DR650 turned into SM, now has a problem
 

One of my coworkers has a Suzuki DR650 that he turned into a SM. He took off the 21" front wheel and put a 17" front wheel on it. Now he is experiencing what he is calling a "tank slapper" like condition when he gets to about 70mph. He cant figure out why. I'm guessing that by going to the smaller wheel, he essentially lowered the front of the bike 2 inches and has fucked up the steering geometry. Anyone have any thoughts on what is wrong? Forks are bad? Wheel is bent? It doesn't have any problems below 70mph.

sounds like wheel balance issues

A wheel out of balance would cause the tire to rotate out of round(up and down) but he is getting the handle bar to shake side to side. Would off balance tires cause a side to side shake?

how tight are the spokes? they might need adjusted or maintained

It was a brand new rim installed by a Suzuki dealer. Spokes should be tight.

i would check the balance out first, if its off the affects would be magnified by the increased speed, or possibly the bearings in the steering stem could be bad

It's the change in steering geometry, dropping the front takes a lot of rake out of the front, and the closer the front gets to up and down, the more likely it is to shake.

How do you fix that, longer fork tubes? He certainly doesn't have 2" of fork to slide back though the triple tree.

or what he said, lol

he could try dropping the rear shock in the lower position - it's easy to do, free and doesn't take all that long.

No guarantees that it will help thou...

The guy is short(5'7") so he says the rear shock is at the lowest setting.

he either needs longer fork tubes to compensate for the 2" change in height, lowering links to put the rear lower and compensate for it, or he's just not going to experience a good ride at that speed.

checking the steering head bearings is a good idea regardless though. at least inspect for good condition and grease appropriately as needed. a steering damper would probably not hurt either.

I'm saying its the geometry. My SM has been lowered a couple inches, and it gets a little wiggly at high speeds. By putting on smaller wheels, the fork angle has been steepened, which makes it much "quicker" on turn in, but also more unstable. A lowering link in the back should fix it up. There should be a ton of lowering links for that bike. Also, a steering stabilizer would help. All these comments are assuming that nothing else is messed up, i.e. wheel, head bearings, etc.

you didn't say what he did for the rear wheel, what was the tire size change. ideally you want to raise or lower both ends roughly the same amount.

Just pull it up into a mad wheelie. Front end not on the ground = no front end stability problems. :D

Sorry, I have nothing to contribute but sympathy for a fellow 'tard.

He was looking at his bike last night and noticed that he had about 2 inches of thread on the shock so he is going to adjust the ride height. Cranking that shock spring tighter is going to make for a real stiff ride though.

I suggested he try to find a rear shock from a DRZ400 SM. I'm sure that is a shorter shock.

He'd want a shorter spring. He can however crank it down to see that it does cure the problem before going out and buying a shorter spring.

Most likely he screwed the trail up, making the bike unstable.

Tell him to check it using a rake and trail calculator

This one has more info
http://www.rbracing-rsr.com/rakeandtrail.html

But this one is a bit easier to use.
http://www.goldammercycle.com/hispeed/rakeandtrail.php

Don't tell me its cruiser/chopper shit. Its all about geometry.

Further for balance issues... go dyna beads and STU
http://lowdownchopshop.com/shopping-cart/14.html

He is going to get some lowing links and drop the ass end 2" to bring it back to the OEM rake.

put some nos energy drink in it

He's not that dumb.

The stock DR650 shock has an extra bolt hole at the bottom, so you can drop an inch just by relocating the bolt.

Frame geometry modification = Baby steps.

JC

He called the dealer and one of the service guys told him that you can turn the bottom collar upside down and it should lower it about an 1 1/2".

Here are the instructions (page 166 and up):

http://www.deakin.edu.au/~mic_adm/DR650.pdf


He may also want to stiffen his front forks a bit...

Also put on a steering damper.