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Old 03-19-2006, 02:29 PM   #1
crankshaft OP
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1981 g/s CR fork project

I'm in the process of retro fitting a CR250 front fork on my 81 g/s and have a few questions for the Airhead guru's. I'm worried about changing the geometry of the bike if the forks end up being a bit longer than the
stock unit. Any words of wisdom?
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Old 03-19-2006, 02:52 PM   #2
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This is a common questions and the answer is yes you will change the geometry of the bike if you put a longer fork on it. Unless you want a chopper you need to raise the rear and on the monolever I believe the only way to do this is to extend the swingarm . This is an involved process as you need to lengthen both the swingarm and the drive shaft. There probably are folks that do this in the US, I don't know who they are though. HPN does it in Germany and will deal with you here. I'm guessing in the meantime you can just raise the fork tubes in the triple trees to level things out. JT
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Old 03-19-2006, 03:19 PM   #3
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I'm planning on doing the swing arm also, but I was more concerned about what I might be forgetting. I'm going to add some gussets to the frame at the head tube and try to find a larger rotor and caliper to slow the beast down.

I'm having a machinist buddy make a jig and lengthen the swing arm for me. If I can find a spare swing arm I might have him make two since he'll be all set up anyway. Is the R80 swing arm the same internally with the exception of the hub (3 vs 4 lug)? I guess I need to find another swing arm.

Thanks
Aaron

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Old 03-19-2006, 03:31 PM   #4
jtwind
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Quote:
Is the R80 swing arm the same internally with the exception of the hub (3 vs 4 lug)?
That's a good question, if the monolever swingarms are the same. Would be easy to pick up another to modify if they are. JT
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Old 03-19-2006, 04:33 PM   #5
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You could also cut the fork springs down a bit to cheat it down. Especially a good move if your 190+ pounds yourself as the stockers are very light, made for a 150-160 lb Japanese guy. Extra BMW heft carries is gotta count for something. Cutting the springs increases their load bearing.

That and sliding them up in the triple clamps a bit might do the trick.
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Old 03-19-2006, 04:43 PM   #6
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Quote:
Originally Posted by cmwoodys
You could also cut the fork springs down a bit to cheat it down. Especially a good move if your 190+ pounds yourself as the stockers are very light, made for a 150-160 lb Japanese guy. Extra BMW heft carries is gotta count for something. Cutting the springs increases their load bearing.

That and sliding them up in the triple clamps a bit might do the trick.
I was 190 lbs about 10 yrs ago Cutting the springs sounds like a good idea.

Aaron
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Old 03-19-2006, 04:47 PM   #7
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Here's some interesting reading on the subject
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Old 03-19-2006, 05:00 PM   #8
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Thanks man!
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Old 03-19-2006, 08:08 PM   #9
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Have you tried to install the triple clamp yet? I briefly tried to install a 1990 KTM 500 fork to my 88 R100GS. The triple clamp is about an inch too long.

Once you know that, you start looking at the HPN frames and realize that they added a welded extension to the headtube. That's going to also put more load on the head tube so they added gussets to the whole area up there at the front.

So have your machinist buddy make press-in collars for the upper and lower ends of the tube and get them welded in.

BTW: did you ever sell that ugly-ass Adventure
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Old 03-19-2006, 08:24 PM   #10
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Quote:
Originally Posted by GregCifu
Have you tried to install the triple clamp yet? I briefly tried to install a 1990 KTM 500 fork to my 88 R100GS. The triple clamp is about an inch too long.

Once you know that, you start looking at the HPN frames and realize that they added a welded extension to the headtube. That's going to also put more load on the head tube so they added gussets to the whole area up there at the front.

So have your machinist buddy make press-in collars for the upper and lower ends of the tube and get them welded in.

BTW: did you ever sell that ugly-ass Adventure
I've been eye balling the fork all day and it looks like it will work. My buddy has a sweet lathe and a nice big Mill, so we can make it work I'm sure.

Looks like the ADV is staying by the way
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Old 03-19-2006, 09:11 PM   #11
tommcbride
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What is that contraption on your wall in the photo?

Please pardon my hijacking your thread Crankshaft, but what is it?
Thanks,

Curious Tom
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Old 03-19-2006, 09:15 PM   #12
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Quote:
Originally Posted by GregCifu
Have you tried to install the triple clamp yet? I briefly tried to install a 1990 KTM 500 fork to my 88 R100GS. The triple clamp is about an inch too long.

Once you know that, you start looking at the HPN frames and realize that they added a welded extension to the headtube. That's going to also put more load on the head tube so they added gussets to the whole area up there at the front.

So have your machinist buddy make press-in collars for the upper and lower ends of the tube and get them welded in.

BTW: did you ever sell that ugly-ass Adventure
Greg


You could just press the stem out of the KTM triple clamps. Machine a new stem the same dimensions as you BMW stem and press it into the KTM Triple Clamps.

Thats what I intend to do with 50mm WP extreme KTM front end into my R80G/S.

a lot easier than have your machinist buddy make press-in collars for the upper and lower ends of the tube and get them welded in.

Pezz :)
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Old 03-19-2006, 09:16 PM   #13
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Quote:
Originally Posted by crankshaft
Looks like the ADV is staying by the way


Damn nice looking bike. I still have to fix the engine in mine. It does look nice sittin' here in the kitchen but warm weather is coming and Death Valley is calling.
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Old 03-19-2006, 09:48 PM   #14
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Quote:
Originally Posted by Pezz_gs
You could just press the stem out of the KTM triple clamps. Machine a new stem the same dimensions as you BMW stem and press it into the KTM Triple Clamps.
Yes, except that the triple clamp would then be gripping the tubes 1" closer together. And that would put more load on the triple clamp.

I thought about both solutions but I figured the HPN guys had to have a good reason for extending the frame rather than shortening the stem.
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Old 03-19-2006, 10:05 PM   #15
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Quote:
Originally Posted by GregCifu
Yes, except that the triple clamp would then be gripping the tubes 1" closer together. And that would put more load on the triple clamp.

I thought about both solutions but I figured the HPN guys had to have a good reason for extending the frame rather than shortening the stem.
Yes, they have done it that way.

I will go with the shorter Stem. The increased load on the steering head would be minimal.

I am also using the older twin shock swingarm extended 100mm.

To each their own

Pezz :)
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