Tyre pressure anomalies

[tolsen]

Specified tyre pressures for 450 models do not seem to add up:

2002 or earlier petrol.
Front wheels (145/65 R15) 1,6 bar
Rear wheels (175/55 R15) 2,5 bar

2002 or earlier Cdi.
Front wheels (135/70 R15) 2,0 bar
Rear wheels (175/55 R15) 2,5 bar

2003 and later petrol and Cdi.
135/70 R15 front & 175/55 R15 rear 2.0 bar
145/65 R15 front & 175/55 R15 rear 2.0 bar
175/55 R15 front & 195/55 R15 rear 2.0 bar

I understand the 0.4 bar difference in tyre pressure for front wheels for 2002 and earlier Smarts. This is obviously due to different tyre width and profile, but makes no sense when examining tyre pressures for 2003 and later.

Can someone please shed light on the vast difference in tyre pressures. A 0.4/ 0.5 bar change is quite significant. Axle weights are practically the same for all 450 Smarts so that is not the reason.

I wonder whether the 2.0 bar specified for rear wheels is Smart sales dpt's attempt of improving ride comfort at the expense of optimum tyre wear and safety?

Any thoughts?

[Evilution]

Re: Tyre pressure anomalies

I think it's because of the leaf to spring suspension changeover and it took smart a while to adjust the tyre pressures to suit.

They probably thought that a narrower tyre needed a higher pressure.

[tolsen]

Re: Tyre pressure anomalies

I think it's because of the leaf to spring suspension changeover and it took smart a while to adjust the tyre pressures to suit.

They probably thought that a narrower tyre needed a higher pressure.

Thanks for your reply. I see the 451 has same pressure as mine, i.e. 2.0 bar front and 2.5 rear and vehicle axle weights are comparable.
I wonder whether those that ride their 450s with 2.0 bar in rear wheels suffer excessive shoulder wear? Ride is obviously greatly improved so I'll give it a try.

[sidle]

Re: Tyre pressure anomalies

when in doubt 34psi all round, works fine on my smart

[tolsen]

Re: Tyre pressure anomalies

I have come to the conclusion that there is no such thing as "correct" tyre pressure and that there is no science behind the published recommended tyre pressures. I shall equip myself with some special white Lidl paint. This to assess tyre wear at different tyre pressures. Can anybody confirm how rear tyres wear when inflated to 2.0 bar?

[J500ANT]

Re: Tyre pressure anomalies

I don't recall the 450 having one pressure front and rear, but IIRC the tyre pressures changed when ESP (mk7) came along, as did the spring rates.

[tolsen]

Re: Tyre pressure anomalies

I don't recall the 450 having one pressure front and rear, but IIRC the tyre pressures changed when ESP (mk7) came along, as did the spring rates.

As per handbook and info on Evilution the 700cc 450 does indeed have same pressure front and rear.

Have been out on a spin test driving my Smart Cabrio with lower air pressure in tyres. 1.6 bar on front and 2.0 bar at rear. There is a considerable improvement in comfort but no noticeable change in handling.

Obviously a softer tyre will be more comfortable as better at absorbing bumps and humps. A bonus is a modest reduction in unsprung mass:

Air density 1.3 kg/ m3 @ -2 C.

Front 145/65 R15
0.4 bar change in pressure
Tyre volume 6.49 litre
Unsprung mass reduction 11.8 grams

Rear 175/55-15
0.5 bar change in pressure
Tyre volum 8.04 litre
Unsprung mass reduction 15.7 grams

Consider above your Xmas puzzle. Verify if above calculated unsprung masses are right.

[Evilution]

Re: Tyre pressure anomalies

Thanks for your reply. I see the 451 has same pressure as mine, i.e. 2.0 bar front and 2.5 rear and vehicle axle weights are comparable.

Yep, same engine so same weight. The diesel engine is heavier than the petrol version.

[coolfart]

Re: Tyre pressure anomalies

I've always found around 30psi (2.1bar ish) front & rear seems to work quite well on mine, which is on the change-over year (2002 mk5 on coil springs), but running wider than standard tyres.

I have been told that the front tyres will always wear slightly more on the inside edges as the car is set to handle better (more toe out), but as I dont do a vast amount of miles replacing the rubber every 4 or 5 years isn't too much of a worry!

[problemchild1976]

Re: Tyre pressure anomalies

Plus you will get a fair bit of variation in pressure with temp (ambient and tyre temp depending on how hard you drive)

....unless you are running nitrogen

JJ

[tolsen]

Re: Tyre pressure anomalies

Plus you will get a fair bit of variation in pressure with temp (ambient and tyre temp depending on how hard you drive)

....unless you are running nitrogen

JJ

The combined gas law applies equally to nitrogen as it applies to air so pressure change with temperature will be exactly the same.

[Evilution]

Re: Tyre pressure anomalies

....unless you are running nitrogen

Normal atmospheric air is nearly 80% Nitrogen anyway.
Nitrogen filling tyres is a con. You can't remove all air from a tyre to fill it with Nitrogen so to fill with Nitrogen would only increase the Nitrogen by about 8% over standard air filled.

[problemchild1976]

Re: Tyre pressure anomalies

Did always wonder about that - so what's the benefit?

I thought it was something to do with the moisture content as well as the increase volume of nitrogen.

JJ

[tolsen]

Re: Tyre pressure anomalies

Yes Nitrofill is just another scam. Just have a look at the claims copied from the Nitrofill site:

Nitrogen in Tires
Increases Safety
Tire failures reduced by 50%
Tread life increased by 25-30%
Improve your fuel economy by up to 10%
Improves steering
Improves handling
Improves braking
Reduces chance of tire failure
Dramatically slows pressure loss from permeation
Improves fuel economy
Reduces tire oxidation
Eliminates interior wheel corrosion
Reduces running temperatures
Decreases false alarms and activation of Tire Pressure Monitoring Systems

[tolsen]

Re: Tyre pressure anomalies

Condensed Nitrofill is same as snake oil.