rear suspension dynamics/geometry question

Thanks mattbnj.
Out of necessity, i'll be installing a new rear shock on my bike next week, but won't be able to do anything to the front for a while. So basically, I'll try to set the rear up based on some info on the board, ride it, and see from there.

Generally speaking, again... if I raise the rear, and during corner entry/mid corner (no throttle or maintanence throttle), and the front pushes, then there's too much weight forward? Is that correct?

 

front push

If the front is pushing you can look at two reasons - either there is too much weight on the front tire, or there is not enough weight on it. The front tire needs to roll smoothly thru the turn. In your scenario, chances are if you've raised the rear, and, as you said, you are at maintenance throttle, i would say it would be reasonable to assume an imabalance where the front tire is being overweighted and pushed thru the turn. If that is the case, you may also find the steering to be too quick - that is, an oversteer reaction at turn-in that pitches the bike to the apex earlier then you the rider wants.

Maybe a true suspension guru can chime in here as well and confirm or deny these assumptions.

 

Not that simple. You'll generally get easier turn in the higher the rear. If the trail is not right you'll push on exit. However most racers use trail braking to steepen the steering angle and get the bike turned. Suspension is complicated.

 

Agreed.
I'm not looking to start a new career in tuning, simply trying to get an understanding of what I'm feeling when things happen.
My explainations are pretty useless when I say, "It don't turn good."

 

front push

you can also think of it this way - if after altering your rear ride height, the bike "races" to the apex, then raising the front to increase trail is the way to go. If however, after raising the rear ride height, you are still fighting to hit your apexes, you can continue to add ride height in the rear - just remember, there comes a point where the ride height can create a overly aggressive swingarm angel and the rear tire will start loosing grip - not good. That's where you have to start evaluating the front of the bike and how you're entering the turn. If you are a straight up and down braker (that is get all your braking done before turn-in) and the bike won't steer to the apex, then i would continue w/ ride height adjustment, or perhaps look to slow down rebound on the front forks. If you're a trail braker, and the bike is still having issues despite raising the rear, maybe you need softer springs in front, or softer compression settings. The idea being the trailing action of the brakes should help keep the nose of the bike down to the apex. If you're too stiff up front, the forks may not be getting down far enough in the stroke to allow the bike to turn the way you want.

 

My buddy picked up suspension for his bike and talked to Roger Bell here in Lexington, KY. some of you may know him. He raced in the Daytona multiple times. Anyway, in short, he told my buddy to raise the rear until it broke traction, then take it down a little. Basically what you've been saying.
Thanks again to everyone who post here on the board. I'm sure it's difficult and possibly even frustrating for experienced guys to try to convey info to guys like myself that are probably asking questions that are 8 steps ahead of our knowledge.:up:

 

Subscribing.

 

i'm no expert, that is for sure, but i have learned to ask questions of those who have been thru it before. the experience of the racers on this board provides a wealth of knowledge, and most are happy to share it w/ their fellow racers. good luck with your setup - you're already heading in the right direction

 

So back on topic...
Is it generally agreed then, that the rear rises on accel?
The question becomes whether or not the amount is enough to overcome the weight transfer caused by accel. If it's not, then the front will push on exit because the rear, for lack of a better term, "squats" and doesn't keep the weight forward?
Good God....Where's the smiley that bangs its head on the brick wall?

 

I still have a stock spring on my bike which is way too soft. On exit the front pushed bad. My triceps were sore which told me I was fighting it too hard. Ended up speeding up the rebound in the rear and slowing it down in the front and the bike turned.

 

Dude... I'm online, but my answer won't mean squat.
From this discussion, it would make sense that the stock spring may not be stiff enough, and allowed the rear to squat, removing weight from the front, causing it to push on exit.
Speeding up rebound in the back would allow the spring to extend with less effort???
Slowing down rebound in the front would mean the lift in the front would happen slower????
Don't know.

 

Speeding up the rear rebound allows the shock to extend easier. Slowing the front end down keeps it from fully extending. The bike was squatting too much and wouldn't turn. Funny thing is I do a lot of drag racing with cars and we tune the suspension a lot to make the cars go. Quickest way to control a wheelie is to slow the front rebound down and limit travel. Two different worlds but both rely heavily on suspension.

 

rear susp dynamics

in the video i posted you can see, w. acceleration, the swingarm pivots up - this upward force pushes on the rear shock (from the bottom of the shock) and transfers it force up thru the shock. This in turn causes the shock to compress and the rear of the bike to squat. So basically, two things happen - swingarm rises, and rear shock compresses. These actions in unison create traction issues, as well as, handling issues i.e. running wide when on the gas from apex to exit. It can also, as you said, affect trail and weight transfer.

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basically, so long as you have an understanding of how geometry affects the handling of the bike, you can apply measured and deliberate changes to your bikes chassis based on the feedback you get while on the track.

Getting the geometry dialed in for a specific track, tire set, and gearing choice will be very important if you plan on getting the most out of you and your bike - once you have a good baseline, assuming you have been recording what you're doing, you can then tweak the valving of the bike to fine tune it even more.

 

rear suspension dynamics

One final thing here - the rear of the bike will only "squat" under acceleration if the force of the weight being transferred under acceleration overrides the anti-squat force opposing it. This anti-squat force is managed via the gearing, rear ride height/swingarm angle, and suspension stiffness. If the overall setup of the bike is sufficiently stiff enough to handle the weight transfer under acceleration, the rear of the bike will extend, not compress.

 

Again, I am not an expert...

BUT, TO SET UP YOU BIKE YOU NEED to understand that things need to be done in a particular order....

Suspension is the # 1 priority...

1.- Spring rates must be with in the suggested range on the forks... Memorize your New spring rate...

2.- Valving must be within the appropiate range for the spring rate...

3.- Fork fluid must be of appropiate viscosity and must be filled to the recommeded level... Memorize your Fork fluid BRAND and VISCOSITY...

4.- Static sag and race sag should be set with the minimum amount of preload possible... Memorize you front and rear Sag Numbers...

Once these 4 basic steps are done...

5.- Set up suggested rear end geometry... Suggested swing arm angle is about 12 degrees... A lot of bikes come stock with 8-10 degrees... This measurement is done on a level ground, with suspension fully extended, and tires barely touching the ground...

6.- Set up suggested front end geometry... This one is difficult to measure... You need to know your bikes OEM RAKE # and OEM TRAIL #... Trail nuimber on most supersport bikes is about 100mm... Suggested trail number for responsive steering with slight stability compromise is about 94mm - 96mm... Most people set up this number by feel...

From here... Adjust and personalize your settings... From here... Tweak you settings from track to track...

Now you can fine tune, Acceleration-Squat and Sprocket set up Anti-Squat... Here you want balance... You can play with sprocket gearing to archive more or less anti-squat... If this does not help, maybe changing one step on the spring...

You can not generally agree that a bike rises or squats under acceleration... There are too many variables such as bike in question, suspension set up, rider weight, etc etc...

Luis



PS: I have heard so many times that an AMATEUR RACER should learn on a stock bike and upgrade only as your skill level overwelms the bike set up...

I am a believer that ANY RACER would perform best if it has a properly set up bike that inspires him/her confidence to ride harder in a controled and predictable way...


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i must say - i just read thru that entire thread again and am thouroughly impressed w. the quality of debate and knowledge sharing. Carry on........