Rear tire on very early 75/5

[umilik]

Ok, here’s my problem. I bought a nice 1970 r75/5 with a lot of rare original feature. Like working original carb 1-2 with the c stamping on top.

I was planning to put new tires on for the new season. Was ready to spent the money on a pair of Avon am26. But i was quite surprise to find what I think is the original thin rear rim on the bike. Those rims are 1.85 inch, the same as the front one ...the problem is that I don’t find any rubber to put on, as avon says thet the 4.00x18 tire i was ready to purchase should not be use on a rim thinner than 2.15 inch!

Anyone with original thin rims and tire suggestions?

Rob

 

[khittner]

Perhaps a better question to the /2 crowd; I have no specific knowledge/experience with tires for the early /5s, but perhaps Heidenau. They're good tires generally, and they offer "classic" sizes, though I can't tell from their website what the rim-width specifications are. https://www.heidenautires.com/tires/motorcycle/vintage/

 

[20774]

Seems like BMWs in the 1970s came fitted with Continental tires...the RB2 and K112:

https://www.continental-tires.com/motorcycle/tires/motorcycle-tires/sport-classic/rb2-k112

Saw it available at Amazon and a few other retailers.

 

[barryg]

I rode a 1971 R50/5 for about 5 years back in the early mid 80's. It had the thinner rims. I ran the Continentals with no problems. 3.25×19 RB2's in the front and the 4.00×18 K112's in the back. No problems with these tires and rims.

 

[barryg]

I guess I should add that they looked right on the bike. They had the early 70's period look. I lived on that bike. Rode it everywhere. The tires did'nt last long. I believe I changed over to Metzlers for longer tire life.

 

[khittner]

I would double-check with Tom Cutter, at the Rubber Chicken Racing Garage, on permissible tire sizing for the narrower 1.85 x 18 wheel you’ve got. The Continental website doesn’t seem to offer info on wheel fitment for their tires, except for their “Classic Attack” radials for older bikes. The wheel size/tire fitment charthttps://blobs.continental-tires.com/www8/servlet/blob/50326/446bf9950bdc2fe106471a0c418988d2/produktdatenblatt-conticlassicattack-data.pdf for those tires indicates that the only tire in that model that would fit a 1.85” x 18” wheel would be their 90/90-18 metric tire for a front fitment, which is, I think, about a 3.50” x 18” inch-size tire. It may be that this sizing limitation is tubeless or radial-specific, and that a bigger 4.00 x 18 would be fine when run with an inner tube on your narrow rear wheel, but before you buy something that’s either marginal or unworkable, I’d recommend reaching out to Cutter. 215-321-7944

 

[1074]

Oh boy! Another tire thread! So just to throw a few shrimp on the barbi:
1. They didn't all come with the Continental k112 and RB2 tires. Mine came with Metzlers that have since been discontinued. There may be other tires mounted at the factory, but I can only speak from my experience.
2. The aforementioned Continentals, although fitting properly, have 1970s technology and IMHO are best suitable for a concourse restoration that doesn't get ridden that often on the street!
3. Although there will be plenty of responses to this thread that insist that the only acceptable sizes are 3.25/19 and 4.00/18, metric sized tires will work well and NOT cause handling problems as long as you keep within the approximate size as the inch sized tires. I've mounted hundreds of 90-90/19 and 110-90/18 tires with NO handling problems, plus, in these sizes, you are more apt to get a modern tire with better handling, traction and longevity.
4. I've lost track of how many /5s that I have owned as well as how many I have restored over the years. The decision of what size and brand of tire you use is, or should be, determined by what you plan to do with the /5 that you have. Just be aware that you'll run into people who admire your bike for what it is as well as those who will criticize you for what they think is a blasphemous change from what they think should be an "original" holy grail motorcycle.

 

[pglaves]

Oh boy! Another tire thread! So just to throw a few shrimp on the barbi:
1. They didn't all come with the Continental k112 and RB2 tires. Mine came with Metzlers that have since been discontinued. There may be other tires mounted at the factory, but I can only speak from my experience.
2. The aforementioned Continentals, although fitting properly, have 1970s technology and IMHO are best suitable for a concourse restoration that doesn't get ridden that often on the street!
3. Although there will be plenty of responses to this thread that insist that the only acceptable sizes are 3.25/19 and 4.00/18, metric sized tires will work well and NOT cause handling problems as long as you keep within the approximate size as the inch sized tires. I've mounted hundreds of 90-90/19 and 110-90/18 tires with NO handling problems, plus, in these sizes, you are more apt to get a modern tire with better handling, traction and longevity.
4. I've lost track of how many /5s that I have owned as well as how many I have restored over the years. The decision of what size and brand of tire you use is, or should be, determined by what you plan to do with the /5 that you have. Just be aware that you'll run into people who admire your bike for what it is as well as those who will criticize you for what they think is a blasphemous change from what they think should be an "original" holy grail motorcycle.

But these same purists installed Leuftmeister or Vetter fairings on their own /5s back in the day.

 

[khittner]

This long-ago posted, apparent extract, ostensibly from the owners manual of a 1970 R75/5, provides useful info that BMW once specified 4.00 x 18 tires for its 1.85” wide /5 rear rims. http://web.eecs.umich.edu/~deroo/bike.dir/owners-manual.dir/owners-manual.html. Click on the “technical data” link, and scroll down to wheel and tire size specifications.

Why this information differs from Avon’s website info showing that their current 4.00 x 18” and 100/90-18 offerings are too wide for 1.85” x 18” rims, I don’t know.

 

[khittner]

FWIW: Here's Avon's current website fitment chart for the RoadRider MkII with recommended rim and alternate rim dimensions: https://www.avontyres.com/en-us/tyres/roadrider-mkii?searchtype=tire&cartype=motorcycle

 

[pglaves]

FWIW: Here's Avon's current website fitment chart for the RoadRider MkII with recommended rim and alternate rim dimensions: https://www.avontyres.com/en-us/tyres/roadrider-mkii?searchtype=tire&cartype=motorcycle

So, according to Avon their 90/90-18 would be acceptable on a 1.85" rim but the 4.00-18 would not be acceptable.

 

[khittner]

That’s what it looks like to me, Paul, and that squares with what OP is saying. Would some Avon tire engineer/product expert have an explanation? Presumably. Would that be interesting to hear? It would be to me (and I don’t even have a /5), and perhaps to others. A good, arcane /5 question for a tire seminar at the MOA National or the next ABC Supertech.

 

[cycleman2]

The profile of inch tires and metric tires are different. Inch tires are taller and are called a 100 % profile, while the metric ones, that we would put on an airhead are usually 90 % profile. So the inch tires are taller. The difference will have a small impact on your speedo and overall gearing, as there is about an inch difference in height between the two profiles.

The older airheads used the inch tires, front and rear, to work on the narrower rims. The newer airheads run with the metric tires. Either will work on your bike. If you use a wider tire, which the metric ones are, the biggest issue is getting the wheel off as it is a tight fit, with the 2 shock models.

 

[20774]

Another issue with using metric tires on the older Airheads is that the side stand might not work as well. On my /7, initially I was using inch-sized tires but began using the metric tires. I noticed right away that the bike sat more upright when on the side stand...I even had it pushed over by the wind at a gas station when on a tour in 2004. I had to be sure that the left side of the bike was pointing downhill anytime that I used the side stand. Not long after that, I went back to inch sized tires and will never to go metric tires again. When I made the changeover, I recall measuring the differences in the radius and it was around 0.5 inches smaller on the metric tire.

 

[khittner]

The profile, inflated width, height, circumference, swing arm clearance, and rear axle spacer issues of using metric, rather than inch-sized, tires have been well-discussed here and in previous similar threads.

OP’s issue really is whether, based on Avon’s current rim-width specifications/limitations, there would be any problem at the wheel/tire bead actually retaining a current Avon 4.00 x 18 tire (or a current metric-sized Avon tire bigger than 90/90-18) on his narrower (1.85”) early /5 rim, despite BMW’s 50yr-old owner’s manual data, and many riders’ long experiences doing so with previously-available tires? Has something changed fairly recently about Avon tires that explains what the current Avon fitment chart shows can be properly fitted to an OEM 1.85 x 18 rear rim like OP’s bike has? Or is there just an error in the Avon fitment chart? It may be overly-cautious to some, but if I was OP, I’d want some clarification about the fitment chart’s info/discrepancies before I’d proceed to mount the 4.00 x 18 Avon tire that he has in hand.

 

[beemeruss]

Bead thickness

The profile, inflated width, height, circumference, swing arm clearance, and rear axle spacer issues of using metric, rather than inch-sized, tires have been well-discussed here and in previous similar threads.

OP’s issue really is whether, based on Avon’s current rim-width specifications/limitations, there would be any problem at the wheel/tire bead actually retaining a current Avon 4.00 x 18 tire (or a current metric-sized Avon tire bigger than 90/90-18) on his narrower (1.85”) early /5 rim, despite BMW’s 50yr-old owner’s manual data, and many riders’ long experiences doing so with previously-available tires? Has something changed fairly recently about Avon tires that explains what the current Avon fitment chart shows can be properly fitted to an OEM 1.85 x 18 rear rim like OP’s bike has? Or is there just an error in the Avon fitment chart? It may be overly-cautious to some, but if I was OP, I’d want some clarification about the fitment chart’s info/discrepancies before I’d proceed to mount the 4.00 x 18 Avon tire that he has in hand.

I run the Avon Road Riders both tubed and tubeless and find them to be excellent. The beads on these tubeless tires are massive as compared to the original tubed type. I wonder whether that was the factor for Avon's omission from the chart. I think that anecdotal evidence from those who have successfully installed them would sway my decision. My first big bike was the same '70 R75/5 that I bought as a kid but I can't remember 5 decades later whether it also had the narrow(er) rims. I do remember that a tire change was a non-issue with the flimsy beads and a pair of the short toolkit tire spoons. Not so much with the Avons but the tradeoff is great tire life and traction if they may prove to be viable.

Russ

 

[umilik]

Thanks! I may have been overly cautious, but since it's my first tire change on this bike I was just on my guard when I read that. I've check bridgestone spitfire 110/90-18 and they said the same thing. Someone told me that he use the spitfire 110/90-18 succesfully on the 1.85'' rim, so I went with it and ordered some. Fingers crossed!
Rob

 

[pglaves]

Thanks! I may have been overly cautious, but since it's my first tire change on this bike I was just on my guard when I read that. I've check bridgestone spitfire 110/90-18 and they said the same thing. Someone told me that he use the spitfire 110/90-18 succesfully on the 1.85'' rim, so I went with it and ordered some. Fingers crossed!
Rob

In that case DO NOT under any circumstances go by the mid-twenties PSI tire pressure recommended 50 years ago by BMW. I would suggest that 36 would be a minimum and 42 the maximum. The minimum is needed to keep the extra wide tire from rolling sideways on that skinny rim.

 

[umilik]

I asked Tom Cutter as suggested.

He told me he's been mounting 4"x18" on 1.85"rims for 50 years without any problem. Concerning the 110/90 option he told me to be really careful because this dimension varies quite a bit from one manufacturer to the other. He told me that is why, for the original 1.85"rims, he prefer using 100/90.

So i just cancelled my order of the metric Brigestone spitfire and ordered the avon am26 in inch size.

Thanks for helping the cautious newbie!

Rob

 

[Woodgrain]

1. They didn't all come with the Continental k112 and RB2 tires. Mine came with Metzlers that have since been discontinued. There may be other tires mounted at the factory, but I can only speak from my experience.

That's right. Metzler Rille 3.25/19 on the front and the C Block 4.00/18 on the rear. Man we're getting old...