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Went out riding with a group of guys through some good twisties for a while today. Something weird I noticed on the way home. When cruising at 90+mph on turnpike with feet on passenger pegs and laying down on the tank the front end would start to wobble, not tankslapper bad, but bothersome bad. If I sat up to the normal riding position with feet on proper pegs and just leaning over the tank, it would be fine. I know I need to adjust my rear sag badly, do you think this could have been the cause? I'm a big guy and I know I was putting alot of weight on the front end when I was in that position. When I would slow to speeds below 90 I wouldn't notice any wobble. When in that position I believe most of my body mass/weight was over/on the tank, not the seat. I checked my tire pressures when I got home. 32 front, 36.5 back, usually keep rear at 39. I have a pretty new Pirelli Diablo Corsa up front, and a Metzeler M-1 in the rear which has about 500-800 miles of life left. I know I need to dial-in my rear shock for my bigger size, do you guys think that is what the problem is most likely?

Thanks

Nic
 

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You definitely made the rake angle more steep by hugging the tank. Check your preload on the forks. You should try to come close to 1/3 of the total fork travel for sag. That should keep your rake correct. You may have to much preload in the rear, although, if you are a big guy, it may be correct for an up right position. Also, if you have to much rebound damping on the fork you could get a packing effect where the fork will slowely sink causing steeper rake. Anyway, you should try and find out why you have steeper rake in the fork when you hug the tank...

Borg Girl
 

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Not to pry, but how big are you?

When you say wobble, are the bars rapidly flapping, or is the bike weaving/snaking fairly slowly? Those are different and distinct motorcycle instabilities, with different implications of problem areas.

Fork oil replaced lately?
Swingarm bearings OK?
Wheel Bearings OK?
Check to make sure that your steering head bearings aren't loose.

Set your preload to achieve the 1/3 sag as Borg Girl suggested.

You're cross branded on tires with the Pirelli/Metzeler combo. I cannot say that it's an influence, but I don't think tire companies test their tires with a mix of other manufacturer's tires.

That, and if your M-1 rear is about finished, it's pretty flat in the center. That never seems to help stability. Might want to slip a matching Diablo on the back.

I just put a set of regular ( not corsa ) Diablos on my bike and they're quite nice. Just did almost 500 miles on 'em in the last 2 days, and, ahem, they were fine up to about 100. ;D
Of course, it a different bike, I'm 6'1"/160, had saddlebags and tankbag on board, etc.

BTW, some folks have said that the corsa (as well as other brands/models of racy rubber) are not very sticky until you get them hot, and it's tough to get them hot enough on the street.

If you can stand it, change one thing at a time and see what solves the problem.
 
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Tire pressure seems high, I run 32f 34r with diablos. They were great on my last track day at 135 down the straights.

Get a new back tire before changing anything else. It could be that simple. If you are happy with the bike in the twisties and it weaves only on fast straight shots in a non-standard ridding position IMO I would not change a thing.
 
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