R75/6 straight line stability

[Abstruse1]

I took my airhead out today for a ~75 mi. ride on the interstate. I was disturbed by what I felt as a rear end squirming.

I didn't notice any steering head wagging, just the rear end. I was perhaps oversensitive from my rear flat a couple of weeks ago, but I don't think I was imagining this. The pavement was mostly good, flat concrete and my speeds varied from 60-75 mph. I had an approximate tail wind half the time, then a head wind coming back. The bike has 51k miles and was carrying around 210 lbs total. I've checked most nuts/bolts for tightness. I'm running "sport bike" handlebars that I cut off by ~1" (each end), and no windshield.

It's possible that I just don't remember how these bikes rode 40 years ago (and I put a lot of miles on airheads back then). Maybe it seemed normal in those days. Nevertheless, I want to eliminate it!

Here are my thoughts on possible causes/fixes:

1. Tires. I have new Avon Roadrunners, 3.50/19 and 4.00/18, running at 28 psi (the pressure recommended back then). By today's standards, they're pretty skinny. Would wider wheels/tires help? Different tires? Different pressures?

2. Bike has a new period steering damper, but that's old technology. Would a Scott or other contemporary damper help, even tho I didn't sense a steering head wobble?

3. I think my swing arm bushings are okay. I can't feel any movement by trying to wiggle them, but I'll need to pull the wheel and the shocks to make sure.

4. Fork innards? I have a Race Tech emulator and springs, but haven't installed them yet. I'm running Ikon shocks, adjustable for damping (rebound, I assume), set to position 3, with springs for my weight set to first notch up from full soft. Would the fork improvements help?

5. Fork alignment. I haven't checked this out but will do so. Steering head bearings seem right, based on trying to move suspended forks (no detectable play).

6. Assuming forks aligned correctly, what about fork brace and/or thicker top triple clamp?

7. What else could I do?

Am I deceiving myself thinking that this bike will be as stable as contemporary bikes?

I've read a lot of the articles by Snowbum, Ausherman, et.al., but would really appreciate others' thoughts. I'd like to start with the most promising things. My thanx in advance.

[khittner1]

Tire pressure is too low. Try 32psi front, 36psi rear, and see if the problem goes away.

[melville]

Tire pressure is too low. Try 32psi front, 36psi rear, and see if the problem goes away.

I agree. Mine is a 75/6 also, no wobbles when the tires aren't worn flat. No steering damper.

[Ken in Oklahoma]

I would also keep in mind the possibility that your new tire is defective. It's probably OK, but maybe not.

When I've watched videos of how tires are made (with a lot of handwork required) I often wonder how the product can be consistent with all of that handwork involved.

Ken

[Abstruse1]

Are the Avon Roadrunner's good in this regard, or are other brands better?

[khittner1]

Get the right amount of air in your tires before sweating a defective product scenario. Hans and Franz would not stop the compressor at 28psi if they were still spitting out /6s, and filling up contemporary sets of ContiTwins. Try 32 front/36 rear---it'll likely be better. You can probably go as high as 35 front/38 rear without causing premature wear or weird handling. The rims will handle it fine, too.

[Abstruse1]

Thanx, khittner1, that's the first thing I'll try.

[ME 109]

Thanx, khittner1, that's the first thing I'll try.

Yep, pump those tyres up. I run about 35/35
If there's still a problem, then fork alignment is next.

Edit, based on running new tyres.

[Major Softie]

Yep, pump those tyres up. I run about 35/35
If there's still a problem, then fork alignment is next.

Edit, based on running new tyres.

Since the tires are new, after pressure, my next check would be tire mounting: a tire not fully seated on the bead. Much more likely with tubeless, but can happen with tube sometimes as well. After that check, THEN I'd go alignment next.

[Rebel]

How are your rear splines?