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p7375
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VBMWMO #8743
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Morning

Put new Metzler Lasertec tires on the the bike.

Front : 3.25-19 M/C 54H
Back : 4.00-18 M/C 64V

According to the 2015 Metzler Technical book the bar for each should be....

Front : 3.25-19 M/C 54H : 2.10 bar (30.45 psi)
Back : 4.00-18 M/C 64V : 2.30 bar (33.35 psi)

My question is what you think the proper (cold PSI) should be?

30 psi for the front and 33 psi for the back seems pretty low?

Appreciate any thoughts!

Pat

schrader7032
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VBMWMO #7032
San Antonio, TX
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Those pressures don't look

Those pressures don't look too horribly low...you could try and see what you think of the feel/performance. I generally run my tires 32/36 F/R and might boost the rear pressure a couple of psi if I were loaded for a trip. I do this regardless of the make of tires.

There used to something called the 10% rule. If the tire increases by more than 10% (cold to hot) then the tire was under inflated...it was working harder and flexing. If the tire increases by less than 10%, then the tire was over inflated...not flexing to produce heat. I think that might not work with today's tires given the change in technology and sidewall stiffness.

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'78 R100/7 '69 R69S '52 R25/2

p7375
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Seems like a better option

schrader7032 wrote:

Those pressures don't look too horribly low...you could try and see what you think of the feel/performance. I generally run my tires 32/36 F/R and might boost the rear pressure a couple of psi if I were loaded for a trip. I do this regardless of the make of tires.

There used to something called the 10% rule. If the tire increases by more than 10% (cold to hot) then the tire was under inflated...it was working harder and flexing. If the tire increases by less than 10%, then the tire was over inflated...not flexing to produce heat. I think that might not work with today's tires given the change in technology and sidewall stiffness.

Yeah, it just seemed to be a few pounds light.

I like the idea of 32 or 33 for the front, and 36 or 37 for the back. Seems like that could work?

Thanks Kurt!

mark_weiss
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Wait, do you mean that you

Wait, do you mean that you think that the engineers at Metzler do not understand the pressures needed for their tires to work?

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schrader7032
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San Antonio, TX
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Hmmm...maybe he should go

Hmmm...maybe he should go with the tire pressures that were stuck to the top of each new bike's fenders!! That's what the service bulletins say. BMW built the bike...they know the geometry of the bike, the weight distribution...seems like they would know. My /7 sticker says 27 front, 25 rear. No thank you!

Tire pressures are just another oil thread...there's no real right answer but what works for the individual. Start with the manufacturer's recommendations. Evaluate the feel, the wear, the performance in different weather conditions. If you're not satisfied, try something else. About the only for certain is don't exceed the cold inflation pressures on the side of the tire.

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'78 R100/7 '69 R69S '52 R25/2

Darryl.Richman
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The problem with tire

The problem with tire pressures, as stated in the BMW manual and stickers on the bike, is that tire technology has changed so radically since these bikes were new. Theoretically, BMW looked at the front/rear weight distribution and chose tire pressures that would provide a sufficient amount of tire contact patch to make the bike safe to ride. This theoretically wouldn't change, regardless of the tires used. With this kind of calculation, it shouldn't really matter whether the bike is a R69S, an R90S or an R nine T -- which all weigh roughly the same.

But we know that recommended tire pressures have been going up over time. You might really expect the opposite, so that bikes would have a bigger contact patch as they got more and more horsepower. It's because modern tires are so much stickier today than they were in the past, even more powerful bikes can get by on smaller contact patches. The problem with having too big a contact patch is that it really heats up the tire and, in extreme cases, can destroy it from the inside.

I once had a flat tire on the freeway on my R1100RS, but because of the stiff sidewall construction of modern tires, I didn't realize it for probably 20 miles or more (until I slowed down and I could feel it wobbling). When I pulled off, the rubber on the tire was globbing up in big, gummy balls and coming off from the heat.

The problem for us is that the tire companies don't test their products with old bikes; they only give recommendations for new models, and maybe some general rules of thumb otherwise. That leaves us to find some compromise values that we feel work for us. I personally run 32/36 on my R90S with Avon tires and find that comfortable with good tire wear.

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mark_weiss
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It is actually the tire

It is actually the tire manufacturer which determines optimal tire pressure. Pressure is based on the weight and forces that the tire will have to bear. Each tire is engineered with a load table, a table of load ratings and the pressures necessary to maintain those loads. The motorcycle manufacturer chooses their preferred tire and uses the appropriate inflation pressure for their intended use.

An example of this is two of my road bikes. Both use the exact same tires but each has different recommended pressures. One bike is spec'd to use 32psi front, 36psi rear, the other bike is supposed to run at 36psi front and 42psi rear. Same tires, different load ratings, different pressures.

Modern tires generally use higher pressures because a modern tire's more flexible carcass requires a bit more pressure to hold proper shape. Too little pressure and the carcass distorts excessively, wrinkles in the contact area, and does not maintain good contact with the ground. Also, there is more heat build up. Excessive pressure and the tire does not conform to surface irregularities and does not absorb small bumps as it should. Likelihood of breaks in contact with the ground become more likely.

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hotei
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Tire pressure

I have been curious about this issue also. I just had continental rb2/k112 put on my '71 R75 and the shop put 41 psi front and rear. I like the way it rides except for rain grooves.

schrader7032
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Hotei - Whatever works. But

Hotei -

Whatever works. But in my opinion, those are pretty high pressures which could make for a rough ride and likely will wear the center of the tread out faster. Not to mention that the contact patch is reduced with high pressures, thus giving you less traction when you might need it.

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'78 R100/7 '69 R69S '52 R25/2

hotei
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Correct pressure for conti rb2/k112

I've been searching around for info on correct tire pressure for these tires and find no info. schrader what pressure do you suggest?

schrader7032
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VBMWMO #7032
San Antonio, TX
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I posted the numbers I run on

I posted the numbers I run on my /7 in the second post on this thread. I run the same on my R69S but a little lower than those on my R25/2...the single cylinder bike doesn't weigh that much.

I remembered I had a post by someone who had asked Continental about these same tires. The tire engineer stated:

Quote:

Your recent inquiry was forwarded to our US offices. Your Conti
Classic tires should be inflated to 30-32 psi front and 34-36 rear.
You can increase your pressures 2 psi for every extra 100 lbs of weight
on the bike (i.e., two up, baggage). Do not exceed pressures stated on
the sidewall of the tires. Pressures increase roughtly 5 psi when tires
are hot.

This was from Janet DeCandia, Continental Tire North America, Two Wheel Division. Not sure if they have a specific website or contact info.

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Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

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