Motorcycle Tire Sizes
by Rick K. for webBikeWorld.com
Motorcycle Tire Sizes: Fitting Wider Tires and Tire Sizes for the BMW R65 and BMW Airheads
NOTE: See the updated information at the end of this article regarding the tire sizes for the 1986 BMW R65 (and possibly other Airheads!)
It won't fit... or will it?
My 1986 BMW R65 was ready for new skins. This is the time when every owner probably thinks the same thoughts -- is there a better tire, brand or size of motorcycle tire that will transform my bike into a MotoGP-style handling machine?
there can be some confusion over motorcycle tire sizing, especially when it
comes to fitment for older bikes. But I was surprised at what I
Not to worry, because the 100-series front is a common retrofit for R65's; I guess it gives a tiny bit more contact patch.
I've heard that the 100-series can slow down the steering a bit, but I hadn't noticed - the bike, with its low bars and the 100 front tire feels like it steers pretty quick to me. And the ME33 was worn nice and even, without any of the cupping that can be a problem with this particular tire.
So I figured a 100-series would make a fine replacement. A 90 just sounds so skinny in these days of 120 fronts and 180 rears!
Note that there's not necessarily a free lunch here -- wider tires can mean that other tire dimensions will change, sometimes not for the best. In the old days, a tire was a tire and the sidewalls were about as tall as the tire was wide. The term "aspect ratio" was unknown. But today, a 90/90-18 means that the tire has an aspect ratio of 90%.
That is, if the tire is (nominally) 90 mm wide, and the aspect ratio is 90%, then the tire is 90% of 90 mm, or 81 mm tall. In other words, the second "90" means that the tire's profile is 90% of its width.
Which means that a 90/90 is a
theoretical 81 mm tall
at the sidewall; a 100/90 is 90 mm tall, and a 110/90 is 99 mm
tall. Here's a table that shows the differences in millimeters and
The problem arises when the tire sidewall heights are dramatically different than stock -- they can start to affect the stock suspension or chassis setup. In this case, if the bike was originally designed for a 90/90 tire, with a sidewall height of 81 mm, changing to a 110/90, even though the width will fit, changes the ride height by raising it a theoretical 18 mm, or .7".
almost 3/4". Is that a dramatic difference? I think
so. This issue can be mitigated somewhat by raising the fork tubes
in the head clamps (or triple-tree). This lowers the front end a
bit, and to a certain extent, can restore the bike to somewhere near factory
However, Speed had a rear but he didn't have the front in stock. He mentioned that a lot of Airheads have been mounting Bridgestone BT45's recently and have been saying good things about them. I know the Metzeler combo works pretty good and is a sure thing, but I'm always willing to try something different. So I figure "why not, I'll go with the Bridgestones".
As we're walking over to Speed's tire racks, he told me that for the BT45, the 110/90-18 front is the correct size, as they run narrow. Was this yet another "urban legend"? Looking at the un-inflated tires on the rack, I could see that the front BT45 did look pretty narrow when compared to other tires of the same size.
So I figured Speed must
know what he's talking about. But just to be on the safe side,
when I got home I did some snooping around on the various tire
manufacturers' websites and with a Metzeler brochure I had picked up,
and found out something pretty interesting.
Even the Metzeler site recommends the 100-series ME33
for the front. But 110? Just looking at the numbers, and thinking
about the warnings I've heard, you'd think you were committing a sin or
something. Common wisdom seems to think - as I did - that a
100-series is a 100-series. But guess what? It ain't!
But Bridgestone does, and here's what I found: The BT45 100/90-18 (V-series) is listed at 3.9" wide. That's interesting, because the Bridgestone Spitfire 100-series is listed at 4.2" wide.
Checking the other sites, I found that the Dunlop K491 100-series is
listed at 4.06" wide; the GT501 is claimed to be 4.15" wide; and just for comparison's
sake, the 100-series Metzeler ME88 and the 880 both check in at
Am I the only one who finds it
interesting that a 100-series tire can really be anything from 3.9"
to 4.2"? And a 110-series can end up being the same width as a 100?
The implications of this are
that you may have more tire choices for older Airheads than you thought
there were. But make sure you take into consideration the aspect
ratio problems as described above -- the height of the tire may be
more of a limiting factor than the width.
Update: An email to Speed confirmed that he had the tires mounted and balanced right on schedule. I had also requested that he replace the front wheel bearings, which was done. After I got the wheels home, I took the opportunity to clean them up a bit before I mounted them. I mounted the rear first with no problems. But I found that I had a bit of trouble mounting the front.
The 110 mm wide front meant that I now had to take off
the single front brake caliper to get the tire on, which meant a bit of
extra work (the old 100 mm wide Metzeler came off without having to
remove the caliper). But everything finally mounted up and went on
ok -- and I was surprised to see that the front tire could spin around
with no clearance problems!
This was a serious problem on the K75, which normally feels top heavy and like it wants to "fall in" to the turn, because after the initial turn-in, the bike would feel like it was falling over, and you would always have to make corrections to keep the bike from leaning over farther than you intended.
This means that you're always having to concentrate on
corrections in mid-turn, which is both dangerous and exhausting.
On the R65, the ME33 gave the same feeling, but it wasn't as pronounced,
so I figured I could live with it. I guess some owners think this
makes the bike feel lively; I think it feels scary.
I can feel the bumps a bit more, probably because of the lower sidewall with less flex, but it's never a problem, and actually makes the bike feel more responsive. As far as the fit goes, it actually went on better than either the 100/90-18 Metzeler or the 110/90-18 Bridgestone. Highly recommended! I've finally found the perfect set of tires for the R65!
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