Motorcycle fork diameter

[Duck-Stew]

Building an LSR bike (M chassis class) which could eventually reach ~225mph.

How small of a fork diameter (RSU forks) would be allowable & safe for a ~550lb (bike & rider) @ that speed?

I'm planning on 2" of suspension travel w/o me on it, & 1" going down the salt.

I've got a 31mm complete front end and am concerned it may be too weak.

[bak189]

31mm dia. on the forks you already have a somewhat small for that weight...However, a fork brace may help.....................................

[panic]

The tube diameter is a bit small.
The problem typically isn't bending, but skewing, where the tubes are tweaked out of parallel.
A larger diameter makes the lock-up tighter in the clamps. Some clamps have enough material to use a locating screw right through the clamp into the tube to prevent rotation.
Does it limit your axle size? You may be able to increase the axle diameter if you increase the wheel bearing size.
How important is easy service?
If you don't care, parallel can be insured by simply welding a brace right to the fork tubes (not the lower legs) as close to the bottoming position as your nerves allow. It makes dis-assembly a bit annoying.

[racefanwfo]

Are you going to use regular forks or upside down type. I would think the smallest size forks you would want to run would be 38mm or larger.

[Duck-Stew]

They're right-side-up forks.  Tire diameter =22" & steering head angle will be ~26'.

The tube diameter is a bit small.
The problem typically isn't bending, but skewing, where the tubes are tweaked out of parallel.
A larger diameter makes the lock-up tighter in the clamps. Some clamps have enough material to use a locating screw right through the clamp into the tube to prevent rotation.
Does it limit your axle size? You may be able to increase the axle diameter if you increase the wheel bearing size.
How important is easy service?
If you don't care, parallel can be insured by simply welding a brace right to the fork tubes (not the lower legs) as close to the bottoming position as your nerves allow. It makes dis-assembly a bit annoying.

So, by 'parallel', you mean that one (or both) of the forks would slide up in the triple clamps?  If so, I'm sure I can come up with a fix for that...

[wobblywalrus]

Duck, the experienced Triumph racers I know use upside down forks for the real fast bikes and 225 mph is fast.  They have been used for a number of years on dirt bikes and it is not hard to find a set.  Triple clamps with different offsets are available for the more popular motocross bike forks and there are all sorts of tuning products for them.  The upside down forks are much more rigid.

[Koncretekid]

31 mm is very small.  I have a 125 Rickman Zundapp with baby Cherianis and I think they are 32mm.  The 26* rake is also very steep. Lots of information on this site about rake angles and most importantly, trail, so do a search.  Sportbikes use angles in this range, with very little offset to achieve fast steering.  I wouldn't want fast steering at 225mph!  Front end "twitchiness" is scary at speed.  I'm curious what sort of "M" class bike would have these dimensions and be a 225mph contender?  It sounds like you're going for extreme light weight, which is not usually a priority in LSR.  I'm also curious about your 22" tire that would be rated for that speed.  What is it?  Probably more questions than you wanted.
Tom

[Duck-Stew]

The option to install the 31mm front-end is based solely on the fact that I already own it and not weight savings.

I'm looking into other options at this point as it appears it may be too small to safely keep me above the salt.  Thanks for the input.

Front tire: 22x2.5x17 Goodyear Eagle

Steering head angle is on an eccentric so I've got from 23.5' to 24.5' available to me from the factory when the bike is at it's OE ride height.  The rear will be lowered a bit more than the front so I'll be higher in the 20 degree range than that, but I may have to rake it out to approx 30' and use the eccentric to tailor the maximum 10% addition to wheelbase.


***update***  Found a good deal on a pair of 46mm RSU ZX6R forks locally.  Now to adapt the Kawi parts to the project...