Tire Pressures - 10/20 Rule?

[El-Twin]

I'm still new to my 1100 EVO and I'm turning my interest to some fine tuning.

I read on a random source that tire pressure should increase 10% front and 20% rear, once fully warmed up from a cold start, if the cold pressure is set properly for your weight. The 20% rear figure is based on the fact that the rear tire works harder than the front.

Is this a valid rule that the forum is in general agreement with?

[Curmudgeon]

The 2.25/2.5 Bar recommendation is accurate. Most people run with no air.  If you want to be fussy, cold to hot, the PSI should rise ~ 3. Reduce PSI if less and add if more than 3. You are overthinking this IMO. Use the 33/36 PSI recommendation from both Pirelli and Ducati and check weekly. You have Rossos, right?

[El-Twin]

You have Rossos, right?

Hey Curmudgeon, I just got back from a putt out to Ojai.   

Yes, I have Rossos. It's easy to get the +3lbs = 10% from the front tires, but I can't get anywhere close to the +7.5lbs = 20% from the rear. Even after reducing pressure by 1.5lbs. Maybe it's not important, and I'm over-thinking it as you suggest. I have a lot to learn, but it's all in good fun.

[Curmudgeon]

Ojai?  http://gallery.leica-users.org/v/lambroving/Miscellany/Ojai/LUF66.jpg.html

33 + 3 = 36 and 36 + 3 = 39, or if you prefer, 35 + 3 = 38.  Forget percentages. It's NOT a car.  Just see what feels better in the rear. On the street I prefer harder. 7.5 PSI ??!!! 

What are you using for a gauge? Is it accurate? I've been using the same German mechanical gauge for 45+ years which can be zeroed out as it wears. (It was good enough for F1  )

[El-Twin]

What are you using for a gauge? Is it accurate? I've been using the same German mechanical gauge for 45+ years which can be zeroed out as it wears. (It was good enough for F1  )

I use a dugital gauge that rounds to 1/2 lbs. Are you saying that creeky old mechanical guage has actually coupled with an F1 valve stem in it's younger days? Prurient pressurized relations?    Mechanical mating madness?

I'd say it's a keeper then.

[Curmudgeon]

The digital gauge you're using may or may not be very accurate, so you may want to check it against a professional mechanical gauge to be sure. I've seen them vary +/- 2 - 3 PSI over actual. Cost seems to have little to do with it either. My son has 5 digital gauges and only two are within 0.5 PSI of reality. 2 PSI is pretty noticeable on a bike..., or sports car. 

The one I'm using is a Drager which I picked up at either the Ring or Spa in 1964. It's dead accurate and zeros out when you release the pressure. You can re-align the scales to zero if they don't agree. One sold on eBay recently but I've seen them sell for much more with the case and in better condition. I'm sure this one works though. They are fairly shockproof. http://search.ebay.com/200741198923 Click on "See original listing".

As a kid I worked with Ray Heppenstahl at NART for both the European F1 and sports car venues in 1964 and 1965. It was our NART entry in 1964 which won Surtees his World Championship. So..., you can be sure that gauge was used on this,  http://en.wikipedia.org/wiki/File:Surtees_Ferrari_158_Goodwood_July_2010.jpg Ray had been Phil Hill's tuner when Phil won his championship too. At that time I had an FIA "driver / mechanic's license" (long story) and it was actually Phil who showed me the line on several of those tracks which were as narrow as a decent 2-lane VA highway today.

Story here if you're interested. Ancient history now... http://en.wikipedia.org/wiki/North_American_Racing_Team and http://en.wikipedia.org/wiki/1964_Formula_One_season

[El-Twin]

Curmudgeon - I have one of those old Drager gauges! I got it for my first stint at riding my CB500 Four back in the 70's. I'll check my digital against it. Great minds think alike. 

Great old history you linked to. I actually remember those days! I still love the raw, elemental beauty of those machines. Wouldn't want to race in one, though.

[Curmudgeon]

Mine's been in constant use and works great. If yours has been buried for 40 years, you might want to check it against an accurate gauge.  With luck, might be fine. Takes some hand coordination to use though. Porsche dealers used to sell them back in the day. Drager still makes precision gauges, just not consumer stuff.

Racing was what it was. The tires were marginal and the cars had suspension. Just the opposite of post ~ '70's race cars. Different skills required and you were always on the edge. We didn't think about it. Three drivers I worked with were killed eventually. Lots of fires... A lot of luck was involved, not in the racing so much as in survival. Most Americans survived and that WAS luck. I bailed when Ford brought stock cars guys to Europe who were killing themselves and everybody around them. Ray quit when he was nearly killed in a King Cobra (Cooper Monaco with big Ford power). Hmmm... http://www.rmauctions.com/FeatureCars.cfm?SaleCode=MO10&CarID=r304&fc=0