Frame Brace Thoughts

[Chuey]

Hi Chuey

something that answers a question that hasn't been asked by the frame

Could you expand on this? as I thought this would be the same question that the CC type braces answer.

Hi Nourish,
I've been away for a few days on my R100CS. I was paraphrasing the expert's statement. Basically, he said that a fore/aft brace would not make a difference in handling except, maybe if one has some ripper brakes, it could help under extreme braking force.

And, from how it looks to me, the CC braces, by going all the way to the front of the bike, would transfer the forces better.

Say, those cool wheels I lusted after aren't for sale, are they?

Chuey

[Nourish]

Hope you had fun and no the wheels are not up for sale but they do come up for sale now and again on EBAY UK.
What stresses do the CC type brace resist if it isn't fore and aft?
Admittedly these braces would work better than my concept but I am unable to use these so really I'm trying to use the engine to transfer the same loads.
White lining - where the bike wobbles over longitudinal lines in the tarmac - what is going on with the frame to do this?

[ME 109]

White lining - where the bike wobbles over longitudinal lines in the tarmac - what is going on with the frame to do this?

The main cause of that imo, is a flattened off rear tyre. When my tyres are worn out, particularly the rear, it will wobble a bit as you describe. It's annoying and looks pathetic from behind.
With new tyres, not a hint of a wiggle.

[Nourish]

I agree that probably causes the wobble but what are you feeling in the seat of your pants? surely somewhere the frame is moving?

[ME 109]

I agree that probably causes the wobble but what are you feeling in the seat of your pants? surely somewhere the frame is moving?

Basically, I think the bottom front of the engine is swinging left to right. As it does so, the forks twist and load up, one way then the other.
This is the gummikah.
I can produce gummikah while stationary, fully loaded for touring, with the bike's full weight on the wheels.
If I shake the bars side to side, the bike flexes side to side like a mofo. This will actually happen up to about 60-70 kph if I shake the bars. I will one day set up my gopro to get a slo mo of this. It will raise some eyebrows I'm sure.
This same horrendous rubber cow is completely gone from perhaps 90 k upwards. At 120, the same loaded bike will not budge if the handle bars are given a shove. I believe the front wheel's increasing gyroscopic effect is the cure.

A strong fork brace and a billet top plate would be my first steps to overcome this type of flex.

[Nourish]

I get your point about the forks flexing - I'm going to use a set of K75S forks with its proper top yoke.
But I don't understand why the engine would be swinging left to right due to what I gather is a side load to the wheels.
If this is the engine flaying around wouldn't a brace from the timing chest to somewhere near the steering head help this problem - tying the top frame rails to the bottom ones?

[ME 109]

I get your point about the forks flexing - I'm going to use a set of K75S forks with its proper top yoke.
But I don't understand why the engine would be swinging left to right due to what I gather is a side load to the wheels.
If this is the engine flaying around wouldn't a brace from the timing chest to somewhere near the steering head help this problem - tying the top frame rails to the bottom ones?

I think the engine moves left to right as a direct result of the forks 'twisting' and setting the front wheel on a new course.
The twist peaks and the stored energy sends the forks/front wheel back the other way, more quickly than the engine can follow.
The heavy engine mass builds up more energy the further it falls behind the speed of the front wheel direction change.
I think the flex begins around the steering head and increases down the length of the fork. I don't think there's much lateral misalignment from the bottom of the steering head down the front down tubes, so bracing there may not achieve much

The fast front wheel's gyroscopic effect is like a steering dampener.

Makes sense to me, but please fill it with holes if required.

[ME 109]

To clarify, the engine swings with the frame. The steering head being the 'pivot' point. I think fork flex is the greatest influence here.
The tail light wiggles side to side, simultaneously with bottom front of frame, but in the opposite direction.

Ow, now my head hurts.

[Nourish]

Mmmmm - if the tail light is travelling in the opposite directon to the front of the frame then surely there is a pivot point somewhere in between? If so isn't that where the CC type bracing would help?
I'm sure my 90S with its side braces doesn't whiteline - it's off the road and so haven't ridden it for over a year.

[Major Softie]

I agree that probably causes the wobble but what are you feeling in the seat of your pants? surely somewhere the frame is moving?

No.

That wobble is caused by the worn out tire causing a difference in tracking direction between the front tire and the rear. The worn out tire is "searching" for a tracking direction. In effect, this creates a constantly changing tire misalignment. This shows up as movement in the steering head, as the relative tracking directions between the front and rear tire keep changing. The forces causing this wobble are tiny (as in: enough to move the bars a small amount), and if that wobble you feel was your frame flexing, then you'd be able to flex that frame all over the place easily in your garage with your bare hands.

Any bike can do it, but some do it much worse than others. I would speculate that the difference is in steering geometry factors - some geometries heightening the issue, and others dampening them (along with just how worn out the tire is).