I Almost Biffed It

[Striker1423]

Took the Yamaha out for a very cold lunch (55F) and on the way home was just having a ball. Did a few test runs to confirm lack of fuel at high RPM's (yup), and decided the last pass would be more fun than usual by going over the overpass before my turn off to the dirt road.

When hooligan me comes out, I've always hammered the gas on the backside of overpasses, never the front. This time I was going probably 60mph or so, and decided to hammer the throttle at the bottom of the overpass. So, I hit the crest of the bridge at say 70-75mph. Possibly faster.

The front tire must have come off the ground a bit over the top deck of the bridge transfer, because I almost had a true speed wobble. I say almost, because just as fast as one started, it stopped by itself and I was able to recover and brake for my turn. It never went full tank slapper, just a few oscillations, but enough to make me clench.

So, no more super sweet overpass speeding on that bike for me. 

[Yamanatic]

Biggest reason (I've found) for inherently stable bikes - like RD's - to 'wobble' when getting light in the front end or the wheel coming off the ground at speed is improper rear wheel alignment. When the wheel comes down full force it tries to align with the rear and is snapped one direction or the other until the two wheels are playing as a team again.

The list of culprits can be anything from worn swingarm bushes, loose rear wheel bearings, loose steering-head bearings, cupped front tires, or improper chain puller adjustments. Checking bearings and bushings is done static, but since I've found adjuster hash-marks that are off, I check alignment at speed - get your body as straight on the bike as possible and let go of the bars; if the bike goes perfectly straight alignment is good. If the bike pulls to one side, move that side adjuster back until it tracks true. If the hash marks are in fact off, remark the puller to match the opposite side.   

Since your wobble went away toot-sweet, it sounds like an alignment issue; most wobbles don't resolve on their own and often get worse, until you get the bike slowed down. Worst wobbling vintage bikes I've found to be short-frame BMW /5's, especially with handlebar fairings; they wobble easiest when the front tire cups - I've has some frightening tank-slappers on Beemers, but so far have never been spit off - 60 years and counting! 

Warren

[1976RD400C]

A steering damper, cranked up, does wonders to get your confidence up that your bike will never spook you again.

[m in sc]

i had that happen once on an sv650, when it was new. wash boarding and a rise and fall over a hill. glad you made it, and yes a damper does wonders. 

[Striker1423]

Biggest reason (I've found) for inherently stable bikes - like RD's - to 'wobble' when getting light in the front end or the wheel coming off the ground at speed is improper rear wheel alignment. When the wheel comes down full force it tries to align with the rear and is snapped one direction or the other until the two wheels are playing as a team again.

The list of culprits can be anything from worn swingarm bushes, loose rear wheel bearings, loose steering-head bearings, cupped front tires, or improper chain puller adjustments. Checking bearings and bushings is done static, but since I've found adjuster hash-marks that are off, I check alignment at speed - get your body as straight on the bike as possible and let go of the bars; if the bike goes perfectly straight alignment is good. If the bike pulls to one side, move that side adjuster back until it tracks true. If the hash marks are in fact off, remark the puller to match the opposite side.   

Since your wobble went away toot-sweet, it sounds like an alignment issue; most wobbles don't resolve on their own and often get worse, until you get the bike slowed down. Worst wobbling vintage bikes I've found to be short-frame BMW /5's, especially with handlebar fairings; they wobble easiest when the front tire cups - I've has some frightening tank-slappers on Beemers, but so far have never been spit off - 60 years and counting! 

Warren

I'm pretty positive my rear wheels been out a bit for a while now. I just couldn't seem to get it perfect. My front forks seem to be sitting a bit twisted as compared to the tank as well. So I'll try readjusting everything again to be safe.

I picked up a Motion Pro fork alignment tool a few months back to check the Suzuki and in reality need to check the Yamaha for sure now.

You just adjust it to your fork size and lay it flat across the inner fork tubes. If it doesn't sit flush then you have to adjust until it does.