The tyre pressure set-up for the Octy II seems, IMHO, to be a bit strange; let me explain:-

I have an Octy II vRS TDI with 225/40 18 tyres. The table inside the fuel filler cap gives pressures for both vRS engined Octys for a 'normal' load (I am not concerned about the full load settings) as follows:-

225/45 17 Front 2.1 bar, Rear 2.1 bar

225/40 18 Front 2.0 bar, Rear 2.1 bar

The strange thing is this. Most front wheel drive cars from my experience have a 0.1 bar or 0.2 bar differential with the Fronts being the higher pressure i.e. Front 2.1 bar, Rear 2.0 or 1.9 bar.

Why then are, for the vRS, the pressures the same or lower at the front, particularly for the TDI when you would expect a higher pressure in the front to counter the extra engine weight?

Can anyone tell me the pressures for other tyre sizes? They are not on my fuel filler cap and not in the handbook.

I am tempted to run my tyres (18s) at, say front 2.1/2.2 bar and 2.0 rear. What do you other vRS drivers do and have you noticed any effect on tyre wear and handling?

I just put 32 psi in all of them.

cc_7up

I just put 32 psi in all of them.

cc_7up

That'll be 2.1 bar all round. Are you on 17s or 18s though?

That'll be 2.1 bar all round. Are you on 17s or 18s though?

2.1 is approx 30.5 2.2 is approx 32

I suspect it makes very little difference on average journey/surface/load.

wheel size shouldn't be different with different size wheels should it?

wheel size shouldn't be different with different size wheels should it?

Not quite sure what you mean?

I think he means tyre pressures should be the same regardless of wheel size.

And he's wrong!

I was also told in the past that tyre/wheel size have no relation to wheel/ tyre size. The pressure remains the same whatever, sure if the tyre is wider more air is needed to get to the required pressure, but its the same.

Having looked at the fuel filler cap on a Fabia there does seem to be a pattern that, on a given vehicle, the pressure required in a bigger wheeled/lower profile tyre is less e.g. front pressure on a 1.4 Furby is 2.2bar in a 185/60 14 but 2.1bar in a 195/50 14.

The Octy vRS take 2.1bar front in 225/45 17 but 2.0bar in 225/40 18. can anyone suggest why?

More importantly, and my initial question, is why does the vRS have lower pressures in the front vs the rear contrary to general experience I have?

I think he means tyre pressures should be the same regardless of wheel size.

And he's wrong!

but why should pressure change tom? as manny has said teh volume will change obviosly but i see no reason why pressure should change in relation to tyre size, pressure normally changes in relation to vechile weight and type of use/conditions in my experience.

Swmbo's 306 TDi has 33psi on fronts, 35psi on rears (but most 306 petrol have higher pressure on the fronts). I think it may just be a way of changing the handling in some way to compensate for the heavy lump at the front - but only on some engine/suspension set-ups. You would expect it to be the other way round to help carry the extra weight but I suspect the pressure difference between front and rear influences oversteer/understeer balance.

(My 105bhp 1.9Tdi has more pressure in the fronts, 2.1 v 2.0bar).

The strange thing is this. Most front wheel drive cars from my experience have a 0.1 bar or 0.2 bar differential with the Fronts being the higher pressure i.e. Front 2.1 bar, Rear 2.0 or 1.9 bar.

I kinda thought the same myself. I just run with 30psi in my Fronts and 31Psi in the Rears.....I've always been meaning to try the other way around for a few days

Could it be that with the extra power in a vRS they want more grip at the front to lessen wheelspin? And that the slightly softer tyre provides that? Just a thought.

The lower profile the side-wall, the lower the required pressure to keep it stable.

Or easier to understand, on a standard "high" profile side-wall the sidewall is flexible and has no structural rigidity (thats why it's called a flat when there is no air in it) - so it needs lots of air pressure to keep the side firmed up (think fully inflated balloon)

However, the shallower the side-wall (ie low profile) the more intrinsic strength the side of the tyre has (less floppy) - so less air pressure is needed to counter the "floppyness".

Technically its shear strength is higher the lower the profile.

The lower profile the side-wall, the lower the required pressure to keep it stable.

Or easier to understand, on a standard "high" profile side-wall the sidewall is flexible and has no strutural rigidity (thats why it's called a flat when there is no air in it) - so it needs lots of air pressure to keep the side firmed up (think fully inflated balloon)

However, the shallower the side-wall (ie low profile) the more intrinsic strength the side of the tyre has (less floppy) - so less air pressure is needed to counter the "floppyness".

Technically its shear strength is higher the lower the profile.

I'd thought about this and it is good to know I was on the right lines.

I have been running my tyres at 2.2bar front & 2.1bar rear for the last 5,000 miles with no detrimental effect on tyre wear, but the front end does occaisionally feel a bit skittish, and traction control gets used quite often; perhaps this is an indication of the fronts being over inflated. :confused:

Last night I dropped the fronts to the 'book' pressure of 2.0 bar. I will use this for a thousand miles or so and see what effect it has.

If you over inflate your tyre the tyre shape will become slightly bowed out (like a doughnut rather than flat faced roller).

This will result in smaller contact patch and more wear to the central part of the tread.

Conversely, underinflation will also result in a smaller contact patch as the tyre will bow in, meaning the car is running on the sides of the tread (so these will wear quicker) and not the middle so much.

Both will reduce your grip and safety.

Well I have run the car for a week with the 'book' pressure of 2.0 bar in the front tyres. The ride was a touch softer and there seemed to be a bit more grip and bite (on turn-in) but when pressed hard the front end seemed to squirm around.

I am thinking that 2.0bar may be OK for the TFSI but with the heavier engine of the TDI 2.1bar may be better. I therefore set the fronts to this on Sunday and we will see.

Does anyone know what pressure the 2.0 FSI Sport on 17's should run at? tried the 2.2 all round as suggested on fuel cap, yet at high speed i feel like im about to skate. I never took note of the original pressure, just saw what was in the fuel cap and put the air set to 2.2 and just plugged in and pumped (wished i took note of the pressure now).

Many thanks!!

Does anyone know what pressure the 2.0 FSI Sport on 17's should run at? tried the 2.2 all round as suggested on fuel cap, yet at high speed i feel like im about to skate. I never took note of the original pressure, just saw what was in the fuel cap and put the air set to 2.2 and just plugged in and pumped (wished i took note of the pressure now).

Many thanks!!

Book pressure (on the filler cap) for the vRS is:-

225/45 17 Front 2.1 bar, Rear 2.1 bar - might be worth dropping to these.

I've now been running mine (vRS TDI on 18s) at 2.1 front and rear for a week nw and seems to be a perfect setup; nice and sharp for turn in, no squirming when pushed hard and compliant on uneven surfaces. I'll keep it at that for now and see how it feels after a couple more weeks.

Now all in need to do is neutralise some of the understeer and (relative) lack of sharpness on turn in 'cos of the TDI engine - stronger rear anti-roll bar is called for I think.

cheers, i did pop out earlier to the car to let my tyres down to 2.1 (30psi) from 2.2 will give it a run later and see.

Seemed better on the straights at speed but back end did slip a bit on the corners, what would be best for the back to add some more air or decrease it?

Seemed better on the straights at speed but back end did slip a bit on the corners, what would be best for the back to add some more air or decrease it?

Personally IMHO I'd keep it at that, it suggests a neutral handling balance instead of the all too common understeer.

Cheers, still doesnt seem perfect as in how the pressures were when i got it, but much better than what it was.