Tyre Pressure Survey

GreatOldOne · #1 17th March 2010, 07:56

Guys - I wonder what everyone is running with. I seem to recall that 22 psi was mentioned way back in the dim and distant past as the Marlin 'standard'

I did have mine at 25psi, but jacked it up to 30 for the IVA test for a little extra self centering.

Does anyone know how to calculate the optimuim pressure? I guess it'd be based on the weight...

Chris Cussen · #2 17th March 2010, 12:00

Well that was an interesting lunchtime diversion. I couldn't find a definitive answer except 'try it out'

The bridgestone website recommends http://www.bridgestone.com.au/tyres/.../pressure.aspx increasing your pressure by 4psi if you have a heavy load, so I'd guess a good starting point would be the donor's tyre pressures minus 4psi.

GreatOldOne · #3 17th March 2010, 12:26

Interesting! Googling the pressures of the Donor (BMW E30 325i) gives this:

205/55V15

Front: 1.8 BAR / 26 PSI
Rear: 2.0 BAR / 29 PSI

So, that would mean a front pressure of 22 PSI & 25 PSI on the rear.

Sorton · #4 26th April 2010, 15:35

I found this thread on returning from a trip away.

I think Mr Google has been economical with the truth! Having owned and driven my donor 325i Sport for many years I always used the pressures advised for 205/55/15 tyres which are given in the handbook:

Front 2.4 bar/34.1 psi
Rear 2.6/ 37.0 psi

(There are also max load pressures 37/42 which I never used)

The handbook has a whole page of pressures for all manner of tyres, wheels etc including the ones you quote but I always used 34/37 which I found worked well. Tyre wear was even across the tread. However, when I fitted lower rear springs I did suffer from wear on the inside shoulder.

Obviously, the Sportster is lighter and has different geometry but in general this profile of tyre probably needs quite high pressures to achieve the correct contact patch.

revesby37 · #5 18th May 2010, 21:28

Bear in mind that the tyre is designed to deflect so as to provide the correct amount of contact with road surface and some cushioning. If you halve the weight of the car, the tyre will deflect much less at the same pressure. Too hard and the car will tend to skip.

Therefore taking a wheel/tyre from a BMW (or any other heavy car), you will need to reduce pressure.

I run at 19psi all round on 195/65/15 and weigh in at around 900kg.

chrislandy · #6 19th May 2010, 05:03

A trick I used to use on my landy was to pump the tyre up until the edge of the tread starts to lift off (reducing the lateral contact patch) then record the psi and try +/- a few psi based on feel.

The Shelsley currently runs at 20f 36r using 195/55/15 and 205/50/15