Fork Rake and Trail on 1974 R90/6?

[Sam LP]

Hello,

Can anyone provide the rake and trail figures for the forks on an early R-series bike?

Thanks,

Sam

explanation: I fitted a 1991/2 K100rs 16V front end to my R90/6 ten years ago or so. The forks are longer than the stock ones and I dropped them through the yokes until it handled right - leaving not much clearance on the fork level beyond maximum suspension travel. Now I want to fit fork gaiters and they seem to take 45mm of clearance up on the fork leg, meaning that I have had to back the forks out.

[robtg]

Trail is 95mm--3.74" From factory manual
Rake ~27deg From memory

[Sam LP]

Thanks very much Robtg

The rake is the same as I measure with the set-up with gaiters fitted. I'll measure trail tonight and see how it compares. I have to finish the exhausts before I can ride it again, but I know that I preferred it when the forks were dropped through the forks: steeper rake in comparison to stock geometry.

Cheers, Sam

[Kurt in S.A.]

Rake and trail for an R75/5 listed in Tony Foale's website:

http://www.tonyfoale.com/Articles/RakeEx/RakeEx.htm

Kurt in S.A.

[vanzen]

Rake will be of little consequence.
Keep trail @ >93.7mm

[robtg]

Rake will be of little consequence.
Keep trail @ >93.7mm

Where did 93.7 come from?

[vanzen]

Why 93.7 ?
A personal preference of "balanced forces" and "neutral handling"
(righting moment vs rider input, directional stability vs self-steering).

Why a non-stock trail # ?
The '91 forks used by Sam offer the option of rake and trail adjustment to improve handling characteristics.

There is NO objective "best" trail # – Too many variables exist and any answer will be context dependent.
IMO: BMW's choices for the old air-heads result in rather lethargic handling.
(for comparison: R1200S rake = 24º, trail = 86.36 mm)

[robtg]

Do you really think that the difference of 1.3mm of trail sitting on your bench is going to be noticed in actual use?

[Sam LP]

Hello,

Back of the envelope calculations bring up a rake / trail of approx 27.50 degrees/88.60mm

I expect this to feel as Vanzen puts it "lethargic". Before I put the gaiters on I was able to drop the forks through the legs to achieve somethings more like the R1200s. It felt very nippy. When I do a test ride, I'll decide if Gaiters outweigh sharper handling.

Sam

[vanzen]

Hello,

Back of the envelope calculations bring up a rake / trail of approx 27.50 degrees/88.60mm

I expect this to feel as Vanzen puts it "lethargic". Before I put the gaiters on I was able to drop the forks through the legs to achieve somethings more like the R1200s. It felt very nippy. When I do a test ride, I'll decide if Gaiters outweigh sharper handling.

Hey Sam, been a while.
What #s had you been running on that beast ?
88.60 will be (close to) the stock trail of an R75 or an R100RS (both at 88.9) –
Should be good handling without shaking getting on the throttle (hopefully) ...
but then the stockers had a 19" wheel and a tad more rake.
To get 93.7 trail with essentially the same fork I'm dealing with a 26º rake !
( Due to differences at the back I will presume )
Curious: Are you measuring trail at right angles to the steering axis or along the ground ?
#s as the R12S may be pushing the frame / chassis beyond it's abilities to keep up ... be cautious ...
In any event, let me know how it behaves !

Cut those damn gaiters to make them fit what handles well, brother !