Tyre pressures/ geometry

[robbosliding]

I posted under "tyre wear RMS and clutch" on the subject of tyre wear. Having spoken to my indie over inflation was blamed for the wear on the centre portion of the tyre. I have run the tyres (Conti Contact 2's) at 44 PSI as per the manufacturers advice. My indies states this is far to high and as standard they run 32 rear and 28 psi front, transforming the handling of the car I was told and increasing tyre life and returning correct tyre wear......Set my cold tyres to this yesterday and........hey presto, did what he said. Much more planted on the rear giving far more confidence when cornering and putting power down from the apex.
New tyres are ordered at what turns out to be reasonable price. I will update on the RMS and clutch episode when resolved.

 

[Richard_Hamilton]

Seems very low to me. Porsche stipulate 36/44 (18" wheels) for a reason. FWIW I always run mine at those pressures and get even tyre wear, so I'm at a loss as to why your indy would suggest a 25% reduction.

 

[robbosliding]

I am not too concerned about this reduction. Whilt it seems a significant reduction is it? Given that all pressures stated are at cold I would allow a10 psi increase for a moderately driven car, resulting in 42 psi when warm. By the same yard stick at porsches recommendation warm tyres will be running at 54 psi.
Not long after I bought the car I had 2 hours at Bruntingthorpe with an instructor who was familair with porsches who put me through my paces to give me a feel of driving a 911 and its characteristic rear end waggle when pushed through gated corners etc. Every now and again we would stop to debrief and check the car over. On the 44 psi setting the tyres were like a drum to tap, the pressure was huge in them, by my modest guest, far too high.
By the same token if I can draw comparison, I have two motor bikes that I ride, both of the identical model, kawasaki ZX10R's, one is a road bike, the other track prepared for which I use for track days. I use the same type of tyres road and track. My rear road pressure by the book is 36 psi. My buddy who sets up British superbike suspension (and he is at the top of the game) sets me up at 26 psi. These tyres get HOT and the tyre pressure increases, I prime them with tyre warmers etc but.....if I started riding at 36 psi it would not be long before my tyre footprint shrank due to over pressure and I'd be off because they would not be working properly.
I am applying the same principle to the car. The evidence is in the wear. An over pressured tyre will wear in the middle and it has done.
Would you be open to try yours for feel at 32 psi before you are set in stone on this one, the difference is smiles better!

 

[Richard_Hamilton]

I've tried lowering the pressures, but (on P-Zero Rossos and Contis) found it incresed the tramlining considerably, and made it kick off white lines and bumps much more. The Yokos I have now seem less susceptible to that, as I did drop the pressures in the snow, and have only just pumped them back up. I understand what you are saying about track conditions, but that's generating a huge amount more heat than on the road.

Each to his own, but I think you might get the opposite wear effect dropping the pressures that low. Just my 2p's worth.

 

[robbosliding]

Thats interesting. By tramlining do you mean weave, as if the tyre is squirming around? I'd have expected a harder pressure to create kick of white lines and bumps etc.
I appreciate that an overly low pressure may be indicated by worn tyre outers (both sides) but staring in the face of a pair of tyres worn unevenly in the centre how else could you explain it? I doubt any geometry error would cause that type of wear, it strikes me of over inflation but I stand to be corrected if anyone knows better, and from my assessment to date from what I have read, thats likely to be you Richard, some really good stuff on here!

 

[Richard_Hamilton]

By tramlining, I mean the effect you get when running on very worn carriagways, where the traffic has created two 'grooves'. This tends to make the car follow the grooves. I found that lowering the pressures made it follow the grooves more, but I can't give you any scienticic reason why. Kicking off lines and bumps (we have cr4p roads around here) made the car very twitchy and nervous, and was an uncomfortable sensation when pressing on. These are just my experiences, YMMV as they say in the US.

Do you always use the same gauge, or the same garage, to check your pressures? If so, try a different gauge - they can vary wildly.

 

[robbosliding]

I have you on the tram lining- top example is lane one on a dual carriage way where HGV's weight provides sunken tyre depressions in the tarmac, rain collects and in the dry wide tyres have a tendancy to "follow" or as you say tram line. Answer to this-remain in lane 2!
Re the above I have found this is true with the recommended tyre pressures, can't say for lower inflations yet. I am not yet sold that my indie is wrong. The "plantedness" with the lower pressures is not minor or negligable, it definately feels better, more planted and less prone to start skipping or twitching when pushing it through corners.
As for the tyre gauge, I have three, two seperate pen types and one on my footpump, they are all within 1 psi of each other so I am happy with that end of things.
I am unaware of the work implication of setting up/checking of geometry. What is a reasonable fee for undertaking this work ie checking it or setting it to factory settings- I am "assuming" that a half wit hasn't inferred with mine and it remains on stock- that said I am the third owner so who knows.....
I am not overly concerned with the warning attached re lower pressures- as I said earlier my bike pressures by manufacturers standards are much lower and I know through experience they work well on circuit, albeit generating much more heat. That said I have run the same pressures on the road and they feel more compliant than the road pressures generally-road pressures feel hard and twitchy- It would though however be interesting to see, particularly on your beast if through torque there was much tyre slippage on the rim with the lower pressures?.....

 

[Richard_Hamilton]

A 4-wheel alignment will cost anything between £100 and £250, if all four settings need adjustment (toe and camber on front and rear). If your car hasn't been done for two or three years I would recommend getting it checked, as it can make a big difference to the way the car feels and performs. The guru is Chris Franklin at Center Gravity if you want to travel, but your local indy can recommend someone local if they don't have the facilities themselves.

I'm sure that's not your wear issue though. Purely as a matter of interest, how old are the tyres? You can get the manufactuting date from the last four digits of the DOT markings on the sidewall. They are in the format of week number/year.

 

[robbosliding]

I will come back to you on that one- they were fitted 2 years ago nearly but that means nothing, I will have a look and come back to you with the digits in a day or so.