Vintage BMW Motorcycle Owners

For a short period of time, I tried a 3.50 on the rear of my R69S. I don't have a lot in my notes, but changed to the 4.00. Likely did that because of the service bulletin...maybe even saw it in Roland's book. I must have a pansy ass because I saw no difference in handling, performance, etc. I agree that the frame geometry didn't change before or after the service bulletin VINs so one would think that the larger tire would raise the rear end slightly. But really how much? I suppose one could measure unmounted 3.50 and 4.00 and see if the radius was that much different. Maybe do the same with mounted tires. Don't even consider trying measurements between different tire brands...just because it says 3.50 doesn't make two tires identical.

Your '66 model would have been before the taller rear end. The 4.00 is slightly wider so I suppose there are some "issues" with rolling the tire to the side. I thought usually that was more noticeable with front tires and the forces it takes to turn the bars versus the contact patch. The rears seem like they come along for the ride. The profile is symmetric which too means less of an impact with forces to lean over. For a metric tire, that's a different story.

jwonder wrote: ↑

Tue Aug 16, 2022 5:39 pm

Kurt, I just read the service bulletins and it is very questionable on why they made the change. One bulletin says they are for sidecar and the other says it's for "higher speed". Read the bulletins below.

See:

https://vintagebmw.org/library_data/249.pdf
https://vintagebmw.org/library_data/261.pdf
https://vintagebmw.org/library_data/243.pdf
https://vintagebmw.org/library_data/267.pdf

The first bulletin (249) doesn't really say much yeah-nay about performance.

Bulletin 261 speaks to 2-ply vs 4-ply.

Bulletin 243 just mentions the VIN change over.

Bulletin 267, the one I referenced, discusses the optimization of power and resistances to motion...yes, does mention higher speed issues. It also mentions an increase in ground adhesion...probably related to the bigger contact patch.

I guess if something feels better to you, then go for it. Most people don't have the ability to try a tire, go for a ride, come back and change to a new tire, and repeat the ride. I sure don't!

I have always heard same size 3.5 front and rear for "neutral" steering on earles models, the US versions specified same front 3.5 but 4.0 for the rear.
S

jwonder wrote: ↑

Tue Aug 16, 2022 4:54 pm

Same size front and rear is a must. Heidenau K34 or K36.

Heidenau Vintage tires for me too.
Same size on both.

Avon RoadRiders. Great handling. Same size front & rear per Vech's advice - who I bought them from.

Do they come in 3.50 x 18?

I just have to jump in
I echo 808Airhead and jeffrey
I tried Heidenau and Metzler on my r60/2. They look period, and are good for riding in a straight line around town.
But for anything else, a modern tire makes a world of difference.
I got two Avon Tire Roadrider MKII Front/Rear Tire (100/90-18 56V) for $128 each. It is like night and day. The bike is on rails.
Same tire, front and rear.
The 100/90 corresponds to 3.50
This size fits good, no rubbing of fenders, and easy to remove rear.
They make an H rated (130mph) and V rated (149mph), but it doesn't make a difference for our bikes.

bellsonracing wrote: ↑

Tue Aug 23, 2022 8:33 pm

The 100/90 corresponds to 3.50

Sort of! It is supposedly 100mm with which is close to 4.00 inches. Plus the tire is lower profile, ie 90% of the width represents the height. 3.50x18 tire is a "perfectly" round profile. YMMV!

Modern tire

Yes,the metric tire is lower profile slightly. Definitely enough to feel a difference in the gearing,at least for me. I have metric tire on my R69S and it revs up a little higher than before at constant speed. I love the tire on there, rides great,but will be looking for the modern profile in 4.00 X 18. for the next round. Bellsonracing, glad to hear you are enjoying the better handling!