Low profile tyres and stability

[ProfJohnL]

Hello Bill,

Lets not get into the realms of truck tyres, as you say they are often inflated to 120psi. Which is 4 times the the norm for most domestic vehicles, so the comparison there is not quite fair.

Most vehicles have some kind of arrangement to try and limit lateral movement of the suspension, and I do not dispute that, but one of the tweaks made to high performance cars is usually much more resilience to lateral movement not only in the suspension , but also in the low profile tyres.

As I said I have no figures and it is gut feeling but I do suspect that the off road set ups that do require a large amount of articulation in the suspension will have a greater movement for a given force than many saloon cars.

To be fair, the Terrano is not rated highly as an off roader, so it may well have better lateral stability, which is partly confirmed by your towing reports. In fact it might be the sort of compromise that works well, having 4 wheel drive for those wet fields, but a stiff enough suspension to cope with higher road speeds.

 

[ProfJohnL]

Hello again Bill,

I have just re-read your first post, and your third paragraph is exactly the issue. If the combination of tyre and suspension allows the rear of the tow vehicle to 'squirm' then it is almost certain to be more susceptible to early onset of instability.

 

[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.

......you describe the drawback of a Panhard Rod located rear suspension.

You think this minimal lateral movement of a 4x4s rear suspension can affect the vehicle's stability under highway conditions .... and I do not.

You will only get the full bounce suspension travel when off road on rough ground.

I do not tow my caravan under those conditions!

 

[Gafferbill]

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.

......you describe the drawback of a Panhard Rod located rear suspension.

You think this minimal lateral movement of a 4x4s rear suspension can affect the vehicle's stability under highway conditions .... and I do not.

You will only get the full bounce suspension travel when off road on rough ground.

I do not tow my caravan under those conditions!

 

[Gafferbill]

Hello again Bill,

I have just re-read your first post, and your third paragraph is exactly the issue. If the combination of tyre and suspension allows the rear of the tow vehicle to 'squirm' then it is almost certain to be more susceptible to early onset of instability.

..........I was referring to the squirm that could be present in under inflated 4x4 tyres on the rear axle of a tow vehicle.

This I agree can cause a problem with 4x4s.... not least because they are often used to tow heavier caravans.

I still maintain that 4x4s suffer no more lateral movement of the suspension than any other vehicle under highway conditions.

 

[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.

You probably don't make full use of the suspension travel if you never go off road, but because the car is designed to be able to do so, the bushings must be also be able to accommodate the full movement and therefore require more compliance than a sports car, for example.

 

[Martin_374319553]

Like someone else mentioned earlier, I would think the extra tyre width had more of an impact than the lower profile. That wasn't a very scientific test changing 2 variables and then stating that 1 of them was the reason for the improvement. Also, only doing the test on one different tyre rather than using the same width and different profiles, different width and same profiles etc.

It's pretty obvious that wider tyres will offer better grip and therefore the rear of the car will be less susceptible to the van pushing it around.

The only conclusion you can draw from the report is that wider and lower profile tyres are better for towing. But then how far do you go?

 

[ProfJohnL]

Hello Martin,

The width of the tyre has very little to do with the side wall compliance and its ability to resist lateral movement by the side wall flexing.

In fact the amount (area) of tyre tread in contact with the road is controlled by the vertical load on that particular tyre and the air pressure within it.

It is also likely that the inflation pressure in a wide tyre will be less than for a narrower profile to keep the length of the tread foot-print correct.

.

 

[Guest]

I am livied to read such rubbish as this.

"euro

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."

I lost my son on the 15th September 2006 age just 19.

he was a passenger in a peugoet 306 .

At the inquest it was reported that two of the tyre were indeed over inflated by as much as 6 psi!and still the police officer said this would not have played a part in the young man losing control!

Where do you get your facts from!

 

[Lutz]

The police officer was right. Loss of control would not be attributed to overinflation, assuming that the tyres were otherwise in good condition, with adequate tread.

 

[Royston]

Ray mentioned blebs on P6000 tyres, what are these, why I question this is because I have pirellies on all 4 axles, early on this year I experienced a drop in air pressure on one of my rear tyres , taking the car in for a slow puncture test, i was informed that I must replace the two rear tyres only, as they were out of round, the Vectra air presures listed within the vauxhall booklet for my model are 32 psi all round normal usage, when towing increase rear pressures to 38 psi. Only used p6000 on this car as they were the on the car when purchased.

Anythoughts as to why this has happened , or was I taken in by a hard up tyre fitter.

Royston

 

[xtrailman]

Royston

I used the word bleb to describe what looks like a bubble in the tyre side wall, or balloon if you like, not sure what the correct name is for it.

I have had pirelli tyres on other cars, and had no problems, only on the 45 profile tyres. In my opinion this low a profile along with the VAG sports suspension, is totally unsuitable to our appalling roads, while the car would corner at any speed you chose, the ride was very hard to the extent that i avoided pot holes and road humps.

Obviously any deformation of the side wall is dangerous and the tyres were replaced, at a cost of

 

[ProfJohnL]

Royston,

It is difficult to second guess the tyre fitters meaning, or the cause for a fault if it really did exist.

Sadly from experience I don't trust just the verbal say so of most tyre companies;

I had a puncture on the way to a job. Being able, I changed the punctured wheel for the spare. Fortunately there was a high street fitter a few hundred yards from where I was due to be, so I left the car with them whilst I met my client.

During the morning i received a call from the fitters, to tell me that I needed two new tyres. I asked why, I was told the puncture was through the shoulder of the tread and couldn't be repaired and another had a bulge on the the inner side wall. Believing what I was told I agreed but added I needed to see the tyres when I pick the car up..

When I got there, they had only changed the punctured tyre, I asked why had the only changed one tyre, and the fitter said we looked again and it was actually aright. in fact there was not even the slightest evidence of a bulge.

It does make you wonder.

Subsequently I always insist on seeing what the fitters are claiming.

 

[xtrailman]

Trust no one, that's my philosophy.

Audi garage phoned while the car was being serviced.

"You have a bleb on the front nearside tyre sir, on the inner wall".

I asked what a bleb was, they explained. "Would you like the tyre replaced sir, (cost

 

[Martin_374319553]

John, I never said the tyre width had anything to do with side wall compliance or it's ability to resist lateral movement.

What I said was a wider tyre would have more tread in contact with the road and would therefore offer higher levels of grip. You can't denied that is true.

I was saying that increasing the tyre width as part of this test would have a positive impact on the results therefore how can they possibly say that it was purely the stiffness of the side walls that gave the improvement? The bigger diameter wheel would also have an effect.

What would actually be a much better test would be to use the exact same size and make of tyre but one in run flat format and the other non-run flat. The only difference then would be the stiffness of the side walls. Tyre size, width, pressures etc wouldn't come into it.

 

[Lutz]

Instability is not a result of poor grip, but a complex interaction of speed, weight, height of centre of gravity, damping, polar moment of inertia, and grip.

A good grip will not prevent sway because the other factors still remain. On the other hand, a poor grip would tend to result in a skid before encountering a sway.

 

[smiley]

I don't know about all the tech stuff but we've had low profiles for 6 years now. They tow at high speeds with no bother. That is at what ever speed you tow at.......

 

[ProfJohnL]

Hello martin,

Well Actually I do deny your contention that a wide profile tyre puts more tread on the ground. It all depends on the air pressure inside the tyres to how much tread is forced into contact with the ground.

If you have a wheel that has a vertical load on it of say 660lbs (300Kg) and you then inflate the tyre to 30psi the tyre will deflect until there are 22 square inches of tread in contact with the road (22x30=660) If you change the tyre to a wider profile, and inflate it to 30psi, you will end up with exactly the same area of tread in contact with the road. the only way to increase the contact area is to reduce the inflation pressure.

Whilst the contact area will be the same, the profile of the contact area will change, and the clever design of tread pattern may help with the tyre to surface grip.

Back to the point of the thread, It is more likely the profile of the tyre (and the result side wall flexing) that caused the differences in resistance to lateral oscillation.

 

[xtrailman]

If the tyres had the same 205 or 235, then the tests would have some meaning, as it is they prove nothing.

Theirs an old yorkshire saying, compare apples with apples.

 

[Gafferbill]

......yes and the test would have to be conducted on exactly the same strip of tarmac with exactly the same weather conditions, wind speed, wind direction etc.

There would have to be at least 10 runs for each set of tyre equipment and the results averaged, for any meaningful comparison to be made.

I would also expect the critical speed-reading to change on every run further complicating the comparison

 

[Lutz]

Well, at least what the magazine was trying to confirm makes sense. That's good enough for me although, strictly speaking, Gafferbill has a point if it were meant to be an in-depth scientific investigation, which I don't think it ever claimed to be - just a hint that the car's tyre equipment can appreciably influence stability of the entire outfit.

 

[David_from_Ashby]

Hi all

I have come in to this conversation rather late, but I think a simple summary is in order (from a practicing vehicle dynamics engineer)

In general

- lower profile tyres will most likely offer better stability than higher profile tyres due to less sidewall flexing

- Wider tyres will offer more grip than narrow tyres on dry roads but potentially less grip on wet or damp roads due to lower contact pressures

- increasing tyre pressure will increase the lateral stiffness of a tyre carcass but may also reduce the area of the contact patch, therefore lessening grip, particularly in the wet

- Same size tyres from different manufacturers will not behave the same

- Same tyres on different tow vehicles will not behave the same when towing as the specific suspension characteristics also need to be considered

What does this mean for YOUR combination? A difficult question to answer, but certainly one that will depend on the type of tow car, the caravan, the loading condition of each, the weather etc

From reading the summary of the magazine test posted here, it seems that the magazine has attempted to show that tyres can make an appreciable difference to stability, and whilst I have to question the logic behind reducing the nose weight to an unrealistically low 15kg, the article provides interesting results that should make us think...

The car and caravan in question (not an unreasonable combination) is demonstrating instability with both sets of test tyres at speeds withing the speed limit for towing on a motorway in the UK. How many people regularly tow a caravan at speeds that could be very close to their combinations critical speed?

David

 

[Dustydog]

David

That's fasinating.I have to bow to your and Lutz superior knowledge .

You said

"How many people regularly tow a caravan at speeds that could be very close to their combinations critical speed"?

Where do I find or how do I calculate the critical speed for my outfit?

It's not listed in my Kia or Bailey handbooks but I guess it must be a lot higher than the current motorway towing limit of 60mph? Or is it a lot closer than I think??

Cheers

Dustydog

 

[Lutz]

The University of Bath study revealed that the critical speed for most outfits is generally round about 55mph. In other words, at motorway speeds it would not be possible to recover from instability withoutout some sort of intervention, either by the driver or by technical means (electronic stabiliser).

Setting the noseweight very low for the purpose of the test described above was a deliberate measure to increase sensitivity, thereby making the results more obvious.

For the same reason, I have never understood why magazines persist in testing outfits at no more than 85% weight ratio although the towcar may very well have a higher permissible towload. It would make more sense to test at the limit or, say, 100%, whichever is the lower. That would make any shortcomings much more obvious, especially as plenty of people choose not to abide by the 85% recommendation. It would be better to know how the outfit handles under more challenging conditions. If it's good at 100%, just think how much better it would be at 85%.

 

[Gafferbill]

.. David in his excellent post has stated the many variables (not least tyre equipment!)

.. so Dusty the Critical speed will vary all the time due to these ever-changing factors.

All you can say is the lower your towing speed the further you are away from instability.