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Old 03-11-2011, 09:25 AM   #1
russbryant OP
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R100GS frame - How tp measure for straightness?

I'm a few weeks away from getting my R100 GSPD back from the insurance company and I'm a long way from riding again so I need something to keep me busy.

It is likely to have a bent frame although I don't know for sure yet. I was on some good pain meds the last time I saw it The forks and handlebars are bent. It basicly needs a front end at the least. If the frame is bent, I may go the straightening/gusseting route with it.

Without tearing it all apart, are there any measurement I can take to figure out if it's tweaked?

Thanks,
Russ
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Old 03-11-2011, 09:52 AM   #2
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BMW makes a frame jig for checking frames. Man I wish I had one! Apparently, the manuals are confusing to some about using the jig. Make sure they know how to use it. The forks and the carbs have to come off.
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Old 03-11-2011, 11:36 AM   #3
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There's got to be some DIY way to check it using laser levels and such. One of you smart guys figure it out!!

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Old 03-11-2011, 12:58 PM   #4
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Dave at Recommended Service has one...checked a few frames for me

Quote:
Originally Posted by supershaft View Post
BMW makes a frame jig for checking frames. Man I wish I had one! Apparently, the manuals are confusing to some about using the jig. Make sure they know how to use it. The forks and the carbs have to come off.
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Old 03-11-2011, 02:06 PM   #5
Rob Farmer
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Loads of places in the UK offer the service. There should be lots in the US?

http://motorcycle-frame-straightenin...aightening.asp

BMW stuff here http://w6rec.com/duane/bmw/frame/index.htm
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Old 03-11-2011, 05:06 PM   #6
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I cant recall but maybe put a straight edge on the steering neck gussets, I dont think they have any radius to them, intentionally. Good luck, and heal up quick, Russ.
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Old 03-11-2011, 11:23 PM   #7
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Check the steering head gussets for flat - if not, the frame could be bent. Another place to look is the backbone - it should be straight, not humped.

Before the 100GS, a wreck would often bend the forks but not the frame. The GS forks are so much stronger that they would possibly not be bent, but the frame would be.

I'm guessing that if the forks bent, then it's highly likely the frame is also.
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Old 03-12-2011, 05:30 AM   #8
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All of the above are good checks.

Here's another test you can perform before spending money on jigs or professional frame straightening services.

It measures the overall picture so if your forks are bent it won't reveal much until you install known straight units.

The BMW frame jig doesn't reveal the whole picture without use of the wheel jig measuing offset. A straight set of forks are required to get the full picture on the frame condition with the wheel unit as well.

Set the bike up on the centre stand and chock the wheels so the are both resting on a surface with enough tension so that the front wheel will retain whatever position you point it in.

Place a spriit level on the rear hub and place wedges under the centre stand until the rear wheel is vertical.

Move the level to the front disc. Position the steering left or right until the front disc is vertical.

Run string around the back wheel and lines forward through the centre stand past the front wheel a few feet in front of the bike.

Make sure the string is resting on the widest part of the rear tyre, you may need to tape it into position to revent it dropping in between the knobs.

Measure the tyre width at the outer edge with a vernier caliper and lock it.

Find a method of securing the vernier in front of the bike, a couple of bricks works OK, and run the string over the inside arms of the vernier.
This ensures the strings are the same distance apart at the front as they are at the rear.

Keep the strings under light tension.

From the front sighting along the strings ensure the strings do not kink or deflect as they run past the leading edge of the rear tyre. Move the vernier from left to right so that both strings are straight and just touching the leading edge of the rear tyre at an equivalent knob to the ones the string is secured to at the rear. It is not required the string be horizontal. (this is so much easier with road tyres)

The distance of the string to the front tyre may be different each side. This is wheel offsett and whilst not desirable up to 18mm is the tolerance.

The distance between the string and the front tyre at the leading & trailing edges on the left side should be equal to each other as should the leading and trailing distances on the right side

So both wheels vertical both pointing straight ahead and not displaced to the side by more than 18 mm the frame is 'straight' or has a combination of bends and twists that cancecl each other out.

Hope this helps but appart from securing straight forks which it sounds like you will do anyway it cost nothing but time.

Get well soon both you and the bike.
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Old 03-12-2011, 09:53 AM   #9
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Send the frame to http://www.motorcycleframeman.com/ Best place in the US You can also look up Z Motorworks in Tucson Frank Zabriskie. He is a BMW tech.
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Old 03-12-2011, 10:08 AM   #10
supershaft
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The frame jig reveals the condition of the frame. The condition of the forks, swingarm, wheels and axles can be found by other means. I think it is much better to at least first test components separately. As I think your advice points out, measuring even the straightest bike completely assembled can have you scratching your head and wondering if your measurements are accurate. There are quite a few very subjective measurements all going on at once. Too many separate components being measured at once as well IMO.

If you had measured a couple of known straight bikes many times over in order to understand what variances you can expect in your measurements and the bikes, I think it would help considerably in determining the condition of a bike of unknowns. Obviously wacked is one thing and figuring if something is barely off is another IMO.
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Old 03-12-2011, 03:37 PM   #11
russbryant OP
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Thanks for all the replies. I think I'll check the backbone and gussets for sure. If they aren't flat then I'll know for sure the frame is bent.

I know Frank and was going to get in touch with him when I get back in May.

I found this place online and they use a 6 axis CMM system to measure frames.
http://www.framestraightsystem.com

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