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.
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. 🙂
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.
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.
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!!! 🙂
Brook: what would you have done if the frame was bent? Could it be straightened?
The frame on my 1975 R75/6 was bent from hitting the rear of car at about 5 MPH and stayed that way for about 20 years. A long time BMW dealer owner and mechanic, Clem Cykowski, had the jig BMW provides to determine if the frame is bent. The jig is also used when straightening it out. He knew a auto frame repair shop that had the hydraulic press needed to straighten it. He loaned the jig to the shop along with the frame.
I really appreciate your posts and videos on Youtube. Thank you!
I’m working on a R75/5 with double discs. It has the ATE callipers. The eccentric pin on both sides are stuck. I’ve screwed in the M8 bolt and can’t wiggle the callipers to get the pins unstuck.
I don’t dare to screw the M8 all the way in and then trying to twist it loose. Afraid to strip the threads. Iv’e drenched the pins with some rust solvent spray, hoping they will come loose next time I go to the garage.
Do you have any suggestions on what to do if they are still stuck?
All the best from Sweden/
Good Morning Philippe,
The eccentric bolts do get rusty if they are neglected. The caliper is aluminum so it may help if you try heating them with a propane torch. Then apply the solvent and it will get sucked into the space around pins as it cools. Of course, this may ruin the seal around the piston, but it sounds like you are rebuilding these calipers anyway. If the pins are still stuck, repeat this procedure several times and eventually the solvent will penetrate completely around the eccentric pins and that should let you remove them.
I’ve had good luck with Kroil which is a penetrating fluid. I have heard another good penetrating fluid you can make yourself is to mix automatic transmission fluid with acetone in a 50/50 mix.
I hope this helps.
Thank you for your reply.
I finally got the pins loose. Used a heat gun, a lot of penetrating fluid. Gave the head on M8 screw some taps with a hammer and at the same time “counter taps” on top of the caliper with a rubber mallet. Repeated this until I saw the pin move out of its stuck position a little. Last part of the procedure was to screw out tje M8 a bit so I culd fit a wrench between the head an the bottom of the fork. Tightened the M8 so the wrench got stuck. Gave the wrench a few punches with a mandrel until the pin came loose 🙂
Cool beans. I’m glad you got the gremlin to relocate. 🙂
Thank you for your thorough explanation.
Recently I crashed my R100/T into a car. Moderate damage to the front end, includes bent stanchions, broken front rim, bent upper crown 🙁
Trying to figure out a way for checking bent in the frame, for the homey mechanic (who doesn’t have this jig). Any suggestions?
Based on your damage, I would expect the frame is also bent. I don’t know how to measure it without the jig.