Went down from a tank-slapper on a CL350...Wondering why.

Tremelune

Been Around the Block
My bike is a damn-near-stock 1973 CL350K5. I was out for my first real ride on the thing. I bought it a year or two ago when the original owner put it away for 25 years and then died, and mostly just puttered around Brooklyn until I could get it sorted. It has 8,000 miles.

It was shod with a new set of professionally mounted/balanced Michelin Pilot Activs in 3.25x19 and 4.00x18. Stock size is 3.00x19 and 3.50x18, respectively. Everything else was, as far as I can tell, stock.

I was riding on a clear day. The bike was behaving flawlessly. No shakes, shimmies, or shifts that I could detect. It felt planted (though still not a fan of the bumps). I was on a dry road traveling in a straight line. I was approaching an indicated 85 mph when the front end began to wobble. I let off the gas as smoothly as I could in the hopes of slowly, calmly decelerating to a speed that had proven more stable. The oscillation increased in magnitude. I then tried to roll on the gas (again, calmly and smoothly) in the hopes that getting the bike neutral would ease things down. The wobble continued to get worse until it was eventually a full-on tank slapper and the front tucked. This was over maybe five seconds or so.

Things I think:

- Even 40 years ago, this bike was designed to recover from front end oscillations at speeds it was not capable of achieving in stock trim, even if the forks where compressed or disturbed.

- The rear tire was noticeably larger in diameter than stock (but I believe the overall change was less than 1" from eyeballing).

- The front tire was probably marginally larger in diameter than stock.

- It is common to put this tire size on these bikes, though I can't say I've seen any speed claims. The CB setup is much more common than the CL setup.

- It is a common mod to put on longer shocks to raise the rear of these bikes to quicken the handling a bit (my understanding being they are conservative in terms of rake/trail from the factory).

- I did not hit a bump or a groove that I am aware of.


Anyone have any ideas as to why this thing started shaking?
 
This was an interesting read:

http://www.advrider.com/forums/showthread.php?t=438034

Unfortunately, it can be summed up by "shit happens; be prepared."
 
Have you addressed the wheel or steering head bearings?
Are you running a front fender/brace? You said stock, but just checking.

How many miles have you put on it?
 
It can be caused by a number of things. I'm curious though, did you have the front fender attached? I don't know if you mentioned it.
 
Any bike that hasn't been ridden in over 20 years I would not be putting on the road.

I checked the steering head bearings and front wheel bearings and they were 100% toast. If I didn't replace these and went for a ride I would have been surprised if something bad didn't happen.
 
CL supposedly has the same lazy steering geometry as the CB and they are reasonably sane to ride, so it's probably not the steering geometry, but they do need to have a steering damper on and working. If that's missing or non functional the front end is prone to flapping.

It is possible the the tires are sufficient different to stock OEM in terms of carcass stiffness to make a difference. Another issue is where were you sitting - froward on the seat or pretty far back. It could also be a combo of work shocks or swingarm bushes or notched steering head bearings OR even just the wheels slightly out of alignment.
 
The oversize tyres probably caused it, what pressures were you running? Too low will have an awful effect on steering, too high gives a tiny contact patch
It's the reason I've been telling everyone to swap rims for wider ones and fit stock tyres in metric sizes (90/90 front, max of 110/90 rear)
If it happens again, rolling off throttle will not load front end enough so make things worse,
I've found it's better to yank on the brake suddenly then let go to get way below the oscillation point, quick stomp on the rear also works
I've had 350's over 100mph without problems with correct rims fitted
 
I've been leaning toward focusing on the 'trail'. I use the RB calculator. http://www.rbracing-rsr.com/rakeandtrail.html

My experience with a Honda CR250 was headshake on hard braking with std setup. Lowered the tubes in the triple clamp to level with the top and that stopped the headshake. This raised the front and increased the trail.

Working on a Motobi 250 - 26 deg rake gives 2.14" trail. Old road reviews call this bike 'twitchy' at speed. I cut the frame and increased rake to 36 deg and lengthened the swing arm to the max of rules - now have about 5.5" trail.

Another racer running a Harley 250 Sprint has his frame raked to 42 deg and ends up with ~6" trail - said it runs like it's on rails.

We also run steering dampers - old style friction - newer style has zero resistance at zero degrees.

Hard braking will angle the bike and reduce wheelbase which all gives less trail. Also, larger front tire and smaller rear tire will reduce trail.

Motocross wants quick steering, ie, less trail. RB says 4.0" or greater is good for roadracing. Bonneville wants a lot - I'm shooting to stay in the 5" trail range.

Would be interested in the trail that you are running.

To me, lt's still a lot of criteria that contributes and there is no "one" answer to correct headshake.
 
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