Rim size vs tire size and /5 SWB fitting??

[jjwithers]

I'm going to lace some Akront rims to some BMW hubs (for two SWB bikes).

I have two different size rear rims and the same 2 fronts.

My front rims are 19 x 1.85 (same as a stock BMW).
I know a typical front tire size is 100 / 90 -19.

I have two rear rims,
one is the stock size 18 x 2.85
and the other is 18 x 2.50

I currently have a 120/90-18 tire on the rear wheel of a stock rim.
Will that same size tire fit on the 2.50 rim?

Does anybody have any ideas how wide of a tire is possible before it starts getting too close to the swingarm?
Does anybody run a 130 sized tire on their rear wheel?

And what about different aspect ratio sizes? 90 seems like it is the maximum size for both the front and rear tire availability. Is it just stupid to run a tire that has a lower profile?

I posted this in Adv rider and didn't get much of a response. Hopefully you guys can help me out. I'm trying to decide which rim will go on which bike and plan on taking them to get laced this weekend.
Thanks in advance.
-Josh

[Kurt in S.A.]

Why do you want to do something like this?? IMO, going tires this wide only has downsides. It's harder to get off and on, changes the overall geometry, lowers the bike making the sidestand and centerstand harder to use, ruins the handling. Personally, I see no benefit going down this path.

Kurt in S.A.

[jjwithers]

I'm customizing these bikes as much as i can. I'm not a BMW purist. I have a stock-like R100S and i'm having fun rebuilding a /5 that would have been sent to the crusher if i didnt recsue it and give it a new lease on life.
I know stock BMWs are built thoughtfully but i want to have some fun with modifications. Both for style and performance.

All i am really talking about here are rims though. There is nothing wrong with a different set of rims on a BMW.

What i want to know is will the same tire ft a 2.50 and a 2.15 rim?
And will it effect the width for clearance issues?

I'm just curious about the different profiles...

[Duane Ausherman]

BMw used two rim widths on the rear of the SWB.

[Major Softie]

Kurt's question has nothing to do with OEM purism, just practical performance and fit.

2.50 is listed as the absolute minimum for a 130/90-18 in the places I can find, so it is definitely not near the optimum size, but it's not outside manufacturer's acceptable range for at least some 130's. But, many 120's won't fit without hitting the swingarm, so finding a 130/90-18 that will clear is going to be quite the crapshoot.

In your original post, you have listed your rims as a stock 2.85 (wrong) and the 2.50. Stock rims were 1.85 and/or 2.15's. I don't know which digit you got wrong, but a 1.85 is NOT considered acceptable for even a 120/90. 2.15 is the bare minimum for some 120 tire models, but, again, nowhere near optimum. Neither stock rim is close to being wide enough for a 130.

I don't know why you want to run the 120, but if I was going to do it, I'd only run it on the 2.50 rim myself, and I'd certainly never consider running a 130 on either a 1.85 or a 2.15 stock size rim.

P.S. The "recommended" widths I'm finding are 2.75 for a 120 and 3.00 for a 130.

[jjwithers]

BMw used two rim widths on the rear of the SWB.

Were they 1.85 and/or 2.15 in the rear and
1.85 in the front?

__________________
And Thanks Major,

Yes, i hit a typo.
The rear is 2.15 (not sure how i screwed that up) x 18.
Sorry to confuse things.
I am currently running a 120/90-18 tire on a stock rear rim. It was the tire size my local BMW mechanic put on. I never specified a size at the time... all i knew was that i needed new tires and I chose the tread, and he chose the size.

What size tire would you consider the optimum size for a 18 x 2.15 rim?

Being that i am currently running a 120 on a 2.15 rim, i am assuming it should be OK to run a 120 on a 2.50 rim with the same clearance near the swingarm?

I just want to make sure i can run the 2.50 rim in the rear, and have a tire that will work (both for performance and fitment).

-josh

[Duane Ausherman]

Yes, MS got the sizes right. If this is art, then do as you wish. If you are going to ride it and know how bikes should handle, then expect to find it quite changed. The SWB /5 is quite sensitive to even small changes. Even using a different tire of the same size makes quite a change. BMW was very particular about the tires on that model when asked about it, as was Conti.

[Major Softie]

What size tire would you consider the optimum size for a 18 x 2.15 rim?

Bridgestone says a 4.00 - 18 or smaller is about all that qualifies for having a 2.15 as the recommended rim size.

Being that i am currently running a 120 on a 2.15 rim, i am assuming it should be OK to run a 120 on a 2.50 rim with the same clearance near the swingarm?

Nope. On the wider rim (which is actually a more correct size rim for the tire) the tire will be a little wider, so just because it fits in the swingarm on the 2.15, that doesn't mean it's going to on the 2.50. On the 2.15 rim, the tire is actually being pinched narrower at the bead than it is designed to run, and that's providing some of your swingarm clearance.

I just want to make sure i can run the 2.50 rim in the rear, and have a tire that will work (both for performance and fitment).

Why? If it's to run wider tires, it may or may not work out. If it's just that you have one of those rims and would like to get some use out of it, it will definitely work on the bike, but you might not be able to run wider than a 110 on it - you won't know for sure until you try. It will also vary depending upon tire model: every different model 120 is actually a little different in width. On the manufacturer's web page, you can usually find the "actual" width. Keep in mind that this width is usually measured on the recommended rim size for the tire, and will change a little on other width rims.

Also, we've only been talking about rim and tire width, but you haven't laced these rims yet, so offset is another factor in the formula.

[jjwithers]

OK. So it sounds like a 110 tire on the 2.50 rim is a safe bet. That answers my question.

Regarding offset,
I was under the impression that the offset pertained to the hub and rim only. You'd want the middle of the rim's width to be centered with the bike.
Are you saying that the offset would vary due to what tire you use?
or just to make sure that the rim is laced with the proper offset when assembling? In other words, you would have to compensate for a wider rim when lacing to maintain the proper offset...

[Duane Ausherman]

Before you cut the old spokes, install the wheel, with or without a tire. Measure to each side of the swing arm. This will tell you just how centered it is. If you need to move it over a bit, calculate it.

Then lay the wheel down on a flat surface. If it is a drum brake, it is very easy. Have the drum side down. Now find 3 nuts that will just fit under the rim. The thickness of the nuts gives you the current off-set. Now calculate in any correction needed for the wider tire.

Now find 3 nuts of the off-set you desire. Hold the rim off the flat surface to give you the off-set. Lace the spokes in by hand and a bit at a time bring the nipples up to snug. The rim is now laced to the correct off-set that you desire. Put it in your tuning jig and finish up. As long as you do equal tightening to each nipple, the off-set won't change.

I have done many dozens this way and it is much faster than using the BMW off-set tool.