Low profile tyres and stability

[Lutz]

The November issue of the German 'Caravaning' magazine includes a feature article on a comparison test to determine if towcar tyre equipment has any measurable effect on stability of an outfit. The result may be interesting to readers of this forum so here's a short summary:

Test car: VW Passat Estate 2.0TDI

Tyre equipment (base condition): Michelin Primacy HP 205/55R16 on 6.5Jx16 wheels

Tyre equipment, modified for comparison test: Michelin Primacy HP 235/55R17 on 7.5Jx17 wheels

Caravan: BÅrstner single axle 1800kg MTPLM, loaded to 1600kg and with noseweight adjusted for the purposes of the test to 15kg (no typo error!) Caravan fitted with a Winterhoff WS3000 stabiliser.

Result:

Critical speed of outfit with the standard tyre equipment: 84km/h (52mph)

Critical speed of same outfit with low profile tyres: 92km/h (57mph)

The critical speed is the maximum speed from which an outfit will recover from a snake on its own without any intervention by the driver or by electronic means (where fitted).

Conclusion:

As can be seen from the results, the low profile tyres, although not that much wider than the standard ones, raised the critical speed by 5mph. Hence, wider low profile tyres certainly do have the opportunity to improve stability and that further improvements could be achieved by deviating from the standard tyre equipment even more than in this test.

What the article does not mention, however, is whether the same improvement would also be achieved on a wet road surface.

 

[xtrailman]

Lutz

from the figures provided, the overall diameter of the two wheels will not be the same, normally if going low profile, the rim is larger in diameter, not the same. So the cars gearing, and wheel arc clearance will be reduced?

Anyway from my own experience of low profile tyres, that's 225/45/17. I found the ride solo extremely uncomfortable, the car tram lined on the motorway lorry ruts, and this resulted in a very unsettled tow.

In addition, the p6000 (if i recall correctly), were prone to tyre wall blebs, over 4.5 years of ownership i had two tyres replaced due to these blebs, i have never had a tyre with this problem, before or since.

My first xtrail had 215/65/16 st20 dunlops, i was really happy with these sensible tyres, and had a comfortable, and stress free tow, and solo ride.

For some reason, presumably pandering to fashion, the new xtrail has 215/60/17 st20, the car has given me stability problems, which i didn't have on the previous car, towing the same van, extra nose weight has helped that situation, the new car tows nose down, the previous model was level, this may be a factor.

But anyway the end result with the 60 profile over the 65 previous, is a poorer tow, NOT an improvement.

Personally i do not like low profile tyres, and neither do tyres fitters!

 

[Dustydog]

Lutz

I always thought the tyres were an integral part of the suspension design?

We deliberately chose our current car without low profiles purely because those on the previous car had a propensity to puncture as soon as the maimsahib hit the curb parking.

Cheers

Dustydog

 

[Lutz]

Sorry, a typo error did slip in there. The low profile tyres were 235/45R17, not 235/55R17, so the overall diameter of the wheels were the same as the standard ones.

The explanation why low profile tyres should be better is very simple; they have a higher lateral stiffness because the sidewalls are narrower (less rubber to deflect).

 

[Guest]

Surely the wheels in Lutz's example are different diameters'

16 and 17 are different diameter wheels are they not?

Does the overall wheel size and tyre Radius not stay the same. If you stiffen up the tyre wall and cut lateral movement stability will be improved.

You don't need a German test to tell you that, it just confirms common sense knowlege! Most of the time the tyres stiffness will make little differaence but at times it could be a positive factor.

Wife and I have matching cars, mine has lower profile tyres but the overall diameter looks the same. But I've never had a tape out to check for sure!

 

[Lutz]

Yes, Euro, a 45 aspect ratio tyre on a 17" rim will have roughly the same overall diameter as a 55 aspect ratio one on a 16" rim.

Nice to know that the findings are common sense to you, but perhaps not to everyone. Maybe others had doubts and welcome the fact that test results confirm theory. That's why I thought I'd share the report with the forum.

 

[xtrailman]

Perhaps the improvement is more to do with the extra 30 width?

The test could have used the same width? and then try different tyre heights?

 

[Gafferbill]

Quote fro above

......."The explanation why low profile tyres should be better is very simple; they have a higher lateral stiffness because the sidewalls are narrower (less rubber to deflect)".

The same result can be achieved by raising the tyre pressures of any tyre.

Ray's newer X Trail may need higher tyre pressures on the back axle?

 

[Lutz]

Yes, Gafferbill, raising the tyre pressure will basically have the same positive effect. I can't say whether it would be as much as what would be achieved with low profile tyres, though.

 

[xtrailman]

I am already using max pressure at the back axle, 38psi. But the max for the front is 41psi, however on the front axle i am using again 38psi.

Do you think 41psi at the front would make a difference?

The reason for the 38 all-round is due to the fact that i don't reduce the pressures while solo on holiday, as 41 at the front makes for a hard ride, two up.

Hence the reduced pressure at the front. Other than that i would have to mess around reducing pressures on arrival at site, then pumping up again when departing.

 

[Gafferbill]

....... as I thought Ray you already know about raising rear axle tyre pressures........ hence my question mark.

I also run at 38psi whilst towing with a Nissan Terrano and do not change when solo whilst away.

 

[Lutz]

It is unusual to have a lower maximum tyre pressure at the rear than at the front, especially when the car is fully laden, but I assume that you have checked the figures correctly. Raising the tyre pressure at the front will probably do next to nothing to improve stability of the outfit (this is best left to increasing the rear tyre pressures), but it could make a difference on how the car handles when cornering.

 

[Guest]

Higher tyre pressures are not an equal for lower profile tyres.

What is the manufacturers guide range for the X trail?

Surely if your tyres are not within that range the tyres are not legal.

I don't think playing with tyre pressures apart from being within the manufacturers guide range is legal for road use.

 

[Lutz]

So long as you do not exceed the maximum safe design tyre pressure shown on the sidewall there is nothing illegal about increasing tyre pressures beyond what the car manufacturer has recommended for the respective loading condition. A higher tyre pressure will only compromise vehicle ride comfort and tyre wear in the centre of the tread, but will not jeopardise safety in any way. However, as you say, Euro, it will never produce results to the same degree as low profile tyres, although one can argue that every bit helps.

 

[Guest]

I'm not sure how the Law in the UK works now. The cases I refer to were some years ago, in one accident tyre were found to be underflated on two of the cars tyres and one was over inflated.

In the other accident a family freinds step daughter died with her partne, the intact tyres were 3-4 psi above the car manufacturers guide. The police accident report blamed the drivers driving in bad weather and the wrong tyre pressures on the wet slippery road.

My understanding is that if your tyres are outside the car manufacturers guide line for the car the insurer may not pay out and you could be liable for the accident for using the wrong tyre prssures.

 

[Lutz]

I specifically referred to increasing tyre pressures. I was not talking about underinflating them, which is a potential safety hazard and would be rightfully prosecuted. The same applies if there is a large difference between tyre pressures on the same axle. Overinflating just reduces tyre life and makes the ride uncomfortable, but it is not safety related.

 

[Sedo]

I can't really see the point here, are you saying we should change wheels and tyres to a low pro set-up to facilitate a more stable rig?

I would have thought too high a pressure would compromise grip to a degree, or tyre manufacturers would have an at least pressure setting, e.g. at least 30p.s.i.

Wouldn't your pressure values be more to give optimum grip with load?

Yes reaction to lateral forces against steel braced side walls are bound to differ due to profiles and pressures and tyre make/counstruction, but as someone's already stated there is a

an optimum for comfort of ride and steering (steering often becomes reactive with low pro's).

If I read correctly certain car makers have already had bit of a caning over their tyre/suspension set/ups.

I would think it's better to keep it in mind if you're changing cars or just drive within your ste-ups comfort zone.

By the way yes I have changed standards to low pro's, altered suspensions and been lucky enough to go around a test track to see how different tyres perform, my Sedona TS has already got the bigger wheels fitted as standard but theres no way it's having lreal low pro's.

Tom

 

[Lutz]

The recommendation wasn't meant to be carried to extremes. It only wanted to point out that lower profile tyres would be better for stability. However, they also have their disadvantages, too, mainly in ride comfort and grip in the wet, so like everything else, it's got to be a compromise.

 

[ProfJohnL]

I am quite pleased that someone has started this thread on tyres.

I have long suspected that the profile of a tyre wall will affect the wheels ability to handle lateral forces, and I also suspect this could be a significant part of why particularly larger off-road 4x4's seem to have more problems when towing at speed compared to other vehicles. The necessary large compliance in the tyres and suspension needed for off-road ability, coupled to the relatively high centres of gravity, and the inherent weight of these vehicles is bound to compromise the handling at higher speeds. Add the effect of the trailer/caravan, and the dynamics begin to add up.

The very nature of the high profile tyres means there is going to be less resistance to lateral side loads so there is likely to be far more movement of the rim compared to the tread. Additional pressure will help to stiffen the tyre but not to the same extent as a lower profile and tighter control of the suspension.

I have no figures to support this yet, but it is guess that where the bow wave of another vehicle that passes you starts to push the tail of the caravan sideways, the tow hitch will be forced in the opposite direction. The scale of the force at the tow hitch I am guessing could be in the order of 100 to 200Kg. If anyone has actual figures please post on the thread.

The other condition where this type of side loading can occur is when turning AND braking. The overrun brake system relies on the fact that that the tow vehicle begins to ****** and for a moment the caravan is travelling faster than the car. This causes the caravan to try and overtake the car, but as the caravan closes the gap the to the car the overrun brake is compressed and that applies the trailers brakes, beginning to ****** the trailer. This does involve the caravan having to push the car just a bit, enough to compress the overrun brake mechanism.

Whilst the car and trailer are in a straight line, there is no problem and the cars brakes easily accommodate the small additional force, however if there is an angle between the car and caravan, some of the caravan push develops a lateral component on the cars rear wheels. This means that to achieve the necessary retardation of the caravan, it has to push harder than if it were travelling in a straight line.

 

[Sedo]

John as an apprentice in the engineering trade many years ago, one the many enjoyable task performed was the pulling and pushing of quite heavily laden 2 and 4 wheels trucks.

If you want a basic idea of what forces are involved in towing it's well worth a try.

All this is going on, on 6-8 wheels supported by technical bags of compressed air.

Tom

 

[Gafferbill]

JohnL.. you are surely not suggesting that a manufacturer of a 4x4 will design a suspension set-up that will allow any significant lateral movement.

Yes there is a greater operating range of vertical movement.but virtually no lateral movement.

My Nissan Terrano has a solid beam rear axle that is solidly connected diagonally to a ladder frame chassis by a 30mm tie rod.

The only movement laterally is within the stiff rubber bushes and is minimal.

Any uncontrolled lateral movement of a rear axle in any vehicle will render that vehicle undrivalable and would be very disconcerting for the driver especially at motorway speeds.

Yes 4x4s have high profile tyres and are less tolerant of under inflation especially when towing because the tyres will be more likely to squirm.

Inflating them to a higher pressure when towing easily solves this.

My Nissan Terrano tyres each have a max load rating of 950kgs at a maximum pressure of 44psi.

I tow at 38psi tyre pressure.

Remember that most trucks have high profile tyres, which can often be inflated to 120psi to cope with the load and prevent tyre squirm.

If indeed 4x4s do have more problems when towing.. then it is more to do with the drivers over confidence, which can be imparted by this type of vehicle

 

[Lutz]

Please let me throw in a couple of words in John's defence here. 4x4's normally have more suspension travel in order to cope with off-road conditions. As soon one wheel is deflected, the whole suspension system twists relative to the chassis mounting points in order to accommodate this movement. The larger the travel, the more it twists. The only way that this freedom of movement is allowed is by providing enough compliance in the bushes. Hence, if the bushes allow a lot of travel they necessarily allow a lot of lateral compliance as well. Even if they only deflect a couple of degrees that can make a big difference in sideways movement at the other end of the suspension arm.

 

[Gafferbill]

... On the contrary Lutz

On the Terrano no lateral movement is allowed by the suspension design as I tried to explain previously.

A substantial tie rod positioned just aft of the rear beam axle prevents lateral movement of the rear axle.

One end is attached to the beam axle just behind and inboard of the rear near side wheel.

The other end is attached to the ladder chassis just behind and inboard of the rear offside side wheel.

Both these attachments use stiff rubber bushes and it is these two bushes that allow minimal movement that is of no consequence.

Your description would be correct if this tie rod was not present.

 

[Lutz]

Even the tie rod does not prevent lateral movement of the axle relative to the body. Because it describes an arc as it deflects from full bounce to full jounce, the axle will move laterally as the suspension goes through its travel. The more suspension travel, the more lateral compliance, and the bushes have got to allow for this. There's no way one can violate the laws of physics.

 

[ProfJohnL]

John as an apprentice in the engineering trade many years ago, one the many enjoyable task performed was the pulling and pushing of quite heavily laden 2 and 4 wheels trucks.

If you want a basic idea of what forces are involved in towing it's well worth a try.

All this is going on, on 6-8 wheels supported by technical bags of compressed air.

Tom

Thanks for your comment Tom. What you say may be true but my point is concerning the normal 4x4s often used by UK towers.