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Old 03-27-2011, 03:54 PM   #1
iansanderson OP
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What tires did a '78 R100 roll out of the factory on?

It's time for new tires on the R100. If I remember correctly, my dad (the previous owner) put on an oversized rear tire. That's great and all, but I want 100% factory size when it comes to tires and tubes. The problem is everything I find in the manual is in inches, but shopping online, everything is metric.

I can't seem to figure out the conversion, so any help would be appreciated. Any brand recommendations?

My bike: 1978 R100/7T. Build date 10/77

Wheels: Stock spoked 1.85X19 fronts and 2.15X18 rear.

Tubes: Unknown?



Here's what the manual says, but, of course, it's in inches again! argh..

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Old 03-27-2011, 04:11 PM   #2
ozmoses
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I switched to Avon Road-Runners/Riders at first for the proper sizing, stayed w/ them for the ride.
YMMV, of course-sorta like oil...
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Old 03-27-2011, 05:05 PM   #3
Lornce
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Bridgestone S-11 Spitfires are a great all round tire that stick well, last respectably, offer good ride quality and steer nicely on an airhead. They're cheap, too. Whatever.

You need a 100/90-19" front and a 110/80-18" rear.


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Old 03-27-2011, 05:14 PM   #4
lkchris
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You're just not shopping hard enough, as there are still inch-size tires available in the correct sizes.

Tubes must be fitted.

BTW, there are ZERO metric equivalents to these sizes.
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Old 03-27-2011, 05:18 PM   #5
bikerfish
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yep, I agree with what lornce says, those bridgestones are great tires for the money, and those are the proper sizes. lots of folks put a 120 in the rear, but you have to let the air out of it to get it between the swingarm and the brake shoes, the 110 will slip through with a minimal of fuss.
trust me, NONE of those bikes will overpower a 110 rear. I thinks its the "big rear tire syndrome", makes folks think they can ride faster.
if you hunt around, you can still find the original equipment style continentals or dunlops, in the proper inch sizes, but they are getting harder and harder to get, and you'll probably pay dearly for them.
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Old 03-27-2011, 06:12 PM   #6
supershaft
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I remember most of them coming with Continentals and some with Metzlers. I don't remember them coming with Dunlops right off hand.

Most conversion charts convert a 3.25 to a 90/90 and a 4.00 to a 110/90. I don't think a S11 comes in 90/90. 90/90 and 110/90 are usually a little smaller than 3.25 and 4.00 and 100/90 and 120/90 are usually a little larger but all that ultimately depends on the specific tires in question since they all vary in size.
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Old 03-27-2011, 06:17 PM   #7
bikerfish
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they do come in a 90/90, I have one in the garage as I type this.
my bad, your right, some did come with metzlers, not dunlops.
I just always order the 100 for the front and the 110 for the rear, been using that combo for years and have no problems. bike is ridden two up and loaded most of the time.
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Old 03-27-2011, 07:20 PM   #8
Cogswell
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Quote:
Originally Posted by Lornce View Post
Bridgestone S-11 Spitfires are a great all round tire that stick well, last respectably, offer good ride quality and steer nicely on an airhead. They're cheap, too. Whatever.

You need a 100/90-19" front and a 110/80-18" rear.


I just spooned a set of those on my airhead last weekend.

Good to hear Lornce


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Old 03-27-2011, 08:25 PM   #9
Tosh Togo
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Quote:
Originally Posted by lkchris View Post
You're just not shopping hard enough, as there are still inch-size tires available in the correct sizes.

Tubes must be fitted.

BTW, there are ZERO metric equivalents to these sizes.

That may be a bit of an overstatement, since the tire's listed size, whether it's metric or imperial, isn't exactly what the tire ends up being in terms of width after being mounted. A 4.00 tire will be close to its' metric equivalent, but it's probably not exactly 101.6 mm wide. It's close enough, and your implication that there are "ZERO" metric equivalents is a bit misleading. In addition, each tire has a range of approved widths, and that will have a little bit to do with the final mounted dimensions.

The tire sizes themselves are nominal, ie: the size on the sidewall is a group size, and the tire fits somewhere within that group's limits. It may be at the high end of spec, (wider/taller), possibly dead-on in the middle of the range, or it may be at the small end...but it's still the same listed size on the tire sidewall. Whether it's sized in millimeters or inches, all that really matters is- does the tire fit your wheels, and does the rider like it?.

Getting back to the OP's question, I have a bike of the same era, with the same skinny rim sizes. Spec tires are a 3.25X19" front & 120/18" rear, and the few current 120's that I've tried are just too wide. The manufacturer's tire spec pages also seem to agree, as the smallest approved rim with for a modern bias-ply 120 is a 2.5" rim.

My best combo so far in terms of overall ride & handling was a 100 or 3.25 front, and a 110 or 4.00 rear Metzler Lazertech pair. Downsides to that approach is that it's an expensive choice, so I've switched to a BT45 rear in the same 4.00 inch size and kept the ME33 front. The Bridgestone rear is ~$35 cheaper, handles just as well, but it's noticeably smaller in diameter than the Metzler, to the tune of about 200 rpm at freeway speeds. The pegs also touch down a wee bit sooner.

I tried a BT45 front once, and didn't like it one bit. Your results may be different, so feel free to try one, as it's about $30 bucks cheaper than the Metzler.

I've got a set of Michelin Pilot Activs, 3.25H19 front and 4.00H18 rear; they're going on next, and fwiw the rear Michelin is noticeably both wider and taller than the Metzler rear. It's design rim width is a 2.15, and it was $172.98 for the pair, delivered.

Whatever you choose to go with, get the "H" speed-rating flavor instead of the "V" variety, and you'll save about $10 up front per tire as well as lose a bit of unsprung weight. My guess is that the lower speed-rating's lighter tire might also be a bit more compliant, but the money difference made the choice for me. I only get about 3.5K miles to each set, and that adds up in a hurry.

Have fun, and when in doubt, skinnier's better.
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Old 03-27-2011, 10:27 PM   #10
supershaft
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Where does BMW spec the rear tire at 120/90? I have only seen 4.00. Most 120's (probably not BT45's since they run pretty small) are too wide with the stock axle spacer.

I find that usually the 90/90's handle better than the 100/90's. Many a conversion chart recommends 90/90's for our bikes. Speaking of approved rim sizes for 120's. Most 100/90 front tires are not approved for 1.85 rims, just 90/90's. It seem like a lot of people make fun of some running a fat 120 rear tire when they themselves are often running a fat front tire? What gives?

I am curious why you think V rated tires are heavier?
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Old 03-28-2011, 05:25 AM   #11
2xdisco
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i have spitfires on my rs, 100/90-19 front and 110/90-18 rear. they fit well and one can remove the rear wheel from the swingarm.. you can find 4.00in in most manufactures line up for bias tires. the 120/80-18 or 120/90-18 is too big for the dual spring bikes. it is however spec for the single side swing arm bikes, like the g/s and 88 rs/rt
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Old 03-28-2011, 05:45 AM   #12
bikerfish
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just based on looks, the 100 front looks closer to the original size when mounted than the 90, the 90 fronts just seem very small, but maybe that's me. you will get quicker steering with a 90 front.
perhaps it all depends on the brand you choose.
don't make fun of my fat front and skinny rear!!LOL!
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Old 03-28-2011, 05:58 AM   #13
jtwind
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It likely came with continentals, something like a RB2 front and K112 rear. You can still buy conti twins in the inch sizes, I run them on my /5, and buy them from American Motorcycle tire or southwest moto tire.
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Old 03-28-2011, 07:30 AM   #14
jdmetzger
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Quote:
Originally Posted by jtwind View Post
It likely came with continentals, something like a RB2 front and K112 rear. You can still buy conti twins in the inch sizes, I run them on my /5, and buy them from American Motorcycle tire or southwest moto tire.
I agree - Conti's seemed to be the standard for most US bikes. Like many others, I run the S11's on my R80/7. When I first got the bike it had some sort of Conti on the rear that was a 120. Even with the air out, I had quite a time getting the thing off the bike. The 110 rubs on install, but not much, and clears everything just fine after that.

I once considered the BT45's (looked to be the next up the chart for "stickiness") but after running the S11 for a while, I don't see the use. I can drag hard parts on the R80 with S11's so traction isn't an issue, and I get much better mileage than I would on the BT45's.
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Old 03-28-2011, 07:40 AM   #15
Jasper ST4
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The Conti twins were the most common here (RB2 & K112) and like a lot of folks I went over sized. The 120 is a bitch to install past a rear brake drum but I recently went back to the stock size and noticed better handling. Looks like bicycles tires though but she's definitely old school at this point.

Those RB2s didn't like the rain grooves in California and I was glad to get away from both of those.
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