1982 R100RS Metric vs Inch Tires

[viejo]

I can almost hear the groans, mine included, that arise from any question regarding tires (we won't even go anywhere near oil)...... but, here goes.

After twenty some great years on a 2003 K1200RS I have finally come back to my senses with a 1982 R100RS. When I took the RS to Martindale Motorcycle Works in Texas for some brake service I listened in on a discussion of "metric" vs "inch tires" and realized I didn't really have a clue regarding the issue. Back when I was first riding Airheads in the mid 70s my thinking was the rims were notated in inches so the tires probably should be as well. Have I missed something/anything in the last fifty years?

Viejo

 

[20774]

Either inch size or metric tires could be installed on rims. Metric or inch refers to the dimension of the rubber. Nominally metric tires have a lower profile than in size tires. If a metric tire is lower in profile it will lower the whole bike closer to the ground and can, although maybe not, affect handling. It certainly makes it more difficult to use the center stand or the side stand.

Your bike more than likely has the snowflake wheels. Then that brings up the big old discussion about tubeless or tube type tires. The snowflakes are not designed to run tubeless, but many seem to like to do it.

 

[chasmrider]

i just bought tires for my 78 R100 RS, got Bridgestone battleax BT 46 3.25x19 front, 4.00x18 rear, and I will be using innertubes. because that's what BMW recommended in 1978. YMMV

 

[PGlaves]

i just bought tires for my 78 R100 RS, got Bridgestone battleax BT 46 3.25x19 front, 4.00x18 rear, and I will be using innertubes. because that's what BMW recommended in 1978. YMMV

They also recommended pulling the transmission every year to lubricate the spline with anti-seize. Then they changed and recommended BMW Red Grease which was Texaco Starplex which melted in a grease gun at room temperatures in the summer.

 

[chasmrider]

They also recommended pulling the transmission every year to lubricate the spline with anti-seize. Then they changed and recommended BMW Red Grease which was Texaco Starplex which melted in a grease gun at room temperatures in the summer.

thanks Paul, blindly following anything, or anyone is probably a bad idea. since the bike is new to me it seems like starting with the factory recommendation is a good idea. I'm not against changing them down the road.

 

[willhawks]

3:25 x 19 Front 4:00 x 18 Rear if memory serves. A relative of mine writes the Tech Talk column for the BMW Car Club of America. In general, his take is that the Factory recommendations for specs, parts and settings etc almost always work. If it came with tubes, I would go with tubes myself. As per Spline lube issues, Tom Cutter of Rubber Chicken Racing is the Guru. Do what he says and you will, not have to worry about it. You should ask him about the circlip issue which may involve your bike. Many good miles to you!!

 

[PGlaves]

I absolutely agree with the use of tubes with stock wheels.

 

[stevenrankin]

My understanding of the snowflake wheel issue is that they did not have a certain portion of the "bead" area of the wheel that is found in tubeless wheels. This could cause the tire to roll off the rim under sudden pressure drop.

Yes, people have been going tubeless on snowflakes for years without problems. I do for the rear wheel but not the front.

Yes, I am gambling I will never have the catastrophic blow out the lack of a tube would have prevented. But, I have had the damned rear wheel punctures. Pulling a wheel off the bike on the side of the road, pulling the tube, replacing the tube (a nail always puts more than one hole in the tube then patches in the repair kit). I am hoping if the next time I get a puncture, I can just plug the hole.

Maybe I am air headed in thinking I will get away with no tubes, that is my choice so far it works, and the worst accidents I have been in (3) have had nothing to do with the wheels and tubeless tires. St.

 

[viejo]

Hola All,

Thanks for the good responses! Inch tires for the 82 R100RS are definitely 3.25/19 & 4.00/18 and I do have snowflake rims so I will most likely go with tubes as per BMW. My next question is- does anyone make a radial tire in these sizes? I've searched all the major tire sites with no success. The Continental RB2/K112 set looks promising (I was very satisfied with Continentals on my '76 R90/6 years ago), but I'm still looking/hopeful for radials in inch tires....... plenty in metric, but I prefer the taller aspect of the OE inch tires.

Viejo

 

[20774]

They do not make metric tires in these specific sizes. To find something close, multiple 3.25 and 4.00 by 25.4 to see what the metric equivalents are. Typically it will be 90/90 for the front and 100/90 for the rear. Some will try to go bigger than that, but you can run into clearance issue if you try to put a 120 on the rear. Also, it's a "myth" to think that a bigger tire is better. It actually hurts the handle as there is some specific geometry at play where the rubber meets the road...it can be slightly harder to "turn in" with a larger tire. To my butt in the seat, I can't say as I've had or would ever feel something like that.

 

[pauls1150]

If there are any bridges near you that have a steel grating, or if there are any roads that have rain grooves cut into them, AVOID the RB2, or any other tire with straight-lined tread in the direction of travel.
"Wiggle" and "squirm" would be polite understatements.

 

[GTRider]

Interesting to note that motorcyclespecs.co.za shows the RT wearing inch size tires all the way through to 1984, while the R80ST came shod with metric size tires from 1982-84: 100/90-19 front and 120/90-18 rear. Selection and availability for inch size tires have been declining for quite some time, unfortunately.

Best,
DeVern

 

[20774]

Interesting to note that motorcyclespecs.co.za shows the RT wearing inch size tires all the way through to 1984, while the R80ST came shod with metric size tires from 1982-84: 100/90-19 front and 120/90-18 rear. Selection and availability for inch size tires have been declining for quite some time, unfortunately.

Single shock versus twin shock??

 

[PGlaves]

Single shock versus twin shock??

Probably. There may be less of a need for skinny tires due to clearance issues.

 

[collingsbob]

I faced the same decision with my 1953 “C” Vincent. I chose to stick with the original sizes, in close to original compounds because the “look” was important to me and the compound was acceptable in terms of grip for the kind of riding that I was doing - I didn’t foresee myself dragging a knee on the bike - the wish to do so had decreased as the value of the bike had increased and the additional grip of a modern tire could easily reveal the weaknesses of the stock suspension.

 

[viejo]

If there are any bridges near you that have a steel grating, or if there are any roads that have rain grooves cut into them, AVOID the RB2, or any other tire with straight-lined tread in the direction of travel.
"Wiggle" and "squirm" would be polite understatements.

"Wiggle & Squirm" to be sure.... I remember my first experience with an expanded metal bridge. In 1977 I was crossing the original Sunshine Skyway Bridge over Tampa Bay in Florida for the first time on my '76 R90/6 with a straight line tread Continental front tire and to this day I don't know how I kept it together on that long, long, long bridge; I damn near fused the grips to the handlebars I was squeezing them so hard. I'm still not overly fond of metal bridges, or grooved pavement, but I have learned over the years to loosen up on the bars, keep a steady throttle, and just trust the bike. So far, so good.

I'm now also considering a set of Metzeler Roadtecs.