Tyre pressures

[phil Dean]

Phoned John Brown wheels to ask about tyre pressures on my 51/2 “ mini lite copy wheels with 195 Yokohama tyres fitted new to my car when I bought it last year. They said look in the manual it’s got nothing to do with tyre width but car weight but the manual says varying pressures dependant on tyres fitted from20 front 24 back  to 24/28 and they are all based on 165  or cross plies  anybody running similar wheels and tyre combinations I wondered if they could advice me  

Phil 

Edited January 31 by phil Dean

[Andy Moltu]

I run my 195/65 Bridgestones at 27 all round.

Triumph opted for lower pressures at the front to give a little understeer. But that was on 165 tyres with 1960s construction and rubber. Even the modern equivalents of the original sizes may benefit from alterations in pressure to suite how you like the car.

Within reason there is no right or wrong, just better or worse for you and the tyres you have.

[ianc]

I ran 195/65s on 5.5J steel wheels from 1993, using 26 front and rear as suggested by Neil Revington, and was happy with that.

Have recently changed to 165/80s on 5" alloys, using 29 front & rear as suggested by Tony Sheach, but too early to comment further other than that my speedo now reads corectly because the overall diameterof the wheel is slightly larger (and as it was in 1962).

The suspension on the Works' TR4 Rally cars is modified at both fron and rear, so these pressures may not applicable for a standard road TR4.

Ian Cornish

Edited January 31 by ianc

[BlueTR3A-5EKT]

6 minutes ago, ianc said:

I ran 195/65s on 5.5J steel wheels from 1993, using 26 front and rear as suggested by Neil Revington, and was happy with that.

Have recently changed to 165/80s on 5" alloys, using 29 front & rear as suggested by Tony Sheach, but too early to comment further other than that my speedo now reads corectly because the overall diameterof the wheel is slightly larger (and as it was in 1962).

The suspension on the Works' TR4 Rally cars is modified at both fron and rear, so these pressures may not applicable for a standard road TR4.

Ian Cornish

Ian, does 4VC have suspension anti roll bars front or rear?

If that higher pressure all round has been found to retain confidence I shall try it, as the steering is quite heavy with low front tyre pressures

I have removed the only anti roll bar from the front of my car and adopted 24 front and 30 psi rear using 165\80x15 tyres on 5 1/2” rims.  My logic was that I need the front to break away first not the rear.   .

[phil Dean]

Thanks for your input I was thinking maybe 26 and see how it feels

Phil 

[Motorsport Mickey]

"Have recently changed to 165/80s on 5" alloys, using 29 front & rear as suggested by Tony Sheach, but too early to comment further other than that my speedo now reads correctly because the overall diameter of the wheel is slightly larger (and as it was in 1962)."

... good suggestion, monitor it and see.

 

"I have removed the only anti roll bar from the front of my car and adopted 24 front and 30 psi rear using 165\80x15 tyres on 5 1/2” rims.  My logic was that I need the front to break away first not the rear. "

...good logic, monitor it and see.

Shows what a mares nest tyre pressures are !  Almost all the TRs on the road these days are different too each other in specification of Springs, shock absorbers and anti roll bars whether at each end, one end or not at all. Why would anybody think that the tyres on all these different cars should be at the same pressure. ? These cars are no longer factory spec showroom condition, and the tyre pressures should evolve to match that.

More thoughts for you, anti roll bars control the roll of the car by stiffening the across car weight transfer, whether at the front or the rear, this trades off grip and changes the handling making it better from one end of the car to the other. This is good, making handling better means the car can be driven faster, (even if still within the speed limits and safely) cornering better and being able to place the car more accurately on the road. 

As a rule of thumb if you drive your car too fast into a corner the manufacturers want it to understeer...(turn the wheel and the car goes straight on) it's safer, and doesn't require any skilful driver input, eventually the car will scrub off the excess speed at the tyres and the steering input will be answered. Whereas any specification that makes the car "tail happy" (oversteer) is avoided, it normally causing cars to spin out into unforgiving scenery or oncoming traffic, a source of law suits claiming unsafe cars. As a given, a solitary front anti roll bar will change the manufacturer ...understeer, into UNDERSTEER, it causing tyre patch deformation on the outside front wheel by weight transfer to worsen, and lose the tyres grip.

Those of you in front of my typing will now be saying ...aha what controls the tyre deformation of the carcass, ? ...tyre pressure. So a crude short cut to better handling would be increase the front tyres in increments of 2 lbs until the carcass stiffens enough not to deform and give enough grip to answer the steering. Alternatively by worsening the rear axle tyres grip (it's grip is exceeding the fronts, hence causing the understeer) more tyre deformation can be caused by removing tyre pressure by increments of 2 lbs, worsening the grip at the rear and handing it back to the front axle. Ideally we fit anti roll bars to front and rear, and by varying the thickness of the anti roll bar, or it's activatiing arm length, (longer is weaker, shorter stronger), we tune the cars handling so that driving through the corner the car can be driven on the throttle, only then are we limited by the tyres. 

Now when many of us started racing we soon found that standard road tyres would likely be pumped up on track work to maybe 40 lbs or thereabout, this helped stiffen the carcass and prevent rolling the tyres off the wheels (possible) and also we could crudely "tune" the handling of the car by balancing the front to rear end grip...which is all what handling is on a car. However tyres and their compounds do not give of their best grip at all pressures, too hard and the tyres instead of slightly deforming and gripping, become concrete like and just roll the tyre material off the surface, loosing grip. As said above, too soft and rolling off the rim and tyre deformation occurs, and grip is lost, so the "Goldlocks" area for temperature gives the best uniform grip in use, and is avidly searched for. When practicing for the race, tyre temperatures taken swiftly will show how the suspension and tyres are working, too hot on the tyres outside edge (the tyre tucking under and the pressure is low, too high in the middle and the pressure is too high, a uniform (within 2 or 3 degrees) temp across the tyre and ! ...please can I have some more porridge !

Now driving on the road should be careful and not cause danger to anybody, but finding out what pressures work best by experimentation, will give you the most satisfaction for the drive, and crudely placing the back of your hand against the tyre tread across the width gives an indication (humans are supposed to be able to judge within about a 1/2 degree on comparison tests) as too whether the tyre has about even temps across it. 

Mick Richards

 

 

 

Edited February 18 by Motorsport Mickey

[keith1948]

2 hours ago, ianc said:

Have recently changed to 165/80s on 5" alloys, using 29 front & rear as suggested by Tony Sheach, but too early to comment further other than that my speedo now reads corectly because the overall diameterof the wheel is slightly larger (and as it was in 1962).

Ian 

My speedo reads high with 24/28 front/rear pressure, 165/80 tyres. Would increasing tyre pressure to 29 make the speedo read lower and closer to what it should? Speedo has 1184 on dial.

Keith

[phil Dean]

Thanks for all your interesting answers I have set the tyre pressures at 26psi. The problem is I don't drive the car enough I always seam to be working on it. I have a few events planned for this summer and over longer miles will try the back of the hand test. I have never raced, but navigated in the past in mostly road rallied something I took up with again last year so I cannot comment on tyres under extreme load generated by high cornering speeds on a race track. However I am a keen cyclist and I have learnt alot about tyre pressures over the years. On My mountain bike for example if I know we are doing a lot of climbing and descending on rough uneven or muddy surfaces I will set the front tyre at 23 psi it helps turn in, something I have learnt with  pain over the years with wash out happening due to lack of grip  On the back I will run at 30 psi ok it doesn't always give me all the grip I sometimes want when climbing but I like the back end to be a bit loose when descending. On canal paths and trails I tend to run 30 on the front and 40 on the back less rolling resistance and faster speeds.  Road riding is probably a better comparison in the winter on the front 85 psi on the back 90 slightly lower pressure promotes heat on colder days and a bit more grip hopefully. in the summer it's 100 on the front and 110 on the back on the same road but 15 degrees warmer you can feel the tyres start to grip in the bends. 

My only other comparison is as a navigator in a trials car were tyre pressures are everything. we can run pressures as low as 8 psi not bad on a car that weighs 1.5 tons especially as 1 ton of that is on the back. sometimes if we go up by just  2 psi we might loose traction and therefore not clean the section other times if its really muddy we drop to 6 psi but usually results in a puncture. So every event we do we make a note of the tyre pressures we have used and what the conditions were like wet, dry, rocky, muddy etc and refer to our notes the next time we take part in that event, sometimes if its a fast section we run at 12/15 psi to help the handling of the car.

So to me tyre pressure can ultimately have a big effect on how a car handles and feels similar to Mick's diagnoses above.  

 

Phil.

[ianc]

The Works' TR4s have an extra full length leaf in each rear spring, which is helpful in that if a leaf breaks, the axle does not collapse onto its bump stop, as happened to my TR2 when the right rear spring broke.

When Neil rebuilt 4VC for me, he found that  the one of full length right leaves had broken, but I had been unaware as everything remained clamped together - which shows the sytem woirks!

The extra leaf int the rear springs means that the rear is VERY fiim, and I think it acts as an anti-roll bar.  The front springs are competition spec and I got Neil to make a special anti-roll bar for the front in 1993.  The car handles well, as Neil can confirm.

Ian Cornish

[BlueTR3A-5EKT]

My own car is fitted with the very same factory uprated extra leaf springs at the rear.   I also have a spare pair in the rafters of the garage ‘just in case’.   Similarly I have the factory spec uprated front coils.   Yes it is all quite firm.

Edited February 1 by BlueTR3A-5EKT

[John Mellor]

Hi folks, some interesting comments here but for the plodders like me who rarely feel much in the way of over or understeer it's the appalling road surfaces that dominate the pleasure in driving the car. Too high and one's fillings come loose due to potholes - too low and slushy feel to the car.

Bog standard suspension and 28 all round was wonderful on the generally fine Spanish road surfaces. Here in Essex 24 all round helps the ride on our awful pot holed roads

Regards. John Mellor

[monty]

5 hours ago, John Mellor said:

Hi folks, some interesting comments here but for the plodders like me who rarely feel much in the way of over or understeer it's the appalling road surfaces that dominate the pleasure in driving the car. Too high and one's fillings come loose due to potholes - too low and slushy feel to the car.

Bog standard suspension and 28 all round was wonderful on the generally fine Spanish road surfaces. Here in Essex 24 all round helps the ride on our awful pot holed roads

Regards. John Mellor

Very much agree John! Not much better here in Gloucestershire! My classic driving day is often spoilt by the disgraceful road surfaces & am sure the car would be used a bit more given decent surfaces. I guess we just have to grin & bear it but the god awful politicians will be about shortly promising pot hole repairs no doubt as they did last time in the local elections! Back to tyre pressures I use 24 on the fronts & 26 on the rears. Minilite 5.5 wheels with 195/65 tyres on my 4A.

[MadMarx]

I guess this is not racing stuff....Then I would recommend  something between 30-36PSI.

That should give a crisp feel of the car.

I never was happy with low tire pressure on radials. My Porsche f. e. has 30 on front and 36 on the back.

My TR4 30 psi all around.