 1 mm left before its as far out as it will go in the triple clamp.

 

thanks

 

 

I'm not a suspension expert but I certainly have experienced this. I mostly agree with the excerpt you posted above about how to tweak it - EXCEPT that if you are otherwise happy with the bike's handling I wouldn't be too quick to mess with ride height. Personally, here is the order of things I would try - check the body position stuff first, before you start changing bike set up stuff, otherwise you can end up chasing your tail with bike adjustments:

 

BODY STUFF

1) first, make sure you aren't stiff on the bars during braking, get locked in really well with your knees so you can be loose on the bars - stiff arms can definitely make the bike weave under hard braking (A Twist of the Wrist II can provide more details on how and why.)

2) Make sure you are not sliding forward during braking. You can also try sitting back farther in the seat (if possible), to shift some weight to the back end - if that is easy and comfortable for you to do that may solve it entirely without any suspension adjustments

3) Relax your back, taking as much braking force as you can in your knees and legs - sitting in a slouchy position with a relaxed back can help absorb bumps and make you help the rear suspension instead of fighting it, and helps keep your arms loose on the bars, reducing back tire skipping or weaving

 

BIKE STUFF

1) I would start with decreasing the rear rebound damping (especially if it is set quite heavy/slow right now) and test it, see if it helps and make sure it doesn't have any negative effects in other areas

2) Sounds like you ARE compressing the front quite a bit, consider adding preload in front or stiffening the front compression so it doesn't dive quite so much - but do one at a time and look for adverse effects; if the bike no longer feels solid or planted in the front that may indicate it has gotten too stiff for your liking. Be aware that significantly increasing front end stiffness can make the front end more likely to slide in corners, so make small adjustments and ride well withitn your limits as you are making changes.

 

Ride height adjustment is a bit more involved; body position adjustments can be done on the fly, and compression/rebound/preload adjustments can be done in seconds in the pits, but ride height takes more tools and possibly a second person to change; I'd be hesitant to mess with that unless the other options don't help. Raising the front ride height could help but usually makes the bike steer slower, personally I wouldn't to slow down my steering response unless the bike was feeling twitchy in other ways (headshake, for example). Lowering the rear can cause the bike to squat and want to run wide in turns.

 

If there is a suspension tuner at the track, sometimes you can get the bike set up inexpensively and often that person will be willing to continue to make adjustments through the day.

 

The back end moving around under MAX braking is a pretty common thing - when you are at max braking the back end is light and will move around some, especially if the pavement is ripply. The Las Vegas track is a good example, the hard braking area at the end of the straight has some wavy bumps (that you can't see while riding) that make you think your rear suspension is having a problem, until you realize that the bumps are there. If it only happens in one place, you might try walking the track and see if that is the underlying problem!

 

Hope that helps...

[Lnewqban]

.......Any opinions?

 

It is just a track day, you are not racing; hence, there is no reason to brake so hard.

 

http://forums.superb...p?showtopic=310

 

IMHO it is not a suspension problem, if the rear suspension is fully extending when that happens.

It is just the result of sticky tires and strong front brake.

Basically, you are lifting the rear tire due to excessive braking (excessive deceleration).

Once the friction of the rear patch disappears, the CG wants to move ahead of the front patch and the tail swings side to side following your minute steering inputs.

 

http://forums.superb...?showtopic=2423

 

I wouldn't follow the advice of the other site: steering geometry has nothing to do with the problem; hence, leave the height of those forks alone (unless you are experiencing steering problems).

 

[Flowboy]

An old thread now, but I found it, looking for something else, so others will! 

Only one answer mentions rear brake. The OP is concerned about floating feel of rear end at absolute max braking. This is quite normal, just watch MotoGP or wsbk. Obviously their set ups are top notch. Even classic racers can experience this with a good front end set up.

However, using rear brake can help. I know many riders say that with their fantastic front discs,  they never use the rear, but they should! If you use it just before you use the front, it lowers the rear, so when the front is applied the rear has further to travel before lifting, or nearly, off the track (which is as others have said, effectively what is happening) The rear gets so light, it wants to overtake the front & starts to rotate around the headstock**, exactly as Lnewqban describes.  It helps to keep your weight further back as you get to the braking zone but obv. extreme front braking will make you slide forward. You will come off the rear before front is at max or a rear slide will occur as the rear rises. Supermoto riders have a technique that involves this slide inducing but it's outside this discussion.

I don't get the relaxed arms in braking comment, certainly not on track. If your knee is moving out ready to lean you have to use arm, shoulder & core strength to brace yourself at your own & the bikes weight transfer. I always know I've had an arm etc workout at the start of the race season as I never brake like this on the road, it would be insane! But it's how top racers get arm pump.

**If you have a bicycle you can see it for yourself. Stand next to bike, front brake full on, now try to push bike forward. The rear comes up & will move right or left, depending on small bar & lean inputs. Put a weight on the saddle or over the rear,  (your imaginary arse😄) now try again. It takes more input before the rear rises. These kind of geometry effects are used by stunt riders.

[Cobie Fair]

True, most have the leg or knee out under braking.  But sometimes top riders have kept the knee on the tank (Pecco and Fabio have done this).

I wonder if some of the reason for the leg out is to block other riders from coming up the inside...does this get discussed by riders?

[Jaybird180]

Going into hard braking, I tend to close the gas before I grab the front lever. I'm thinking this has a similar effect as using rear brake to alter the rear height...at least on my "analog" bike, it doesn't freewheel, nor do I use the clutch, so I get a bit of an engine braking effect.