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Did My Preload Adjustment Cause My Crash At Sears?


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I ran the Saturday Sears Point AFM practice and was doing very well, especially after the CSS Level 1 I took on Monday. I had my lap times down to 2:13 (I'm on an ex500), which is the fastest I've gone at Sears. I had some serious brake dive on corner entries so I asked my suspension guy for some input and he recommended adding preload to my F3 forks. I added 8 mm at the beginning of Sunday practice and the dive was significantly reduced. I was slowly working my pace up that morning due to the cold track and cold tires and my first lap was 2:30, my second 2:19. On my third lap I lost the bike on the entrance to T7. I was going pretty fast on the dragstrip to get heat in my tires so I braked pretty hard for the turn. I was still more or less up and down - maybe I was 5 degrees leaned - when the bike just went. It felt as though I hit a patch of oil or something. The cornerworkers said there was nothing on the track there, and nobody else had the same problem. The strange part is that the bike and I both went down on our LEFT sides.

 

I hadn't grabbed the brakes, but was easing them on.

my tires, when the bike finally made it back to my pit, were at 23 psi front and rear, which is where they should have been according to the Bridgestone tire guy.

I can't identify any mistake I made in riding that would have contributed to the crash.

My rear tire had only the CSS day and the Saturday practice on it. The front had an additional 10 laps from a 600 supersport before I bought it.

Apparently the Bridgestone slicks take the longest to warm up an any slick, but the Bridgestone guy said 3 laps should have been fine. Also, I don't have tire warmers.

 

The only thing that seems to have possibly played a part is the preload. I tested the front end when the bike came back by pushing down on it and it was awfully stiff. Since the bike sustained minor damage (peg, bar, engine case, slider) I don't thing the forks are twisted or sticking so the stiffness is probably a result of the preload adjustment. Before the adjustment I had 37 mm of rider sag. The tires also looked a little funny. Cold tearing and just general raggedness - I don't really know what else to call it.

 

I am truly in the dark here and want to at least come away with some idea of what happened and what I can do in the future to keep it from happening again.

 

Does anybody have any idea?

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Well, I'm FAR from an expert so I will be very curious about what more experienced folks have to say, but I wonder how old the front tire was before it was first used? Rubber does have a "shelf life", and it can also vary depending on what conditions it was stored in. I know there is a lot of data on this forum regarding heat cycles and Dunlop tires in particular but I also wonder how many heat cycles the front had gone through before (and after) you bought it? Of course I don't know if that would've done it but perhaps that combined with an uncompliant suspension contributed to the problem.

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Also no expert, but sounds like the tyres could be part of the problem. Second hand slicks - who can know what conditions they had been stored in and for how long, whether they had done too many cycles, and especially no tyre warmers......no point having slicks if you don't have tyre warmers - I reckon you'll just get cold tear when trying to warm them up if you're going fast enough, if your not going fast enough you'll never get them to proper operating temperature, so its catch 22.

Of course that's IMHO.

Regarding preload adjustment as a cure to fork dive under brakes - this would be a stop gap measure at best. Has your body weight suddenly changed? Correct stiffness springs would be a better cure. By adjusting the preload only, you'd be raising the ride height of the front of bike in the process of taking out the springs compliance. This could have been a contributing factor. Unless your compression damping was maxed out already, I think that should have been the adjustment to make to reduce the dive, of course with rebound damping adjustments to suit. If your damping was maxed out, then the problem would have to be the spring rates.

Suspension adjustment is a tricky thing, my advice would be to go and get your spring rates set for your weight, with proper static sag adjustments , and with appropriate valving changes for the different rate of spring, and then experiment with rebound and compression damping settings, one variable at a time, to learn what each variable does FROM YOUR IMPRESSIONS and then attempt to set your bike up from there. Getting a good base setting from a suspension expert is vital, and recording what changes you make religiously so you can return to your base setting is crucial also.

 

regards

 

db

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Maybe I should have included more info. My forks are from a CBR F3, so I don't have compression damping adjustments, just rebound. The forks were revalved and sprung for my weight by a local suspension shop. My front sag, prior to the preload adjustment, was 35mm.

 

As you point out, I did raise the ride height with the preload adjustment, which is exactly whey I compensated by lowering the triples on the tubes the same amount. A racer told me that this was a huge no no later on.

 

Also no expert, but sounds like the tyres could be part of the problem. Second hand slicks - who can know what conditions they had been stored in and for how long, whether they had done too many cycles, and especially no tyre warmers......no point having slicks if you don't have tyre warmers - I reckon you'll just get cold tear when trying to warm them up if you're going fast enough, if your not going fast enough you'll never get them to proper operating temperature, so its catch 22.

Of course that's IMHO.

Regarding preload adjustment as a cure to fork dive under brakes - this would be a stop gap measure at best. Has your body weight suddenly changed? Correct stiffness springs would be a better cure. By adjusting the preload only, you'd be raising the ride height of the front of bike in the process of taking out the springs compliance. This could have been a contributing factor. Unless your compression damping was maxed out already, I think that should have been the adjustment to make to reduce the dive, of course with rebound damping adjustments to suit. If your damping was maxed out, then the problem would have to be the spring rates.

Suspension adjustment is a tricky thing, my advice would be to go and get your spring rates set for your weight, with proper static sag adjustments , and with appropriate valving changes for the different rate of spring, and then experiment with rebound and compression damping settings, one variable at a time, to learn what each variable does FROM YOUR IMPRESSIONS and then attempt to set your bike up from there. Getting a good base setting from a suspension expert is vital, and recording what changes you make religiously so you can return to your base setting is crucial also.

 

regards

 

db

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Aaah well, what can I say then? I wouldn't have gone on had I known. Sounds like better forks with compression damping could help, but at what cost? As for dropping the forks through the triple clamps, I've heard of it being done before, but if someone reckons its a big no-no I could believe that. I saw it mostly on drag bikes. You'd have to reckon it would alter the basic suspension characteristics such as trail, and who could know what the effect of that might be? Judging by your experience I think you've changed too many variables at once to try and stop this fork diving.

 

Could it be that you're being too aggressive on the brakes at the expense of being prepared to concentrate on your cornering? TOTW advises to smoothly brake and set your corner speed more accurately (and higher) rather than charging in, braking like mad and then upsetting the balance of the bike as well as focussing all your attention on the braking....

 

Anyway, I reckon you'll get to the bottom of it. Get some warmers for those slicks or go to supersport grade street rubber.....some blokes at my last ride day had warmers on their street tyres anway just to be sure!

 

good luck mate

 

regards

 

db

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You mentioned your sag was set at 35mm, is not recommended race sag 25 to 30mm, and higher for street use?

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Maybe I should have included more info. My forks are from a CBR F3, so I don't have compression damping adjustments, just rebound. The forks were revalved and sprung for my weight by a local suspension shop. My front sag, prior to the preload adjustment, was 35mm.

 

As you point out, I did raise the ride height with the preload adjustment, which is exactly whey I compensated by lowering the triples on the tubes the same amount. A racer told me that this was a huge no no later on.

 

If I understand this correctly it would seem that with no compression damping you are basically just running an unrestrained spring (?) and your front wheel may have just literally bounced off the ground, leaving you with no contact area. Am I way off?

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If I understand this correctly it would seem that with no compression damping you are basically just running an unrestrained spring (?) and your front wheel may have just literally bounced off the ground, leaving you with no contact area. Am I way off?

 

 

It would have compression damping in there, it would have to, but no adjustment, as I understand it.

 

db

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On my third lap I lost the bike on the entrance to T7. ..................... The strange part is that the bike and I both went down on our LEFT sides.

 

Hello twitchmonitor, T7 at Sears Point is a double apex right turns, wonder why you land on left side. You were at only 5 degree lean.---do you brake hard enough (even you just easing THEM on) such that the bike turn around completely, more or less change direction with rear wheel going first and then flip to left?

Wonder whether the side of your front tire, and the middle of your rear tire have been scuffed in well enough, such that they are still slippery at certain areas.

Hope to have someone with experience on similar crash condition to analyze. Glad that you are OK.

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  • 2 weeks later...

You say your sag was 35 mm before the crash and that you

incresed the preload to some unknown value immediatly prior

to the crash? I got that right?

Your sag with you and your gear should be 40 mm front 30 mm rear.

with an outside of 35 mm front and 25 mm rear. If in your case, if I have it correctly,

you decreased your sag even further from say 35 to 25 mm in the front.

I would hazard to say yes, the preload did contribute to your crash.

To have your bike just come out from under you while you are basically straight

up and down is really strange to say the least.

You can adjust the compression rate more or less by changing the wieght of oil and the amount

of oil which reduces the air volume inside the fork tubes. This air inside acts like a spring, an air spring.

The more air the softer the spring, the less the stiffer.

I would seek out a professional and have your bike looked at and setup

before you crash again.

Sounds to me like the thing is all whacked out.

Glad you did not get hurt!!

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I am by no means an expert at this yet but I enjoy trying to understand the physics of how a motorcycle works :P . I just have a hard time seeing how pre-load can cause a wreck that way. For the most part all it does it change your ride hight. More pre-load = higher ride (and higher center of gravity). Depending on what you're rear pre-load is set at, in theory more front pre-load can actually make the rear tire come off the ground easier because of the higher CG. That makes it harder to lock up the front tire under braking.

 

I would think its more of a tire issue. Were there any cars that raced on the track before you? It looked like you might have picked up some loose rubber or something on that front tire. I can tell you from experience if you have a decent amount of grip and your front tire locks, you'll definitely know about it. If it just fell out from under you like you're explaining I don't think you had much if any traction in the first place.

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  • 1 month later...

Twitch,

 

Lets look at this for a moment:

 

3 laps in, tires should have been warm.

Hard on the brakes as you said you were going fast.

Started to lean the bike, had some lean angle in there.

Crashed on the left (in a right hand turn).

 

You didn't mention anything else about the way the bike handled, so assuming it wasn't a real issue.

 

35 mm sag is very close, know some fast guys use even more.

 

Here is what we have noticed on the Brake Bike: most riders get the bike to buck and move around, due to pressure on the bars during braking. I might guess that with the incresed preload, the bike didn't seem to be diving as much as you are used to, but in fact still a good amount of brake was happening, turned in, slid a bit, tightened up on the bars and had a little highside.

 

How's that seem to you?

 

CF

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