Evidence Frame May Be Bent

There is significant damage to the front top center panel that goes around the headlight shell and the top fairing bracket tab that is bolted to the steering stem is sheared off. That makes me wonder if the frame is bent.

There are ways to check if a frame is straight that do not require a jig. Here is a good reference.

• BMW motorcycle frame alignment tests-Duane Ausherman

I decided to call a friend of mine who retired after being a BMW mechanic for many years to find out how he checks a frame for being bent. He offered to let me borrow a frame jig that he has. Cool beans, access to special BMW tools. 🙂

Frame Jig

I picked up the frame jig which consists of four parts: a steel frame, a steering stem steel rod with a locking nut, and two locating pins with graduated markings that fit into the rear of the jig and align with the swing arm bushings.

Font End Removal Before Installing Frame Jig

I need to remove the front end of the bike. I previously removed the body panels, wheels and fenders for repair and painting and you can read how I did that here.

• 46 BMW 1977 R100RS Remove, Disassemble Bodywork

With the body panels removed, I remove the brake calipers, the Telefix fork brace and fender bracket, the lower fork sliders, the control perches, the handlebar grips with their wiring for the heating elements, the handlebars and mounting brackets, the steering damper, the top clamp and finally, the steering stem. This work is similar to what I did on the R75/5 tear down and when I replaced the fork seals on the 1983 R100RS forks. You can see how I did this work here:

• 1973 BMW R75/5: Remove The Handlebars, Controls and Fork Tubes
• 1983 BMW R100RS: Remove Front Wheel
• 1983 BMW R100RS: Remove Fork Brace and Fender
• 1983 BMW R100RS: Remove Fork Gaiters
• 1983 BMW R100RS: Remove Handlebars and Fork Springs

Here are some details of this work on the 1977 R100RS.

Remove Disk Brake Calipers

The front calipers on the 1977 R100RS are ATE units and are secured with an eccentric pin. I remove the rubber cap and the nut with enclosed spring to expose the eccentric pin. I use an 8 mm bolt and catch a couple threads inside the eccentric pin so I can pull on the pin while wiggling the caliper back and forth to free the pin.

Remove Fork Brace

The fork brace on the 1977 R100RS is a Telefix. It’s secured by several bolts. Then the stock fender brace is removed from the fork sliders which frees the lower brake line brackets from the lower fork sliders.

Remove Lower Fork Sliders

The lower fork sliders on the 1977 R100RS attach the same way as the R75/5 sliders. The damper rod secures the lower fork slider with a 10 mm nut on the end of the damper rod. I like to use my cordless impact driver to remove the nuts.

Remove Heated Grips

Another difference is in removing the handlebar grips since this 1977 R100RS has heated grips. My handlebar on/off switch was relocated to the lower left sub-frame mounting bolt and a splice was placed in the wiring to that switch. I had to cut the wires of the splice so I could remove the wires with the handlebar grips as the wires are routed inside the handlebar.

Remove Handlebars

As shown in the R75 write-up, I remove the handlebar mounts, the instrument bracket and the handlebars.

This exposes the top steering stem nut. I remove it and the top plate.

Remove Steering Damper and Steering Stem

The R100RS has a hydraulic steering damper. The damper is attached to two pins–one on a bracket mounted to the frame and the other on a plate mounted to the bottom of the lower triple clamp–with a wire clip to secure each end of the damper to a pin. Once I remove the wire clips I pull the damper off the pins. I remove the two Allan head screws that secure the damper adjustment housing on the bottom of the bottom triple clamp and remove the cover with the internal rack gear, locating pins and springs. Be careful as the springs can sprong quite a distance if the pins come out of the bracket.

I use a plastic hammer to drive the steering stem down and out of the steering head. I support the lower triple clamp so it doesn’t crash to the floor when the steering stem comes loose.

Install Frame Jig & Measure Straightness

After I remove the steering stem, I remove the carburetors so the legs of the frame jig can fit next to the swing arm pivots.

Now I can install the front end of the jig in the steering head.

I secure the jig to the steering head by tightening the round nut on the bottom of the rod to snug the jig tightly to the steering head. I check that the jig is solidly mounted in the steering head and does not move when I push on the jig.

I insert the two swing arm locating pins into the holes at the rear of the jig next to the swing arm bushings.

I orient the right pin so it fits inside the swing arm bushing and is centered.

Then I adjust the right and left pins so that the distance on the graduations on the pins is equal distance from the inside of the jig leg.

When I am sure the two pins are at the same distance from the inside of the jig frame, I look at the left pin to see if it is exactly in the center of the left swing arm bushing. It’s about 2-4 mm off center toward the rear.

This is a minor deviation and does not indicate that the steering stem nor frame spine tube are bent. GOOD NEWS!!! 🙂