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I'm getting mixed messages from shop manuals and a couple riders as to tire pressure specs. The bike is a new-to-me 1978 R100S. Tires are Metzler Lasertec with 110/90 front and 120/90 rear. One R90 rider tells me the front tire should have more psi than the rear. Say what?

What's the conventional wisdom? I need to get this sorted before getting too frisky in the twisties. Advice appreciated.

I wouldn't put any faith in the shop manuals. Those manuals were written decades ago when motorcycle tires were very different from today. I don't know what the tire pressures should be, and they are likely to vary from manufacturer to manufacturer. I would put a lot more faith in what Metzler has to say about pressure.


Ken

I use 32psi front 34psi rear in my Bridgestone Spitfires

If you want to get frisky then I would consider changing to the Michelin pilot range of tyres, using 4.00 rear and 3.25 front, with pressures around 40 rear and 36 front. Modern tyres seem to require higher pressures, the narrower tyres will speed up the handling and e Michelin tyres have very good grip wet or dry and are stable but agile.

I have found that those metzlers are too prone to white lining in the wet for my taste. Your and others experiance may be different.

I run 35 & 40 but my bike is pretty well loaded with just me

When in doubt, I check the tire sidewall info for psi.

dwerbil wrote:When in doubt, I check the tire sidewall info for psi.

No, don't do that.

The tire sidewall shows the maximum PSI the tire can hold. That has nothing to do with what pressure you should be running when the tire is mounted on your motorcycle, it is just the number you should never exceed.

And whoever that guy was who told you to run the front at a higher pressure than the rear, as you suspected, there's something wrong with him.

Major Softie wrote:

dwerbil wrote:When in doubt, I check the tire sidewall info for psi.

No, don't do that.

The tire sidewall shows the maximum PSI the tire can hold. That has nothing to do with what pressure you should be running when the tire is mounted on your motorcycle, it is just the number you should never exceed.

And whoever that guy was who told you to run the front at a higher pressure than the rear, as you suspected, there's something wrong with him.

Thank you, but I'll keep doing that. I've used the factory psi my entire life....on my cycle tires, car, truck, wheel barrow, what ever. Up to and with in a few psi. The factory psi on the sidewall is perfect info Major for what to run, as it always says to "Never exceed". Otherwise, wouldn't you say one is just guessing.

Edit to add...seems most all cycle tires have a never exceed of 40 psi front and back, as far as I recall.

What about the % rule (I forget what % is used).

But you start with cold tires, at your best guess. Go for a long ride and warm them up. Stop and check the pressure when they are warm. If they show too much pressure increase, I believe that means the tire pressure was too low to begin with. (Tire flexing causes heat that causes air pressure to rise.)

If your pressure check shows no increase in pressure, they were probably too high to begin with.

All, feel free to critique my logic.


But, like Tim, 32/34 was what I ran on my R65.

dwerbil wrote:
Thank you, but I'll keep doing that. I've used the factory psi my entire life....on my cycle tires, car, truck, wheel barrow, what ever. Up to and with in a few psi. The factory psi on the sidewall is perfect info Major for what to run, as it always says to "Never exceed". Otherwise, wouldn't you say one is just guessing.

Edit to add...seems most all cycle tires have a never exceed of 40 psi front and back, as far as I recall.

I'm sorry dwerbil, but you are quite simply incorrect. If you trust the manufacturers so much (a perfectly reasonable trust), then just check their sites, and they will inform you as to your error.

In addition, may I correct my earlier statement about it being the maximum pressure for the tire. That was incorrect as well. I believe there was a time when tires showed max. inflation (as bicycles still do), but that is definitely not the case now. Now, the pressure on the side of the tire is the pressure at which their maximum load rating has been tested. If I may reference the Avon Motorcycle Tires "Tires 101 for Consumers" .pdf document:

A tire inflation pressure is on the sidewall of every tire showing a tested pressure under load. This is neither the recommended pressure nor the max inflation.
http://www.avonmoto.com/download/Tires1 ... sumers.pdf

In other words, the only thing the pressure on the sidewall tells you today is that this is the pressure at which the tire was tested in reference to its maximum load rating.


On your second point (in your edit) exactly zero of the 8 mounted motorcycle tires in my possession list 40 lbs as their max load rated psi.