What are the correct tire pressures for a 1979 R100RT?

[Kurt in S.A.]

One downside is the loss of contact patch. Think what higher pressures do...they expand the tire (slightly) and don't compress as much due to the stiffer action with the additional air. That results in reduced contact patch and less traction when you might need it. Also, a high front tire pressure is going to make for a stiff/hard feeling front end. You'll end up wearing out the center of the tire sooner, as well.

40 seesms excessive to me on the front end. 40 is not too bad for the rear, but again seems high.

Kurt in S.A.

[Seth]

The Worldwide Michelin site recommends pressures.
For a 77 thru 84 R100RS and RT, they recommend 1.9 bar (27.5 psi) front and 2.0 bar (29 psi) rear.
Interestingly, they show the same pressures for both 1-up and 2-up. Not sure I'd believe that!

http://moto.michelin.com/tyre-selector#s=0

Metzeler lists 2.1 bar front (30.5) and 2.3 bar rear (33.4) for both their older Lasertec and new Sportec Klassik tires (same bike).

http://www.metzeler.com/site/us/product ... logue.html

[ME 109]

Michelins recommendations are too low. They might be ok for a gentleman tootling around the city, but not for a loaded touring bike.
For me, 36 rear and 34-35 front. I give tyres a maximum workout.

[Seth]

While I agree that Michelin's pressure recommendations seem low, especially for 2-up, I'm not sure I'd go above 35 psi in the rear for 1-up. I think the Metzeler recommendation is more reasonable.

Overfilling the tire, you not only wind up with a smaller contact patch, but also a loss of spring action. This is especially important during cornering where tire and chassis flex are needed for handling road irregularities.

Underinflation, with more tire flexing, causes additional tire heat and possible failure, especially at high speed. Additionally, it can allow the contact patch to distort more during corning.

Bottom line. Tire inflation is dependent upon a number of factors, including load, riding style (corners vs. highway) and speed.

[barryh]

Those Michelin pressures are too low.

The website doesn't recommend an actual tyre when I put my bike in. I think there is something wrong with the site and it's defaulting to some minimum pressures. I've never seen a manufacturers website that recommended anything lower than 2.1/2.3 bar and most are a little higher.

[jagarra]

I think I would be more concerned about the excessive drag than the ride height right now. I have read somewhere not to tighten the axles with a longer length wrench than what is supplied in the tool kit. When you think about it, the axles are not going to move fore or aft so excessive tightening will only screw the bearings up.

As far as ride height, I have felt differences when I changed to different brands of tires, went from close to flat footed to tippy.

[SteveD]

Twin shock R100RS...if my Michelin tyres are overinflated it can be immediately felt in how the bike corners and how it feels over bumps. It's about knowing the feel and being aware of it.

I recently rode the 110kms to Grants and checked my pressures cold before I left. They felt high in the first corner. I checked them again when I arrived. My tyre gauge had reached the point of wear & inaccuracy I expect. It said 36 & 38 cold. The front tyre was high 40's psi hot! The rear was also too high. I would expect a 10% rise in pressure but not 25%! What should we expect from cold to hot?

If you ride your bike often enough you should be able to get some feel for what's right. Then there's the tyre gauge!

[barryh]

I would expect a 10% rise in pressure but not 25%! What should we expect from cold to hot?

You hear two versions of how much the pressure should rise. One is that both front and rear should rise 10% and the another is that the front should rise 10% and the rear 20% on the grounds that it's working harder transmitting the power.

My experience is that the rear always does run hotter in my normal riding and when I've done the hot pressure check I'm much closer to 10/20 than 10/10.

If your pressure gauge is the old pencil type they can be recalibrated provided you have some known accurate pressure to compare it against. Unscrew the top from the body and withdraw the mechanism which is a simple piston and extension spring that can be adjusted for active length. If the gauge is reading low lengthen the spring and if high shorten it. The adjustments are quite sensitive so small changes in the spring length are all that's required.

[SteveD]

Thanks barryh.
The pressure gauge is the one on my compressor which has been good up until recently.
I still use the compressor but I'm recalibrating my head to know what's what with the gauge, using a small digital gauge to ensure reasonable accuracy. The pencil gauge in my jacket pocket still co-relates to the digital one.
Too many gauges!
It pays to be a tad pedantic though.