GT6 Tyre Pressures

[dellyend1]

Hi All, apologies in advance as have tried looking on forum threads and google for tyre pressures for a GT6 and lots of different figures quoted.

I've just put on a set of 175/70/R13 tyres and the most common GT6 stated pressures for tyres  i've found on the net are  24 front and 28 rear or 21 front and 26 rear ? 

I appreciate it may actually need some fine tuning following road tests but an thoughts / previous experience would be gratefully recieved.

 

[Pete Lewis]

modern tyres need higher than the book pressures from times past 

many add 4 to 6 psi over the book  or the sidewall is unstable 

and common in most handbooks of the day was for motorway up 6psi was quite normal but modern radials are very different to 1970 stuff 

its very trial and error testing to get decent sharp handling and a comfortable ride  

modern cars use suspension to tune the ride  on old triumphs the tyres had a much bigger input to ride quailty 

its wise to keep the front rear pressure balance similar also helps  keep the understeer controlled 

so its time to have a play 

Pete

[Ian Foster]

I'm currently using 28 front/25 rear on my Mk2 with 175/70 R13 tyres.

If you think about the weight distribution on the GT6 the bias is definitely towards the front, so higher pressures in the front tyres should result in similar contact area front and rear.

I also have a fairly small steering wheel so the slightly higher front pressures reduce steering loads at low speeds. High speed stability and cornering seems fine.

Ian

[Colin Lindsay]

30 all round on the GT6; I actually dropped it from the 32 the tyre fitter had originally. There's slight understeer but I'll put that down to the front anti-roll bar.

[Dick Twitchen]

13 minutes ago, Colin Lindsay said:

30 all round on the GT6

Same for me.

[dannyb]

What about a Spit 1500 same tyre size. 

Danny

[Pete Lewis]

there  Danny  you have the advantage of not having a ground anchor under the bonnet 

but its test and try is all you can do , the old pressures listed have a wonderful range 

like courier van is shown as 15 at the front .....not these days 

Pete

[PeteH]

Original Herald Owners manual First Edition Jan 1962 (bearing in mind this would likely be Cross Ply`s) Front 19PSI. Rear (2up) 24psi  (4Up) 28psi.

Pete

 

[dellyend1]

Hi All,

Thanks for all the feedback. I've managed at last to dodge the rain and go for the 1st drive since  I bought the GT6 2 weeks ago. I had all tyres on 29 psi and whilst the front was fine the rear made a rather alarming wobble. Not round a corner, but just in a straight line, intermittently when going down a country lane with lumpy surface. I then stopped , put the rears up to 32 psi and it is better but still an unnerving sensation as not used to it. Even at 32 psi there is a lot of lateral movement in the side wall if I move the rear end from side to side , so wondering if I need to inflate further. 

Trouble I have is that having never owned or driven another GT6 before I am not sure what are GT6 quirks and what is not right. When I test drove it it was on R14 175 / 65 and these certainly did not produce a wobble. I'm also thinking that maybe the geometry has been tinkered with and if this needs sorting. Is this something straight forward for a local garage to do on the GT6. 

Apologies if this is all a bit numpty but don't have anything to compare too.

Best Wishes

 

Jeremy

[dellyend1]

[Colin Lindsay]

4 hours ago, dellyend1 said:

I had all tyres on 29 psi and whilst the front was fine the rear made a rather alarming wobble. Jeremy

They will jump due to the live rear axle; mine yesterday was jumping sideways on bad bumps - always the driver's side, too - so it's normal up to a point. Excessive jumping about means something's not right, but you can't really tell until you drive the car in question.

[Ian Foster]

5 hours ago, dellyend1 said:

the rear made a rather alarming wobble. Not round a corner, but just in a straight line, intermittently when going down a country lane with lumpy surface.

Jeremy

It does sound as though the rear suspension is moving around more than it should. You haven't said yet what Mk of GT6 you have, but each has its own idiosyncrasies. I don't think it will just be tyre pressures as you are in the ball park (a bit high if anything) in terms of what we are typically using. 

If the back end is moving around on a straight piece of road, then something is either loose or worn, which is allowing the back end to steer the car. If the rear tracking was seriously out, you might expect the car to steer on and off the throttle. Worn dampers might be another culprit. If everything is is in good condition it may be that the alignment is just 'off' and a proper four wheel alignment on modern equipment would be a good investment and will ensure you get maximum benefit from those new tyres (what brand did you go for BTW ?)

My Mk2 really tracks and steers very well for a 50+ year old car and actually deals with some surfaces much better than might be expected.

A bit more information on the car and a detailed description of the issues would help.

Ian

[dellyend1]

Thanks Ian and Colin for comments.

it’s a 1972 mk3 and the suspension was overhauled / rebuilt by the previous owner, the bits I can see have all got purple poly bushes in and it has a new pair of shocks on. It may be the case that something is loose from the rebuild or as you say may be alignment issues.

I think it alarmed me as I thought the wobble was a wheel coming loose ! It feels tight in a corner and there is no clunking of anything . 
 

many thanks 

Jeremy

[Badwolf]

I have 26psi all around in a 1300 Spit. Types are 3 years old Yokos on standard rims. Appears to handle well, should I tinker or just leave it alone?

[dellyend1]

21 minutes ago, dellyend1 said:

Thanks Ian and Colin for comments.

it’s a 1972 mk3 and the suspension was overhauled / rebuilt by the previous owner, the bits I can see have all got purple poly bushes in and it has a new pair of shocks on. It may be the case that something is loose from the rebuild or as you say may be alignment issues.

I think it alarmed me as I thought the wobble was a wheel coming loose ! It feels tight in a corner and there is no clunking of anything . 
 

many thanks 

Jeremy

Tyres are Yokohama bluearth es32

[JohnD]

There is no "correct" tyre pressure - there is one to match your driving style, the amount you load the car and  where you drive it.

Measure the depth of the tread - if there is more wear in the middle then more pressure and vice versa.     Can also indicate if you need more camber, if the shoulders are unevenly worn.

John

[Colin Lindsay]

33 minutes ago, Badwolf said:

I have 26psi all around in a 1300 Spit. Types are 3 years old Yokos on standard rims. Appears to handle well, should I tinker or just leave it alone?

If it's handling well, why tinker? The only other option is to inflate / deflate the tyres and then it's too late...

[Nick Jones]

Should be rotoflex rear end with the lower wishbone I think. These should feel pretty stable at the rear as they are much less affected by track changes as the suspension moves through its travel, which can make the swing-axle cars feel a bit skittish on bumpy roads.

For either type I’d highly recommend getting a 4 wheel alignment done, bit of toe out on the rear makes things way too exciting!

Nick