GT6 Tyre Pressures

[Taggart65]

Afternoon all , just bought 1971 Mk3 GT6 a few weeks ago and was wondering what pressures people run the tyres at .

It's been fitted with Uniroyal Rainexpert 155/80 on 13" rims which I think are the original wheels for the car . 

Being used to driving something a bit more modern i'm finding the ride to be rather harsh , feeling every little hump and bump that I hardly notice when driving my other car 

 

Edited August 17, 2023 by Taggart65
Adding picture

[Pete Lewis]

modern tyres need higher pressures than whats in the manual , many add around 6 psi but throughout the range pressures have changed a lot 

on reading  some say 22 some say 26  depends on age and model/ year   you should attempt to keep the front rear balance 

its a bit of trial and  error to find whats nice

these older cars use the tyres as a high proportion of ride quality   a modern has silly tyres and relies on the suspension a lot more 

heavy steering can be you have a small handwheel  or low trye pressures 

your tyre size is spot on compared to the original spec 

Pete

 

 

[JohnD]

oThe "correct" tyre pressure is very a down to your driving style.     Two ways to check:

1/ Quick.   Go for a test drive ( a track day is best)  And as quick as you can, measure the tread temperature with an infrared 'gun'.    Lok for even temp across the tread.

2/ Slow.  Note tread wear with a tread depth gauge every week.    You might need several months to get an  answer, but again you're looking for even wear across the tread.

Hotter, or more wear in the middle, pressure too high.    Heat or wear on the shoulders, too low.

John

[dougbgt6]

The majority of us will, eventually, be changing the tyres because of age rather than wear. So it's trial and error for the best ride comfort, I've settled on 30F 28R. But, the thing that stopped the involuntary teeth chatter for me was new shock absorbers.

Doug

[68vitesse]

I swop a dash cam between a modern and a Vitesse, the default G setting protects all the files when in the Vitesse but none in the modern.

It just sees the ride in the Vitesse as a series of shunts, l run my tyres, standard size, at the Triumph recommend pressure, never had a problem with the wear pattern.

Regards

Paul

[Stratton Jimmer]

My 1971 Mk3 has 165/13 fronts and 175/13 rears which I set at 25psi all round. The ride is pretty good and the handling too. As I have said before, the plan is to fit 165/13s all round.

[Pete Lewis]

the general replacement to be close to the original  155/80 /13   would be 175/70/13  as used by the many 

Pete

[Nigel Clark]

Based on over 20 years ownership, I've found 26psi front and 28psi rear best on my 1972 Rotoflex GT6 Mk3. It's running 175/70-13 tyres on 5.5" rims.

Tyre west is quite even across the tread width, handling and wife are gonna for a 50 year old car, though harsher than a modern.

Hope you enjoy your GT6.

Nigel

[Ian Foster]

I run 27 front and 24 rear on my Mk2 GT6, on the basis that there is more load being carried on the front axle.

Tyres seem to wearing evenly and the steering (even with with my 12" wheel) is quite manageable.

Ian

[Gully]

There are various threads on here regarding tyre pressures. Triumph had the GT6 MkIII pre-1973 cars with the fronts at a lower pressure than the rear, but reversed that for the 1973 swing spring cars. From the owners manual (based on 155 SR-13 radials):

Pre-'73 Front: 24psi, Rear 28psi

1973 Front: 30psi, Rear 26psi 

Gully

[johny]

I always thought that must be a mistake (not unheard of in the manual) because surely the engine is heavier than the backend so more pressure will always be need at the front to keep the correct tyre shape and contact area....

[Chris A]

Just to butt in on this, my handbook for the 13/60 gives a lower pressure at the front than the back - never really thought about it till now. Will I sleep easy tonight due to this ?

[johny]

For the Herald it might make sense as the weight distribution is better but a Vitesse😳

[PeteH]

Most original handbook pressures, certainly pre about 1965/70 will be for Crossply tyres. In the past it was considered prudent to up the pressure 2-3psi for Radials.

Haynes BTW. For a Vittesse give Cp/R 21F and Cp24 R26 psi?. But still have 21/24 for both Cp and radial? On the GT6 Curious?.

The 13/60 Handbook, gives 21F and 26(2up)28(4Up) psi.

Pete

 

Edited August 18, 2023 by PeteH

[Gully]

On 18/08/2023 at 17:05, johny said:

I always thought that must be a mistake (not unheard of in the manual) because surely the engine is heavier than the backend so more pressure will always be need at the front to keep the correct tyre shape and contact area....

I always assumed it was a RWD thing. RWD Escorts also had higher rear pressures than fronts. My experience with my '71 GT6 is if the fronts are pumped too hard, understeer appears - inevitable with the boat anchor of an engine mounted partially in front of the (virtual) front axle - and if the rears are too soft I can feel the sidewalls 'slopping'. Given that the GT6 Mk3 handbook was updated for the late swing spring models (so it covers both suspension designs), I don't believe the earlier pressures were a mistake.

Michelin's period fitment guide for the TR6 was 22psi front and 26psi rear...

Gully

[johny]

Yes checked my vitesse and 22 certainly feels enough at the front so maybe the weight distribution isnt as bad as I thought....

[cliff.b]

I know I have a Spitfire but have 185/70s on the rear and at 25psi, they seem to act like additional suspension, smoothing out the bumps and generally pretty comfortable. A small change to 27psi or above and the ride is far harsher. 

[johny]

Yes its a trade off between comfort and tyre wall flexing which doesnt help handling (why moderns have ultra low profiles and higher pressures)...

[daverclasper]

On 18/08/2023 at 08:53, dougbgt6 said:

But, the thing that stopped the involuntary teeth chatter for me was new shock absorbers.

Did the old ones, still pass the bounce test Doug? 

[johny]

On 18/08/2023 at 09:53, dougbgt6 said:

But, the thing that stopped the involuntary teeth chatter for me was new shock absorbers.

Doug

I thought old shocks would be too soft not too hard🤔

[Pete Lewis]

have you seen Dougs Teeth   ha !!!

Pete

[dougbgt6]

12 hours ago, johny said:

I thought old shocks would be too soft not too hard🤔

I saw an episode of Fuzz and his lanky mate where they took the shockers off a truck completely. They filmed a couple of likely lads in the back trying to eat fish and chips with shockers and without. With, just about do-able. Without, chips everywhere. 

Doug

[Pete Lewis]

how many cars and vans have you followed and noticed the rear wheel/axle pounding up and down in a uncontrolled frantic manner all due to a shot shocker 

the drivers seem unaware of the noise/vibration  this must make .  (but not  good old Doug )  

Pete

 

[johny]

Ha, yes I saw the programme and thought they should have also tried substituting the shocks for rigid bars to show the opposite effect - now that would shatter teeth!

[Stratton Jimmer]

Typical of my luck... Out this afternoon in the GT6 and I had a blowout on the nearside rear. Luckily I had checked the spare's pressure this very day and fitted it in no time but I have decided to take this as a chance to replace all of the tyres with new as those fitted are getting on a bit. Which brand of 155/80 R13 would anyone recommend?