Technical Tyre Pressures with Bigger Boots on!

[Bleeding Knuckles]

Greetings, Just starting to get a Spring Service organised while waiting for parts and I have a slight problem with my larger wheel/tyre combination and the best pressure for normal road use? I'm running 165/60 R12 71H Yokohama A539's on a 'minilite' type alloy of 5J x 12 Anyone out there using the same tyre or have the best numbers? I'm looking at 28psi rear 26psi front - ish??????

Ian.

 

[IanEmery]

Form a previous life when we used motorhomes over many years I've often phoned the tyre companies and spoken with their technical departments to get their advice over what tyre preasure to use. With motorhomes there is a huge variance in the loading that people carry and hence different preasures required. If you give them the weights for both front and rear axles plus the car's total they should be able to help.

Hope this helps you.

 

[jjacob]

I think that it is important with rear engine cars to maintain the correct difference between the front and rear tires as originally specified by the manufacturer. This is really important for rear engine cars to prevent over steer. I noticed for the 126p that the front inflation is 1.4 bars (19psi) and the rear is 2.0 bars (28-29psi). So I would think that where ever you end up on your rear pressure, the front should be 0.6 bars (8-9psi) lower.
John

 

[Adry500]

Normal pressure for the 500 would be 1.3 bar in the front and 1.6 in the rear.
On those tires I would go 1.7/2.0 and see what happens...
I think my car would barely move with those wheels ehhehehe

 

[the hobbler]

Evening Ian;
Whilst I accept that I am running 13in wheels, as against your 12in wheels, I think that you will find that 26psi fronts will give you a very 'light' feel--possibly too light! I consulted MBG as Anton (their little Dutch foreman) also runs 13in wheels on his VERY quick 500. The pressures that Anton runs his tyres at really surprised me, even if he does weigh a bit less than me (their dig at me!), so I have upped the pressures a bit and now run my tyres at 15psi front, and 23psi rear---note the 8psi difference. This gives a nice 'feel' and it doesn't feel as if the front is floating, which it did with 20psi at the front. I ahave done a reasonable amount of mileage with tyre set to these pressures with no problems---don't forget, the 500 is considerably lighter than the cars these sizes of tyres are normally supporting.

 

[stevehg]

I was running the same tyres, IIRC after playing around with pressures, the fronts were about 16psi and rears about 23-24psi. Chalk test showed this was giving even wear.

As per Tom's comments, too high on the front affected turn in and stability at higher speeds.

Cheers

Steve

 

[F123C]

Afaik, if you go up in the width of tyre used, tyre pressures need to be reduced slightly not increased. I'd agree with the other posters about maintaining the pressure differential front to rear and not going too hard with the front tyre pressure. Dunlop e.g. used to recommend lowering tyre pressures slightly and reducing camber angle to nearer 0 degrees. You could also reduce castor angle as the wider tyre gives more self-centering action to the steering.

I like the idea of doing a 'chalk test' but it doesn't measure contact pressure in the middle tread area, nor does it take into account the effectively reduced tire width contact patch on the rear of a 500/126 due to the significant amount of +ve camber employed.

Me, I just drive through a puddle on an otherwise dry road or watch my tyre tracks in the rear view mirror on a wet road.

Am I the only one who tries to follow in the tyre tracks of other cars in the wet?

Don't forget to allow for tyre growth at speed (like on dragsters).

Al.

 

[Adry500]

While it is true that the 500 is very light, the original tyres with the inner tube were specifically built to use such low pressure.
I own a classic mini with 12" minilites and the reccomended pressure is 2.0 bar (29 psi). The mini is a bit heavier, but not a whole lot.

 

[the hobbler]

Depending on ones source of information, the classic 500 weighs 500-510kg; the classic mini weighs between 617kg and 686kg, with the engine contributing 150kg alone--and directly over the front wheels. The weight on the front wheels of a 500 with 2x14 stone people (and a little bit of luggage) is approximately 220kg. The weight over the front wheels of a mini with the same 2 people in it (and again a little bit of luggage) is going to be approximately 360kg---hence the need for much higher front tyre pressures on a mini.

 

[Adry500]

Depending on ones source of information, the classic 500 weighs 500-510kg; the classic mini weighs between 617kg and 686kg, with the engine contributing 150kg alone--and directly over the front wheels. The weight on the front wheels of a 500 with 2x14 stone people (and a little bit of luggage) is approximately 220kg. The weight over the front wheels of a mini with the same 2 people in it (and again a little bit of luggage) is going to be approximately 360kg---hence the need for much higher front tyre pressures on a mini.

As I said before, you DO need lower pressure on the front tyres, or the car will have no grip at the front and you'd risk to crash. Still, newer tyres are not made to work with pressures as low as 1.3 bar, as the original ones were (which was my point ).

 

[Bleeding Knuckles]

Greetings Gentlemen, Thanks for all the comments and information - appreciated.
I have to admit that the rather high pressure figures I came up with were from a Mini using the same tyre and as I remember my car was a tad 'twitchy' on the front!!!!! So I shall go with 24 psi rear and 16 psi front and see how the tyre wear goes thanks again.

Ian.

 

[Bleeding Knuckles]

Evening Ian;
Whilst I accept that I am running 13in wheels, as against your 12in wheels, I think that you will find that 26psi fronts will give you a very 'light' feel--possibly too light! I consulted MBG as Anton (their little Dutch foreman) also runs 13in wheels on his VERY quick 500. The pressures that Anton runs his tyres at really surprised me, even if he does weigh a bit less than me (their dig at me!), so I have upped the pressures a bit and now run my tyres at 15psi front, and 23psi rear---note the 8psi difference. This gives a nice 'feel' and it doesn't feel as if the front is floating, which it did with 20psi at the front. I ahave done a reasonable amount of mileage with tyre set to these pressures with no problems---don't forget, the 500 is considerably lighter than the cars these sizes of tyres are normally supporting.

Hi Tom, Can you tell me the wheel and tyre sizes you run please. Also the offset would be handy as I'm researching 'space saver' wheels as an option!!!!

Ian.

 

[thepuddlejumper]

I have 165x50x13's on my Gamine, the Gamine is lighter than the standard 500, and I run the front at 15 with the rears at 20. I have used this set over the last 11 years without any problems. I found if I had the fronts any higher it felt like I had no suspension with very light steering.

Ralph