Technical Tyre pressure

[Kiltedboarder]

Hi all
1 more very simple one. What true pressure should I be running at. Just standard tyres fitted to a 500l

Thanks

 

[Goldnrust]

Lower than you might think!

Factory pressure for the front tyres are between 16-19psi and rears 23-25psi, depending on the years and exactly models. So that's the kind of ball park.

When I first got my 500 on the road, I thought 'thats way too low, let's put 26 front and rear and go from there', I'd had other vehicles where the owners consensus was to ignore the very low factory suggested pressures, as modern tyre technology made them irrelevant. But clearly this is not the case for the 500.

I can confirm that the handling was definitely worse at the higher pressure, the front end especially felt more wayward at speed. Dropping back to the factory pressures, was a good improvement.

 

[Kiltedboarder]

Right o, way lower than I would have thought. I’ll go check later and adjust them. Thanks a lot for the fast reply

 

[smart51]

Tyre pressures are related to the weight of the car and the size of the tyre contact patch with the road. The 500 is a super light car. Think about it like this. If your tyres have no pressure in them, the weight of the car pushes the tyres flat. There is a lot of tyre in contact with the road. As you pump the tyres up, the air pushes the car up and the tyre starts to become round again. There is less tyre touching the road. Eventually, you will have huge pressure in the tyres and almost none of the tyre will touch the road. Why is this?

The pressure in the tyre and the area of tyre that touch the road exactly equals the weight of the car. This is one thing where imperial measurements are easier to understand than metric. The 500 weighs about 1300 lbs including the driver. The tyres' contact patch is about 4.5 inches across because of the width of the tyre. You might decide that the ideal contact patch is circular, so each tyre has a contact area of about 16 square inches, 64 square inches in total. 1300 lbs divided by 64 square inches is 20 pounds per square inch, so the tyre pressure should be 20 PSI. Now the 500 is heavier at the rear than the front, so the front tyre pressures should be a bit less than 20 and the rears a bit more.

 

[bugsymike]

Interesting little formula.

 

[Pugglt Auld Jock]

The Hillman Imp was another I believe the recommendation on cross plys was 15psi in the front and around 30 in the rear. On radials they seemed to go well with 20 in the front and 30 to 35 in the rears. However if you fitted a decambering kit to the front you could up the front pressures a bit to good advantage. If you drove it "stupidly" in the wet it was very easy to end up with so much understeer you would be very quickly inspecting the nearest ditch.

 

[bugsymike]

I only had one Hillman Imp, I preferred the two Renault 10s for more practical rear engined cornering, though the one I fancied was the tuned up Renault 8s the boy racers had in the South of France many years ago.

 

[fiat500]

Right o, way lower than I would have thought. I’ll go check later and adjust them. Thanks a lot for the fast reply

I suggest that you buy a very accurate tyre-pressure gauge. You definitely can't trust anything from an automated pump and the gauge on any manual inflator will not be accurate at these low pressures.

I bought an analogue gauge that is intended for aeroplane tyres, which alows me to feel confident that the pressures are correct. I have had the same experience of @Goldnrust in finding that the "factory" pressures are the best for handling. Having tried other makes of tyre I also consider that the Pirelli Cinturato is the best for handling.

 

[Dougal Cawley]

Hi

Out of interest this is our weeb page on the subject of tyre pressure.

Classic Fiat 500 Tire Pressure guide - Longstone Classic Tires

Fiat 500 - Tire sizes and fitment guides from Longstone Tires. Period fitment guides and tire sizing recommendations for every classic Fiat 500

www.longstone.com

It has lots of period fitment guides from tyre manufacturers. Of course including Pirelli who made the original tyres in period, and are making them again now.