What tyre pressures?

[whitenemesis]

ok, here's a question for you (or anyone else actually) on this subject...I have the exact same car as White Nemesis, and am currently running 32F & 34R but have noticed that in the wet weather she likes to slip and slide a bit and I was going up the motorway the other day and she did a little aquaplane - which kind of freaked me out because in all the time I have owned ad driven cars I have only had that happen once and that time it was because I was driving too fast.

Sooooo my question is, I usually have two 6 year olds in te car and me and one other adult passenger - that's the average load, so do you guys recon my tyre pressures are too low - would that cause aquaplaning??

Thanks
Emma

I have never experienced aquaplaning in my current car and I've driven through some atrocious standing water.

I tend to run my tyres at slightly higher pressures but always within the recommended range.

Intrestingly my driver in Germany (a police persuit driver) always recommends I run at the highest pressures for stability and control. I tried it for a few thousand miles and yes it works but it tends to blur the vision!!

 

[Dieselman]

No, similar thing happens when over-inflated except you lose the edges of the contact area rather than the centre. Under-inflation is a MUCH more common problem, which is why they go on about it.

jadefox pressures are not over inflated. An underinflated tyre will build more water up in front of it and so will aquaplane, a harder tyre will cut through the dam better so will resist aquaplaning better.

The reference to overinflated tyres is really about the tyre having a reduced contact patch generally, which 34 psi (2.3 Bar) won't do.

jadefox, check the tread depth as that has a pronounced effect where water is concerned.

The real answer is that you were going too fast.

 

[Bobby Dazzler]

DM is spot on as always, you will aquaplane when the tyre can't dispel the surface water quickly enough. Nothing to do with tyre pressures.

Water travels through the grooves in the tread and exit at the side wall or more likely through the circumferential grooves. Hence when you follow a vehicle in heavy rain, water appears to shoot up rearwards at an angle from beneath the contact patch of the tyre - wider tyres demonstrate the point better as they must dispell more water.

This allows the tread blocks to be in contact with the road surface and hence grip is retained, albeit less than in dry conditions.

As your tyre wears the grooves become shallower, and have a reduced capacity to move water. The faster you go, the harder the tyres work to dispel more water it comes into contact with.

If the tyres reach the limit of how much water they can dispel, and there is still water standing on the surface, the wheel will rise above it meaning there's water between the tyre and the road surface. This is aquaplaning - a disconcerting feeling as you're no longer in direct contact with the road!!

 

[Dieselman]

Any chance of a detailed explanation, Bobby.?

 

[Bobby Dazzler]

Any chance of a detailed explanation, Bobby.?

I don't do detail. I'm a blue sky thinker...

 

[Uberwagon]

Perhaps the OP should consult his manual, and perhaps his MB dealer to obtain more advice than is printed on the inside of the fuel flap.

There is another cautionary note to sound. Many like to reduce the rear tyre pressure, and increase the fronts - for tyre wear reduction reasons. This is a very bad idea, as it moves the natural bias of the car more towards an oversteering characteristic**. This is hinted at on the Michelin web page posted above.

Peugeot 405 - Lower front pressures, raise rears = more oversteer
MK1 Golf GTI - Lower front pressures, raise rears = more oversteer
MK4 Golf GT TDI - Lower front pressures, raise rears = more oversteer

It's the same for the Merc, up to a point though.

I absolutely hate understeer in every possible way, although I don't like snappy oversteer either, i'd rather feel the car start to come round and reduce lock than feel it push wide.

My experiences....

I had a slow puncture on my Merc and round a long sweeping bend I had a long progressive slide, if it was on video it would have been amazingly good to watch, but this was very much luck, I never knew about the puncture until I eased into the bend and then felt the pogoing effect that the low pressure gives. By the time I was at the garage a couple of miles down the road after checking it, the pressure was showing about 10PSI.

This fits in with what they say for low rear pressure = oversteer, in my experiences on different cars it needs to be at the extremes in regards to pressure.

If I run my front tyres at 38psi (I have tyre OCD and at the time was single ) then my turn in is improved but ultimately understeer will happen sooner, if I raise my rear pressures and leave my fronts as normal then I again reduce understeer.

I found the difference more noticeable in a FWD car, my Peugeot was the most sublime car to do LOO in, incredibly easy to catch, easy to provoke, and by raising the rear pressures up high and dropping the fronts it was far far easier to do. In comparison my MK1 GTI was more a cse of going, going, GONE!

Despite all my fiddling (Which if you aren't silly isn't going to kill you) I run 34psi front, 36 psi rear pretty much in line with the reccomendation from Merc.

Dave!

 

[Bobby Dazzler]

The 205 was known for it's lift off oversteer - and lots ended up in hedges.

 

[BTB 500]

When I checked the tyre pressures on my Vito a few days after I took delivery they were massively over-inflated ... above 60 PSI IIRC.

In fact the stated tyre pressues have an enormous range:

Why the 3 psi difference (unladen) between 225/55 R17RF 101H and 101V ?

Mine has the H rated tyres (130 mph, which seems pretty adequate for a van) and 33 PSI seems exceptionally low for a vehicle of that size. I run them at 35 PSI (roughly mid-way between the light and heavy load figures). The same tyre pressures are given in the handbook btw, so it's not a mis-print.

It's also interesting to see that the official tyre pressures vary from 33 PSI (mine) up to 42 PSI, depending on the wheel & tyre. Yet I thought tyre pressures were meant to be the same regardless of this?

 

[Will]

Why the 3 psi difference (unladen) between 225/55 R17RF 101H and 101V ?

Mine has the H rated tyres (130 mph, which seems pretty adequate for a van) and 33 PSI seems exceptionally low for a vehicle of that size.

At a guess, I imagine it's because theoretically the V rated tyres are suitable for higher speed driving.

Whether or not you would choose to or whether the vehicle is even capable of over 130MPH I don't know, but I guess that's what they deem correct.
I guess it's not so much a case of V-rated tyres for speeds in excess of 130MPH, more a case that you'd fit the V-rated tyres if you were constantly driving at high speeds to give a greater comfort zone?

Will

 

[BTB 500]

Possibly. As an aside though there's no Vito which is capable of doing over 130 mph ... the top speed of mine is 122, and the 3.7 petrol V6 version does 125.

Mine came from the factory with the 17" alloys and H rated tyres. V tyres are rated to 149 mph.

 

[jadefox]

jadefox, check the tread depth as that has a pronounced effect where water is concerned.

The real answer is that you were going too fast.

Will check tread, thanks.....but DM.....too fast???!!!!!

LOL I never drive too fast what you on about??!

Actually in my defence, i was only going about 50 on a stretch of motorway which had this one dip of the road where the water was sitting, i did feel i was being careful going 50 - but the car still did a nice aquaplane for a second or two - was the most obscure feeling, even for me (who did skid pan training etc years ago)

 

[wheels-inmotion]

A huge topic so my intention here is to explain an overview not the entire package.... I think you will understand what i mean as you read this explanation.

For the domestic vehicle the standard "suggested" pressure is determined by the vehicles weight, the tyres aspect ratio and contact patch.

A predetermined internal pressure is suggested to compliment the tyres deliberate structure to enhance the handling properties desired by the manufacturer, tyre and car respectably.

So why the variants?

Air is not an efficient form of inflation!.... Within the composition of air some of the various molecules are susceptible to heat, in particular "water".

Since the holy grail of tyre technology is the transition of heat into the silica to accelerate traction properties the failure of air to maintain a constant value seems ridiculous..... But that's the way it is and the reason for this explanation.

Ok so what do we have then?..... Most manufacturing guides will offer three ranges
1> Normal pressure
2> Loaded pressure
3> High speed pressure

Why

As i have explained air is inefficient and the silica needs heat for traction, it's unreasonable to expect the owner to stop and change the pressure during any journey, although it is possible to predict how the air will expand and suggest a initial cold pressure to cover the fields above.

How
Manipulate the tyres contact patch.... All three fields have known parameters so it's possible to manipulate how hot or cold the tyre will be-become within those fields..

Pressure stages

If for example our imaginary car is intended for town use then the suggested pressure may be 28psi, this comfortably assumes 30mph and a full tyre contact patch with minimal temperature/pressure gains

If the same car was intended for motorway use an initial cold pressure of 34psi may be suggested to reduce the size of the tyres contact patch thus reducing the thermal internal expansion to a pre-calculated average level.

The loaded pressure is a harder topic
Since the guidelines for loaded "motorway" pressures is very vague the normal intention is to aid the tyres construction and maintain a full contact patch after the internal expansion.... It's also assumed the speed limit will be around the 70mph mark so centrifugal forces are expected to distort the higher pressure suggestion but the exact level is theoretical.

Non-standard tyres
Changes in the tyre width or aspect ratio will belay all guidelines offered by the manufacturer. It's reasonable to assume the replacements are not to distant from the OEM fitment so it's wise to adopt the original pressures as a template then read the handling/wear traits and adjust accordingly.

Reading the signs
Tyre wear due to inflation levels is easy to observe and correct (see image at top of page) but handling traits due to tyre modification needs a little more understanding.

A common modification is to go low and wide, so how do you judge the pressure now?

If the pressure is to high at the front the low aspect ratio tyre will have a tendency to "tram-line". This is due to the inflexibility of the tyres sidewall. If the rigidity is to high then the transient grip can no longer be absorbed within the sidewall and perpetually steer the car.

Additionally if the pressure is to low then the pneumatic slip angle will allow the wheel to transit between Yaw and the actual tyre contact patch. The effect is a numb, lazy feel to the steering.

Finally there is a need to balance the tyres footprint and sidewall rigidity front and rear despite the obvious weight disparities.. If for example the rear footprint is low due to high pressure then the car will over-steer...To much footprint then the opposite can be said

All examples can be manipulated by small changes in the cold tyre pressure, examples of "over-under-steer" by manipulation.

In summery
As you may have noticed i keep saying "suggested" pressures? The reason for this is that no manufacture can cover all examples, so it's up to you to decide the correct pressures for your own unique example