RT A-Arm

shreddr · shreddr's Garage

What is the thought on the shorter a-arm for the R1200S? I know you can buy the RT a-arm which is kind of pricey. What exactly is different with the RT item that makes it shorter? I was looking at the a-arm on my bike, and where the casting is machined for the lower clamp "Jesus" nut, it looks as if there is room for a range of spacing. It seems as if a reasonably talented machinist (Motoyoyo?) ought to be able to machine that hole out to an oval and use stainless steel spacers for fill the gap and give a bit of adjustability. That should be a lot cheaper than the $700+ the new a-arm would cost. Any thoughts?

jduke · 03-18-2009, 02:11 PM

It's a few mm shorter so it shortens the bike length which quickens the turning.
At least that's the way I understand it. If I'm wrong, I'm sure it will be corrected quickly....

SergioK · 03-18-2009, 02:21 PM

Buy a used one off beemer boneyard.

signit98 · 03-18-2009, 03:23 PM

The A-Arm is actually the same length, just the tap-point is about 10mm further backwards.

I am sure that one could manufacture and excenter that would make the whole thing adjustable... but why would said person mess with such an incredible amount of liability???

This is America, you get sued for looking crooked at your neighbor and incredibly stupid people (read Jury members) award them incredibly stupid amounts of money...

Yeah, the last stand of rugged individualism, I know... LMAO!

SergioK · 03-18-2009, 03:48 PM

Watch what you say Ralf or I'll sue you!

shreddr · shreddr's Garage

Quote:

Originally Posted by signit98

The A-Arm is actually the same length, just the tap-point is about 10mm further backwards.!

where is it different? is the clamp hole relocated or the swingarm pivot?

signit98 · 03-18-2009, 05:33 PM

I can't tell you, Jeff or Sergio is going to sue me...

signit98 · 03-18-2009, 05:34 PM

Quote:

Originally Posted by SergioK

Watch what you say Ralf or I'll sue you!

Hey, wait a minute... you are not my neighbor...

shreddr · shreddr's Garage

Here is a picture of a GS a-arm (same as R12S) and the RT a-arm. You can see the slight difference in position of the bolt hole if the castings are the same.

GS

RT

BMathison · 03-18-2009, 09:07 PM

Quote:

Originally Posted by signit98

The A-Arm is actually the same length, just the tap-point is about 10mm further backwards.

I am sure that one could manufacture and excenter that would make the whole thing adjustable... but why would said person mess with such an incredible amount of liability???

This is America, you get sued for looking crooked at your neighbor and incredibly stupid people (read Jury members) award them incredibly stupid amounts of money...

Yeah, the last stand of rugged individualism, I know... LMAO!

That is when you market them as "for off road use only".
I think the eccentric idea would be a perfect and reasonable priced alternative. It is something that is already used on swing arms from many different mfgs, so why not?

signit98 · 03-18-2009, 09:15 PM

Different forces at work... I think it would work well and - on top of that - would give you the opportunity to adjust to the type of riding you plan on doing...

However, as opposed to the excenters you find on swing arms, this one would only give you two useful positions, 180 degrees from each other. So, you could do a 10mm offset like it is with the RT/S a-arm... for example.

Twisties/Track = Short setup ... for the rest, you go long! It would invovce major surgery on the a-arm, however and I am not sure that all to many folks would go for that!

motoyoyo · 03-18-2009, 10:36 PM

heya shreddr, good questions as always. I only play a machinist on TV, I am actually...well let's not go into that right now. I see from your pics and Ralfs comments that all the R1200 telelever arms are made from the same cast aluminum parts. From the looks of it they had allowed for some fairly heavy modifications at the factory as well.

The conical bolting of the ball joint imposes some complication to the adjustable slot approach to varying the steering head angle. However, you could have a machinist do a stepped slot and make some conical eccentric spacers something like this to change the ball joint position.

Something like this with a mating half to fill the slot:

shreddr · shreddr's Garage

Quote:

Originally Posted by motoyoyo

heya shreddr, good questions as always. I only play a machinist on TV, I am actually...well let's not go into that right now. I see from your pics and Ralfs comments that all the R1200 telelever arms are made from the same cast aluminum parts. From the looks of it they had allowed for some fairly heavy modifications at the factory as well.

The conical bolting of the ball joint imposes some complication to the adjustable slot approach to varying the steering head angle. However, you could have a machinist do a stepped slot and make some conical eccentric spacers something like this to change the ball joint position.

Something like this with a mating half to fill the slot:

Dude, you rock! Now thats what I'm talking about!!

When will you start taking orders? Shall I send you my a-arm now, or do you want to play with a beemer boneyard unit first?

motoyoyo · 03-19-2009, 09:44 AM

Actually it would be more stable to do a one piece unit and make them at different offsets as needed. Not sure how much material there is to work with on the R1200 telelever arm. You would have to check that out first. You could probably take the telelever arm and a couple sketches to any decent machinist and have the parts made and telelever modified. Pretty straight forward as long as the mods don't weaken the telelever by ending up with too small a wall thickness at any point.

Stock offset (not to scale)

10mm offset (not to scale)

Jeff Williams · 03-19-2009, 09:02 PM

Quote:

Originally Posted by signit98

I can't tell you, Jeff or Sergio is going to sue me...

not me i hate lawyers almost as much as i hate dentist. ????? i hate me

Jeff Williams · 03-19-2009, 09:03 PM

Scotty watty do daugh day,
you coming to San Diego my friend

Joe the Viking · 04:18 PM

A friend of mine has just done this mod to his R11S with a litte help and encouragement from me, What do you think?

http://bmwklubben.se/viewtopic.php?f=218&t=7996&p=50097#p50097

I know the text won't do you much good but the pics tell the tale exept for the changes in rake etc:

Hole was moved just 4mm in this mod.

Rake has steepend by about 1 degree

Wheelbase got 10mm shorter.

Rideheight got 4mm higher up front.

The tendency to "rake out" when compressing the suspension is reduced by 0,3 degrees from 5,8 degrees between fully compressed and extended suspension to 5,5 degrees.

Bob Hancock · 03-25-2009, 03:56 PM

This is not difficult machine work. But I don't understand how the offset insert would hold itself in the elongated hole.

signit98 · 03-25-2009, 04:13 PM

large washer, bob

shreddr · shreddr's Garage

Quote:

Originally Posted by Joe the Viking

A friend of mine has just done this mod to his R11S with a litte help and encouragement from me, What do you think?

http://bmwklubben.se/viewtopic.php?f=218&t=7996&p=50097#p50097

I know the text won't do you much good but the pics tell the tale exept for the changes in rake etc:

Hole was moved just 4mm in this mod.

Changes in rake:
Normal riding position: -0,35 degrees
Fully compressed position: -0,7 degrees
Fully extended position: -0,4 degrees

Plus:

Wheelbase got 5mm shorter.

Rideheight got 6mm higher up front.

The tendency to "rake out" when compressing the suspension is reduced by 0,3 degrees from 5,8 degrees between fully compressed and extended suspension to 5,5 degrees.

Are you sure about that wheelbase change? Seems like a 4mm offset of the pivot halfway up the forkleg, ought to give more than 5mm at the axle