Topic: Anyone want to talk about suspension tuning and the limits of traction

[550_ko]

Alright.  I've got a cb550 cafe racer which has got a slippy front end.  A couple or few weeks ago I spilled the bike  because of what I thought was old hard tires.   I mounted new tires (avon super vemons), washed them carefully and scuffed them a bit with steel wool and sand paper.   I put in the manufacturers rec tire pressure and adjusted it to the lighter bike and my tastes (about 4-5 psi lower).   Yesterday (after a week of break in), I went out on a local road with 11 miles of turns and switch backs and going in to the first turn the front end slides out.  I stay smooth on the the throttle and a millisecond later it gets traction again. 

Technique wise I think I was OK.  It was a down hill, slightly banked turn with a smooth road.  I was leaned over at 45 degrees (definitely not to fast for the turn),  looking through the turn, rolling gently on the the throttle and the bike felt level.  So I am wandering what else it could be.   

The new front tire has a slight out of roundness to it .020  -  .030.   The rim was within spec on the ride (.030 to .040 out) but I trued it afterwards to within .010 because the front end felt bouncy. 
10wt fork oil
Teleflex brace
Progressive springs with a bunch of preload (perhaps too much?)  I have had the front end bounce over big bumps before.
Tapered head bearings.


Any ideas/ thoughts on what would cause the bike to behave like this. The front end has slid out like this consistently for a while now.  I'm going to start adjusting stuff so any input would be appreciated.  My guess is springs are too stiff for the out of round tire/ rim. 

One other related thing is that with the new tires the bike likes to dive over at about 10 degrees of off perpendicular, which doesn't inspire confidence.  I thought I'd adjust head bearings (it has been a while). . . and see what happens.

[GroovieGhoulie]

Sounds like the springs are too stiff and don't have enough compliance to keep the wheel down.  A bump is transmitted right to the frame, which jumps, and unloads the front wheel.

[crazypj]

You have very limited tunability , spring preload (with spacers) and fork oil plus you need to balance front and rear. If the rear is too soft it will give similar symptoms to front too hard.
 Need to set up suspension. (you will need someone to help with this)
This is how we set up race bikes, MX or Road.
Its going to be a compromise at best.
You need to have roughly one third of suspension travel as sag when your on the bike.
Unload front, (wheel off the ground) measure from fixed point( bottom yoke pinch bolt or similar) to axle. This will be L1.
 Sit on bike with a helper holding it upright, measure same two points (L2) take second measurement from first and see if your close to 1/3 travel (L3).
 Do the same for rear.
Now comes the trick part. get off bike, have it sitting on its wheels and check the measurements again front and rear.( not on side stand) should not be more than 1/3 of the L3 measurement.
If its less, springs too soft, if its more, springs too hard.
 The L3 sounds backwards but isnt.
For a 550 your going to have an L3 in he 20~30mm range (R~F)
You can also check if the front and rear are moving somewhat toigether (balsnced) by pushing on front of seat when its on its wheels bike should move roughly straight up and down, not one end first/more than other
PJ

[tsflstb]

I agree it may be a combination of springs/fork oil weight that's making your forks too stiff.  I can't tell from your picture, but if you have clip-ons, you could try to slide the fork tubes up a little to put more weight over the front.  That would also make your bike steer much quicker.  Maybe good or bad.  You could just leave it as is and try some 5 wt oil or go back to the stock springs.

The new tires probably have a more triangular profile compared to the old ones, which will want to make the bike lean as soon as you tip it in.  I have to get used to this with new tires, as mine get squared off with age.

Just curious...it sounds like Deal's Gap is a local road for you?  You're one lucky dude.

[bwaller]

It does sound like you need to address how much preload is too much. What was the temperature? If it was a bit cool your tires may not have been at a temp to give you the performance you needed if you were "at speed". I had a set of Pirelli  Phantoms once and they broke loose often which is a little unnerving for sure.

[GroovieGhoulie]

Deals Gap is a local road for me, but it's about an hour away.   

LUCKY!!!!

One of my dreams is to shoot the gap.  What is it?  320 corners in 10 miles or something?  Heaven.

[TwoTired]

Are you using oversize tires on stock width rims?

What rim width does the tire manufacturer say your tire will work on?

A wide tire with a narrow rim pinches the parallel tire bead closer together.  With standard tire pressure, this amplifies the tire crown.  reducing designed tire contact patch.  To regain contact patch, reduce tire pressure.
However, the narrower rim width also changes the sidewall angle, which allows the tire tread to wander laterally on the wheel with low tire pressures due to lack of sidewall support.  To mimic side wall support, tire pressures have to be HIGHER than recommended, reducing tire contact patch size with the road surface.

One other related thing is that with the new tires the bike likes to dive over at about 10 degrees of off perpendicular, which doesn't inspire confidence.

I think this is also a tire crown profile issue.  However, this is related to the front to back crown width difference.  I'll try to explain.  First, you'll notice that the bike was originally designed for a tire with a round tire tread crown whose rear radius, derived from its width, was only 0.5 inches different front to rear.  During leanover, the tire contact patch moves sideways on the tire crown.  If the lean angle and gravitational forces remain centered on the contact patch then steering remains neutral during the lean.  More importantly, neither fork receives more force to effect position neutrality.  However, a wider or flatter tire tread crown profile, allows the tire contact patch to move out of line than the lean angle and gravitational forces. resulting in more pressure being placed on one fork leg than the other.  This makes the bike dive into the corner and you have to apply counter steer to maintain your line.
Also, if a wide rear tire has a wider crown, the patch centerline shifts into the direction of the turn.  It's as though your rear wheel was shifted toward the turn and you fall into it faster as you lean over. The front tire crown radius need to be close to the rear tire crown radius to negate the dive in.

Hard to explain.  Hope this makes some sense.

Cheers,

[bistromath]

Even though they were scuffed, new tires take some time to start gripping appropriately. This might exacerbate your problems. Bike dealers typically tell you to take it real easy in the twisties for the first 200 miles to let the tires start gripping.

[etn4runner]

Deals Gap is a local road for me, but it's about an hour away.   This one is north of Knoxville about 20 minutes from my house and has no weekday traffic on it. 

Whereabouts are you talking about. I live in clinton. Plenty of twisties around here.

[MRieck]

Race Tech lists a Gold Valve for the 550. It would be an excellent improvrment with a set of they're springs. I'd guess the springs would be in .80kg to 90kg depending on your weight.