purpose of a torque arm - K-Bikes.com - Excellence in Motion
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post #1 of 10 (permalink) Old Oct 28th, 2005, 5:54 pm Thread Starter
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purpose of a torque arm

My buddy and I where having a conversation regarding my bike and I mentioned about switching out the torque arm. He asked me what the thing does and I kinda fudged through an answer. Basically he said don't mod the part if you don't understand its function.

What does the torque arm do? I am under the belief that it keeps the final drive in a proper position. Can someone clarify this parts function with better detail.

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post #2 of 10 (permalink) Old Oct 28th, 2005, 7:06 pm
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The torque arm forms a parallelogram with the actual driveshaft housing to apply the torque developed by the ring and pinion into the frame, instead of causing the rear of the bike to rise like it used to do pre duallever.
hope this helps
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post #3 of 10 (permalink) Old Oct 28th, 2005, 7:33 pm
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Yeah, what he said.

But the ring/pinion interaction can also result in squat when decelerating.

This is probably the worse of the two effects - go into a turn too fast and roll off the throttle and a pre-Paralever bike will squat, possibly using up whatever ground clearance was left - ouch!
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post #4 of 10 (permalink) Old Oct 28th, 2005, 10:20 pm
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What HGP3 said is correct. The question one might ask is why you'd want to change out the factory torque arm for an aftermarket (perhaps an adjustable unit) unit.

It would seem to me that making the torque arm longer or shorter would change the pinion angle and that might just cause accelerated wear on the universal joints. It might also cause lift or squat on acceleration, depending on the adjustment.

There are people on this list that are running an aftermarket adjustable torque arm but I don't know if they've adjusted it to stock dimensions or have played with the length. I also don't know how long (in miles) they've had it installed. Most that I've seen are beautiful in design and have been machined from aluminum.

As for me, I'm not willing to take the chance that something might go wrong if I go aftermarket with the torque arm and I'm also not going to bet against the engineers at BMW getting the design wrong.
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post #5 of 10 (permalink) Old Oct 28th, 2005, 11:35 pm
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I know Pirate offers one on his site and he states it is purely cosmetic. I am assuming that it is a direct bolt on. He does not make mention of it being different in length. I was thinking about swapping mine just for the look. Does anyone know if that one is o.k. to swap out with. I am not looking to change any setup just liked the look.
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post #6 of 10 (permalink) Old Oct 29th, 2005, 8:26 am
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I have an aftermarket torque arm, purely for cosmetic reasons (as seen below).


The stock stamped-steel piece does seem kinda chintzy by comparison.

The purpose of the adjustable-length arm is to vary the wheelbase. Shortening the wheelbase should make the handling "quicker".

It's been discussed whether small changes in wheelbase could affect the angles of the u-joints enough to make a difference in longevity. I doubt it, but don't have any data to back that up.

BTW, how do I get the image to show up in the body of the post? I'm putting the link between "img" tags but still just come up with a clickable link, not the image.
Eddie: Your question conserning picture in 'body' of text, use 'the insert picture' icon and you will get a box to add the url. Good Luck, Locus.
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post #7 of 10 (permalink) Old Oct 29th, 2005, 9:15 am
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adjustable torque arm

FWIW I'm certainly no expert on the subject, but had I briefly considered putting an adjustable torque arm on my previously-owned R1150GS when I was doing a lot of 'super-moo-to' mods (a big pig for the street-only)... basically, my understanding of the purpose of altering the torque arm's length is to raise the rear ride height and thereby quicken the steering response and shifting the weight bias a bit more forward. I ended up keeping the original torque arm and specifying a longer rear shock (Wilbers) to achieve the same net affect, which on the GS was an excellent street mod! (along with fitting a set of Bridgestone BT-020's)

I've since switched to an '04 K12GT (last April) and just last month picked up an '05 DRZ400SM for throwing around the corners... Riding both bikes back-to-back I can see why you might want to quicken the K's steering response (kinda' steers like a yacht compared to the suzooks wave runner-like cut-and-dive antics) but I'd be real afraid of changing the K's steering geometry and upsetting the bike's awesome stability at speed... just my .02...
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post #8 of 10 (permalink) Old Oct 29th, 2005, 2:14 pm
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A popular mod for Oilheads is to use the shorter torque arm from an R1100/1150GS or R1100S with sport suspension on any of the R/RS/RT/S Oilheads. It is approx 20mm shorter and serves to quicken steering by slightly reducing the wheelbase. It is a noticeable difference and the bike drops into turns easier and steering response is much better. The angle of the final drive is not changed drastically and I have had no ill effects on my R11RS after 20K plus miles. If you get a used stock BMW short unit, it is one of the cheapest and most effective mods you can do.

Mike Figielski
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post #9 of 10 (permalink) Old Nov 12th, 2005, 11:47 am
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BTW, one example of an adjustable torque arm can be found here:
The Verholen site mention a 15mm adjustment (shortening) from the stock paralever geometry for good results on a K12RS/GT.

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post #10 of 10 (permalink) Old Nov 12th, 2005, 5:19 pm
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Smile torque arm

i have after market torque arms on both my 99r1100s 7 years old 30,000 miles 02 k1200rs 28,000 both from the pirate both look great both bikes work good, they are both same as stock

99 R1100S BLACK
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