1978 R100S rear tire size

[germancarguy]

My 1978 R100s came with a fresh set of Bridgestone Battlax BT46 on its snowflake wheels. The rear is a 120/90 X 18 and when I tighten up the axle the tire rubs the drive shaft housing. (not by much). Also I was surprised that the caliper mount does not float and it is part of the axle stack so it will not allow the caliper to align. Thoughts ??

Thanks
Paul

[schrader7032]

The 1978 model should use the inch-sized tires all around. That would be 4.00x18 in the rear. 120 metric is just too big.

[srankin]

I don't know anything about Brigstone battleax tires except what I see in various tire threads. 120 is too large.
My current project bike is fitted with a 110 90 18 tire. Over the years of running Continental or Dunlop tires I may have used that size on my 78RS and 84 RT. The past few years I have been running Metzler Roadtec 01 tire in 400 100 18 size with no noticeable difference in handling, I just get better mileage out of the Metzler tires.

There have been hundreds of tire threads in various forums touting the pros and cons of metric size versus inch size. Also there has been mention of the Battle ax tires. I have no opinion of then as I have never used them.

I am not quite understanding your question about the rear caliper? LOL, then again, I am the wrong person to ask about the disc brake set up on these bikes as I HATE it.

Good luck with fitting a proper tire, St.

[NMBeemer]

My 1978 R100s came with a fresh set of Bridgestone Battlax BT46 on its snowflake wheels. The rear is a 120/90 X 18 and when I tighten up the axle the tire rubs the drive shaft housing. (not by much). Also I was surprised that the caliper mount does not float and it is part of the axle stack so it will not allow the caliper to align. Thoughts ??

Thanks
Paul

Paul,

First - I have Bridgestone Battlax BT46s in 3.25-19 and 400-18 on my snowflakes. The rear is v-e-r-y close installing it: I have to reduce the pressure to ~25 psi, then wiggle-wiggle and on it goes. Once in place, there's no rubbing, though it is close--but no problem in operation.

Second - I found that I needed to insert a washer on each retention bolt between the caliper and mount to avoid dragging. Technically, this shouldn't happen as the pucks should retract away from the rotor evenly--that is, not in relation to their respective bores but in relation to the rotor surface.

Regardless, washers solved the issue simply and inexpensively. They were standard-thickness washers: i.e., not 'wave' washers or other thin ones. So, maybe 1/16" or something like that.

Hope that helps,

Joel.

[chrishea]

I have had a few twin-shock bikes. I also recommend “inch” sized tires on the twin shock bikes (except for the R45/65) this size is a 3.25-19 front and a 4.00-18 rear. There is a 1.5 mm wider spacer (36 31 2 301 737) that can be installed under the right hand wheel bearing seal that offsets the wheel slightly to the left, moving the tire away from the swingarm. Many bikes already have that installed.

I don’t understand why people complain so much about the rear disk brake. There are linkages on the right side than can be adjusted to improve the feel and pedal height. To bleed the brake, the caliper does need to be removed to have the bleed screw higher. The casting is sandwiched between the axle, I’m not sure I see a reason for it to float. The wheel, disk and caliper remain in the same relationship to each other as the suspension moves.

[germancarguy]

I have ordered the correct 4.00 X 18 tire and will replace the rear brake pads then give the rear brake a proper bleed and see how it works out. A thought about the spacer for a wider tire, wouldn’t that reduce the drive spline contact?

[schrader7032]

I wouldn't worry about the spacer. I've never had one on my R100/7. It's typically only needed if you want to run oversized tires...which you should do anyway!!

[srankin]

As Chris mentioned the caliper has to be off the bike and tipped so the air can escape the bleeder. A block of wood between the pads works to replace the rotor. St.

[germancarguy]

I want to thank Chris for the pointer on BMW wider spacer to help with the rubbing of the rear tire (36 31 2 301 737). I ordered and received a new 4.00-18 tire from Amazon and decided to return it, they processed the return and told me to keep the tire. Its only 17 degrees here this morning so I have time to sort things out.

Thanks everyone for the advice
Paul

[Werner1111]

What would happen if one were to adjust the pivot pins for the rear swingarm so that the swingarm were slightly offset to one side? I'm not suggesting doing this, but perhaps it's doable. I've never tried it.

A 1978 R100S was my first BMW motorcycle, back in 1993, and I still have it. Back then, in college, I wanted a bigger rear tire, but quickly ran into this issue with the swingarm.

Glad you're finding your way forward.