Tyre Pressure Question

[js456]

The handbook for my Mercedes Sprinter 208cdi SWB van recommends tyre pressures of 2.3bar front, 2.5bar rear for an unladen vehicle.

At these pressures the front tyres look under-inflated so I am running 2.8bar in the front.

When unladen most of the vehicle weight is at the front so why would the recommendation be for a lower pressure in the front tyres?

[Tony1963]

Ask Mercedes’ and the tyre companies!

Tip: it’s unlikely that you or me know more about tyre pressures than the manufacturers, so stick with what they advise. Emergency braking in the wet with overinflated tyres could prove ‘exciting’…

[GreenV8S]

js456 said:

look under-inflated

Unless the tyre is grossly underinflated, you won't get any indication of whether the tyre pressure is right just by looking at the tyre.

If you're using the manufacturer's recommended wheel and tyre sizes then you should run them at the manufacturer's recommended pressure. Setting the pressure too high can reduce grip and impact the handling and stability of the whole vehicle.

[js456]

GreenV8S said:

Unless the tyre is grossly underinflated, you won't get any indication of whether the tyre pressure is right just by looking at the tyre.

If you're using the manufacturer's recommended wheel and tyre sizes then you should run them at the manufacturer's recommended pressure. Setting the pressure too high can reduce grip and impact the handling and stability of the whole vehicle.

Thanks for the replies.

Front tyres look underinflated compared to the rears. Quite a big difference. Going to continue running the fronts "overinflated".

[InitialDave]

Is your gauge reasonably accurate?

[js456]

InitialDave said:

Is your gauge reasonably accurate?

Yes.

[Aunty Pasty]

The fronts looking a bit under inflated is quite normal. It's true in my car and most front engined cars as that's where the weight is. You can't just go by how round the tyres look.

[E-bmw]

Aunty Pasty said:

The fronts looking a bit under inflated is quite normal. It's true in my car and most front engined cars as that's where the weight is. You can't just go by how round the tyres look.

^^^^ Wot 'e said.

[paintman]

Mine's a 2002 208CDi SWB.

The stick-on label on the side of the driver's seat gives the following. These are for laden.

195/70 R15 C tyres:
Front 3.3bar/48psi Rear 3.5bar/51psi

225/70 R15 C tyres:
Front 2.3bar/32psi Rear 3.0bar/44psi.

Mine came factory fitted with the 195 tyres & I run at those pressures as the van is a mobile workshop & fitted out with shelving, workbench plus equipment so laden but not heavily so. Never had a problem.

For empty it advises to see the handbook. Don't have that to hand ATM.

[js456]

paintman said:

Mine's a 2002 208CDi SWB.

The stick-on label on the side of the driver's seat gives the following. These are for laden.

195/70 R15 C tyres:
Front 3.3bar/48psi Rear 3.5bar/51psi

225/70 R15 C tyres:
Front 2.3bar/32psi Rear 3.0bar/44psi.

Mine came factory fitted with the 195 tyres & I run at those pressures as the van is a mobile workshop & fitted out with shelving, workbench plus equipment so laden but not heavily so. Never had a problem.

For empty it advises to see the handbook. Don't have that to hand ATM.

My tyres are 225s and I agree with your figures. For unladen the handbook says 2.5bar in the rears.

[paintman]

js456 said:

My tyres are 225s and I agree with your figures. For unladen the handbook says 2.5bar in the rears.

Thank you.

Think I'd go with MB's recommended figures as they're more likely to be for actual use than aesthetics.

Edited by paintman on Friday 10th June 19:44

[js456]

paintman said:

Thank you.

Think I'd go with MB's recommended figures as they're more likely to be for actual use than aesthetics.

Edited by paintman on Friday 10th June 19:44

My question was not really about the aesthetics, more about the science/ engineering aspect.

I still do not understand why the recommendation is to put less pressure in the front tyres compared to the rears when the fronts are carrying more weight than the rears.

The Mercedes recommendation does not make sense to me.

Are there any engineers out there with any ideas?

[finlo]

js456 said:

paintman said:

Thank you.

Think I'd go with MB's recommended figures as they're more likely to be for actual use than aesthetics.

Edited by paintman on Friday 10th June 19:44

My question was not really about the aesthetics, more about the science/ engineering aspect.

I still do not understand why the recommendation is to put less pressure in the front tyres compared to the rears when the fronts are carrying more weight than the rears.

The Mercedes recommendation does not make sense to me.

Are there any engineers out there with any ideas?

Because the reverse will be true when you load a couple of pallets of bricks into the rear!

[paintman]

^^^^^What finlo said.

Axle weights.
Commercial vehicles for carrying loads will have a manufacturer's maximum load weight for each axle.
This will be much higher for the rear axle as that's where the payload sits.
Whilst the load on the front axle will increase when fully laden it won't be anywhere near the increase on the rear axle.
This is why the rear pressures will be higher than the front.
As you said, there is a considerable difference on yours on the rears laden/unladen.

Important to bear in mind as axle weights will be taken into account if you are checked for overload by the Police or DVSA.
The whole vehicle will be weighed & then each axle separately.
Depending on how far over you would be likely be given a prohibition notice which will stop you taking the vehicle anywhere until the load is corrected, the vehicle re-weighed & the notice lifted.

Quick google of Ford Transits shows the same lower pressure in the front tyres than the rears.
The Transit labels show tyre pressures but also the max laden axle weights.

Quick google of Sprinters didn't bring anything helpful up so I expect it will be in the handbook.

I would think if you want a full scientific breakdown of how they arrived at the figures you'd need to ask Mercedes.

ETA FWIW out of curiosity I checked the figures for my car - '86 Range Rover Classic 3.5EFi. 205R16 tyres. Factory workshop manual gives 'Normal on & off road use. All speeds and loads'
Front axle: 1.9bar/28psi
Rear axle: 2.4bar/35psi
Which is what I've always run it at.

Made me wonder if that would apply to all estate type cars as well as vans.
Another quick google of Ford Mondeo estates showed higher - not by much - pressures on the fronts of all tyre sizes when unladen but higher on the rears when laden.
www.tyrerunner.com/find-tyre-pressure/FORD/mondeo

Edited by paintman on Monday 13th June 00:31