Speed Wobble

[webbie89]

Hello all,

I've been doing some extra work to my '82 over the last few weeks, but one issue is cropping up that I just can't seem to fix: speed wobble.

The entire bike just feels unstable, compared to my other early-80's bikes. Regardless of speed, I can wiggle the handlebars at a frequency that'll set the bike into a wobble. At low speeds, it damps out on its own after a few seconds. At high speeds (55+ mph), it takes much longer; 5-10 seconds of wiggling around before it settles down.

Sweeping curves are the real problem. At 65 mph, I get a wobble going around LEFT-HAND turns only. It doesn't get really bad, but it doesn't go away until I either goose the throttle and sit further back on the seat, or grip the bars and do my best steering-damper impression.

The kicker is this: the wobble only started after doing a bunch of chassis work last season, including new steering head bearing (tapered roller), swingarm bushings (bronze), and resealing the front forks.

I'm flummoxed. The fork components didn't seem overly-worn, though I didn't replace anything but the seals. I tightened the head bearing. I've checked for swingarm slop. I tightened spokes (front and rear).

I have a few things to try, and I'd love any feedback on them, or additional ideas.

- Use heavier-weight fork oil
- Install fork brace
- Replace rear shocks (seems unlikely to make a difference to me...)
- Check rear-wheel alignment (again)
- Check spoke-tension (could unevenly tightened spokes cause this kind of behavior?)

Thanks in advance for any ideas you can provide. She's rideable, but I'd like to be able to take the freeway with greater peace-of-mind.

-Casey

[pablosixfivezero]

check your wheel bearings, tire pressure, triple tree bolts are torqued properly, are your forks straight in the triple tree(slightly twisted triples?) axles tight?

[Burns]

your new head bearings may have seated-in and that would loosened up the steering stem. Make sure it is tightened properly.

[xsleo]

On your spokes, improper tightening may have pulled the rim out of true. Check the rim runout, should be less than 2 mm up/down and left right. That's the book max. Easy to get better.
On your swing arm you need the pivot bolt torqued to at least 50 ft/lbs to get the frame compressed enough to hold the pivot tube so the swing arm pivots between the pivot tube and bushings, not the pivot tube and pivot bolt.
The steering head tapered bearings need proper adjustment. This is called fall away. You need 1 inch fallaway. To determine the fallaway lift the front wheel off the ground. Put the bike up on center stand, jack up front of frame to lift tire.
Once you get the front tire off the ground with the wheel straight forward, hang a string with a weight off the center of the back of the fender. Have the weight hang just off the ground. put a mark under the weight. Put another mark 1 inch each side of the first mark.
Now with a gental push on onehand grip push the bars, as you push the bars at some point the wheel will fall over to the side on it's own. This is fall away.
You may have to unhook the clutch cable or anything else that keeps the bars from turning easily.
The point it falls away the weight should be over the 1 inch mark.
If the fallaway occurs before the weight is over the 1 inchmark the bearings are too loose. If it falls away after the 1 inch mark the bearings are too tight.
Adjust accordingly.
The wheel bearings are not adjustable. Check for loosness and wear by runout, Mount a dial indicater to the forks with the end against the side of the hub, try to push the hub sideways, any play means you need new bearings. Put the indicater on the edge of the hub, try to lift the hub up down. Agsain any movement means new bearings.
Tires, if old or worn they can cause wobbles. Do you know how the tell how old your tires are? On the sidewall is a DOT number. The number has a oval at the end of the number with 3 or 4 digits in it. The digits in the oval indicate the week and year of manufacture. The 3 digit numbers were made before 2000. As in 248, the 24 is the week of the yyear and the 8 is the last digit of the year as in 1998.
2000 and after have 4 digits in the oval. As in 4306, this means the 43 week of 2006.
The tire makers reccomend NOT using a tire over 6 years old.
On my 81 when I got it it had good looking tires on it, still had some of the mould tits on the tread. They were dated 03 and 02 for the years. The bike had a death wobble at 35-40 mph. As in get it up above this speed and let go of the bars and let it cost down through the 40-35 mph range and the wobble was so bad it almost throws you off the bike.
I put the front wheel of my 75 with a new tire and the wobble was gone, put the 81 wheel and tire on the 75 and death wobble.
I swapped some wheels of the 82 parts bike that had newer tires but worn and full of cracks and such. The bike was ok.
Check your tires if old, replace them.
On your forks, low or uneven oil amounts can effect handling. The best method for filling the forks is the 6 inches down from the top method. To do this remove the fork caps and springs, compress the forks fully. If empty put about 8 ounces in the forks, [ump up and down to bleed out any air, now with the forks compressed use any tubibg you might have. I use 3/16 inch clear plastic hose. Put a mark six inches up from one end. Now put the six inches down into the fork, hold the mark at the top of the fork. Use vacuum to draw off the extra oil. I use my Miti Vac. Your mouth or an old syringe will work.
Put the springs and caps back in.
A fork brace won't hurt, especially if you don't run a fender, the stock fender has a fork brace built right in. An add on brace even helps with the fender.
As mentioned check your fork mounting bolts, loosen them all fender mount bolts and axle too, and start at the top and work down. Your repair manual has the torque specs on the bolts.
Leo

[Burns]

I put XSleo's post on my Internet Favorites under "basic chassis tuning"

It should be made into a stickie up there with Curley's charging sytem thread and Griz's VM tuning masterpiece.

[grizld1]

Right you are, Burns--done! Leo, thanks for a fine summary.