Questions about suspension and tire wear - Chicagoland Sportbikes
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post #1 of 9 (permalink) Old 09-07-2005, 10:28 PM Thread Starter
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Questions about suspension and tire wear

So I'm still thinking about all the factors leading to my crash cuz I'm hoping to eliminate as many as possible for the future. Hopefully the answers will also help others new to the track.

When I rode the full track at Autobahn, I rode 5 full sessions pretty hard. In comparison to Blackhawk where I think I rode faster, I think I spent more time at extreme lean angles at Autobahn cuz the turns seemed longer and track was wider.

In both cases, my tires wore to the edges but I didn't have the characteristic balls of rubber or melted rubber ripples on them. They looked more like they had been "shaved." Kim's tires looked shredded even though we both know that she wasn't carrying as much corner speed. Should this have been a sign that I was headed for trouble?

I had BT012s with about 2000mi @ Autobahn and had virgin Pilot Powers @ Blackhawk. I know my suspension set-up was wrong but here is what it was: stock rebound and compression, 1-1.5 lines of more fork preload, and .5-.75cm of more shock preload.

I now know I should have set the sag correctly. Also I will also upgrade fork and shock springs before the next time. I was told that .95-1.0 fork springs would've been a better set-up for my weight (260+). As far as rear shock, I was told to go with a complete unit (shock+spring). Does anyone know of people who just upgrade the spring?

Hope some of the experienced guys can clarify.

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post #2 of 9 (permalink) Old 09-07-2005, 10:56 PM
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dude, come by and we'll set your sag right and get you some baseline damping settings to work from. dont go out with random settings!

also bring the 50 by and lets put those springs in.

Chris
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post #3 of 9 (permalink) Old 09-07-2005, 10:58 PM
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To measure sag you need two people but three is way easier. you need to find points on the bike that you can easily measure from and make a note so you can repeat at later time. Keep a note book so you can easily refer to adjustments and measurements later on.

step one- get the front or rear completely off the ground and measure, this number is very important because it is used to calculate free sag and rider sag. (full=325mm)

step two- set the bike down on the ground, push the suspension down, SLOWLY let it up and measure. Take this number and subtract it from the first number (315mm).
full 325mm- 315mm = 10mm free sag

step three- get on the bike, have someone balance the bike so you can get into full race position, have that person push down and slowly let it up while your in race position and have a third person measure(295mm).
full 325mm- 295mm= 30mm rider sag.

10mm free sag and 30mm rider sag are good number for the back. If the spring is to soft than you'll have to crank the preload to get the 30mm rider sag and then you'll wined up with little to NO free sag. If you spring too soft than you loosen and loosen preload til you get to 30mm but then you'll wined up with say 15-20mm or more free sag.

The last thing to measure is "Stiction". Lift the suspension and slowly let is down, measure (318mm) then subtract free sag measurement (318mm=315mm=3mm stiction). This is the amount of resistance in the suspension. 3-5mm is acceptable.

The reason most people replace the rear shock is because it cost around $400 to revalve plus $100 for the spring totally like $500 when you can pick up a penske double clicker with the correct spring for $700-800.

Jeff

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post #4 of 9 (permalink) Old 09-07-2005, 11:08 PM

 
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Talking suspension is part but

I've had the pleasure of working with a CR from Nesba who shall remain nameless. None the less we've been learning a lot about tires and suspension.
One thing for sure is the number of heat cycles regardless of the tread depth.
We found out the hard way, "the tire may look good but sticky went away".
This can happen on one side only or the whole tire. The number of miles don't mean anything compared to the number of times a tire has been heat cycled. Typically on the street, you will not get the tire hot enough to be concerned.
But the reduced pressures run on the track, with extra stress on the body, will definatively cycle the rubber compound.
Do you run warmers? As I recall, you do not. This means as you go faster at reduced pressures, you will be cycling the tire every session. After so many cycles, the rubber looses its tackiness or stick. We've got a stack of take offs that are all good on the street but one side has lost that tackie feeling at temperature. My son's multiple lowsides are a result of one side going away.
We've dialed the suspension for specific tracks only to find one side going away before the tread shows any indication of a problem. Every manufacturer and compound will react different for every track relative to the conditions. The real goal is to learn how to read the transition from traction to no traction.
Two keys to reading the traction are the rider feedback and feeling the tire surface right after a session. Rider feedback may be, " ya, it seems like the tires are squashing or feel like rocks". The surface on one side will glue your butt to the wall but the other side is like fresh grease.
Pressures are parimount with appropiate suspension settings to equalize those effects. Yet the temperature and humidity of the day can cause change as well.
All this aside when you put an expert rider on a bike. The riders skill to adapt to such varying set of conditions is truely the miracle of speed.
If there is a way to formulate the ideal condition, and I find it, I'll share with you all. For now, I can say with all confidence, stay with one variable at a time and learn to adapt as the weather changes with your riding techniques.
Sorry to be so long winded.
This is a very complex subject.
As far as your crash is concerned, don't look for a single cause, it will drive you nuts. Like me.
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post #5 of 9 (permalink) Old 09-08-2005, 11:06 PM Thread Starter
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Thanks for the info/advice guys. I plan on being more prepared mentally, physically, setup and equipment-wise next time. I know there are many factors to take into consideration now.

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post #6 of 9 (permalink) Old 09-09-2005, 09:06 AM
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At your weight you will not be able to get the sag you need with the stock stuff. I would try to find a used Penske, Ohlins, or Elka for a rear shock. Look on ebay and wera there are usually good deals to be had.

The valving is just as impotent as the spring being the right rate for your weight. I am close to the same weight and I run .95 springs in the forks. The rear is more bike specific so you just need to talk to the manufacture of the shock for their recommendation. At your weight you will have a lot of tire wear in a short time. The tire is being loaded more because the shocks are not able to keep up with the loads they are receiving. This is what wears the tire out faster.

Brent
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post #7 of 9 (permalink) Old 09-09-2005, 06:07 PM
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Quote:
Originally Posted by Nolan1300

In both cases, my tires wore to the edges but I didn't have the characteristic balls of rubber or melted rubber ripples on them. They looked more like they had been "shaved." Kim's tires looked shredded even though we both know that she wasn't carrying as much corner speed. Should this have been a sign that I was headed for trouble?
I'm not sure what you mean by shaved, other than that they are smooth. What you want is the tires to look smooth with even wear, with little balls of rubber at the edge. If you see someones tire and they are all shredded, have ripples, waves or scalling, then the suspension is incorrect. Those marks indicate that tire is working to hard because the suspension is not doing it job, usually caused by to much damping. either compression or rebound.

Alot of people say heat cycle are the problem and they can be. but there are warning signs of them being heat cycled out and unless you are turning some serious laptime you can run a tire thru tons of cycles and past the wear marks. if you suspension is set correctly.

Wprk on just suspension slowly and you will be rewarded greatly. remember the less damping the better, you want to ride the spring not the oil.

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post #8 of 9 (permalink) Old 09-16-2005, 12:24 AM Thread Starter
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Zeus: Suspension will be my winter project for sure.

Duc: My tire wear was even but no rubber at the edges. They didn't look much different from what they look like on the street, just a bit "coarser." I thought the lower tire pressure and softer pilot powers would have worn more than the bt012s but they didn't look like it. I think I might try a "race" compound tire or corsa next time.

Thanks again.

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post #9 of 9 (permalink) Old 09-16-2005, 09:54 AM
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Nolan... those little balls of rubber isn't actually from your tire. You pick those up from the track. I always thought it was but one conversation with Steve Brubaker at Dunlop set me straight.
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