Frame Brace Thoughts

[ME 109]

Gidday Steve, welcome!

I can see that alloy brace preventing the rear end of the top tube moving to the left or right. If that situation exists, that brace configuration would certainly help.
If the situation is the bottom of the frame/engine swinging left to right, The brace would be far less effective.

Imo only.

[Ken in Oklahoma]

. . . Fabricating and welding of gussets, braces, tubing etc is the easy part. Knowing exactly where the flexes occur is the baseline. . .

That's one of the things that concerned me looking at the gusset (with the lightening holes) welded to the bottom of the "sub frame". From reading Tony Foale's book I've come to understand that one wants to do the welding on the stress center line of a tube since the "bottom" and "top" of the tube of the tube flexes markedly and the center line doesn't. Metal fatigue, of course is the issue. We see that "center line" welding a few places on our stock BMW frames.

Ken

[steve]

What's the consensus on why/where the flex occurs? I keep hearing that airheads have some flex-induced wobble, but I've never heard exactly why, or what part of the bike is most responsible for it.

I'm building a 1976 R90 right now (my first BMW), and I'd like to try to address any inherent handling problems while I am putting the bike together. So I'm trying to target the "weakest link", and not just thoughtlessly spend a pile of money on the standard fixes, like San Jose frame braces, high-dollar shocks, billet triple clamp, or billet fork brace. I want to target my work and my money on the one thing (or group of things) that do the most good. What do you suggest as the best plan for minimizing wobble? (Let's assume a bike in good nick - no mis-adjusted steering head bearings, no loose subframe bolts, no misaligned fork tubes, no worn-out tires, no worn-out shocks, etc.)

Just for reference: here's an old article from Cycle Guide, they tested a R90/6 back in the day, and said it wallowed slightly in 50 - 60 mph turns, no matter how they set the steering damper and shock preload. http://www.motorcyclespecs.co.za/model/ ... 6%2074.htm.

[Bamboo812]

Here's a video from the 1970s discussing the cause and cure of wobbles and weaves: http://youtu.be/z3OQTU-kE2s

[ME 109]

In the quest for optimum handling, one must start with a straight frame, perfectly set steering bearings, perfectly aligned forks, front and rear suspension correct, good tyres at correct pressure.
No bracing should be needed when all these things are in order.

A better top plate and effective fork brace should eliminate any problems associated with road riding.

Further bracing is beneficial on a racetrack where the machine is pushed to its limit.

[melville]

In the quest for optimum handling, one must start with a straight frame, perfectly set steering bearings, perfectly aligned forks, front and rear suspension correct, good tyres at correct pressure.
No bracing should be needed when all these things are in order.

A better top plate and effective fork brace should eliminate any problems associated with road riding.

Further bracing is beneficial on a racetrack where the machine is pushed to its limit.

Speaking of limits, I was doing this corner the other way today (a lllloooonnnnngggg uphill right):



Somewhere well over the suggested speed. The bike was solidly planted without braces or steering damper. Much faster would have been 'go directly to jail' fast and imprudent due to the limited sight lines.

Of course, then the dope grower passed me on the outside mid corner in his Toyota pickup.

[Major Softie]

Somewhere well over the suggested speed. The bike was solidly planted without braces or steering damper. Much faster would have been 'go directly to jail' fast and imprudent due to the limited sight lines.

Of course, then the dope grower passed me on the outside mid corner in his Toyota pickup.

Clearly you are using the word "dope" here as an adjective.

[melville]

Somewhere well over the suggested speed. The bike was solidly planted without braces or steering damper. Much faster would have been 'go directly to jail' fast and imprudent due to the limited sight lines.

Of course, then the dope grower passed me on the outside mid corner in his Toyota pickup.

Clearly you are using the word "dope" here as an adjective.

Don't you have agribusinessmen over to Tahoe?

[Toga]

Of course, then the dope grower passed me on the outside mid corner in his Toyota pickup.

Wow... thats great performance... cant help but wonder now, if he had extra frame bracing on the Toyota

[Major Softie]

Don't you have agribusinessmen over to Tahoe?

Indoor only. We don't have the climate for it.