Mk VI / R-Type tyre pressures for radials

[Paul Bennett]

My 1954 B143WG is fitted with good thickness tread 6.50" x 16" radials (that are pehaps designed to look more like cross-plys.)
After some inconvenient punctures I replaced the existing thin innertubes with 2.5mm heavy duty reinforced Michelins.
I keep the tyre pressures as per the original handbook, ie. 24 / 33 psi cold.

However, I now remember reading somewhere that if radials are fitted to these wheels, (that were designed for cross-plys,) the pressures should be increased by 10 psi to ensure, (I think,) no movement of the tubes or covers? The article also said that this was because the steering would be heavier at low speeds with radials than cross-plys. (I do find it hard work parking, especially in front of an audience of pedestrians!)

Does anyone know about this or have any views on the subject, eg. would the tyres wear more in the centre and/or have less grip in the wet?

Thanks
Paul

[John Murch]

My understanding is that pressures should be increased when changing from cross ply to radials - but how much! I would suggest a call to Vintage Tyres at Beaulieu ( 01590 612261 )
They are usually very helpful on this type of thing.
John

[Mark Taxis]

I have a Mk6 Special fitted with radial tyres, by trial and error I have settled with 35 psi all round. I have tried a higher pressure but found that the ride was too hard, but this is a special which is considerably lighter than the original car.
_________________
Mark Taxis
Sunshine Coast
Queensland 4560

[Martin Webster]

I’m running Blockley 6.50 x16 on my HJM MkVI. They are inflated to 34 psi front and 36 psi rear. They have now completed over 5000 miles including 2000 miles to and from and over the Swiss alps this Summer. The handling of the car is perfect at all speeds and I feel that the tyres would be difficult to improve on. The car is fitted with a 12/41 rear axle and is cruised at 80 mph when possible.I have experimented with different pressures at different weights and speeds and those quoted give the best results for me.
B88HR

[Paul Bennett]

Thankyou John, Mark and Martin for your useful advice.

Martin, I am impressed with your cruising speed, having never even dared to go above 70 mph top speed in mine, (even though the Autocar tests of the day achieved 104 mph with the standard rear axle!) I am not as brave without seat belts as years ago. So at 80 mph you are probably only doing 3,300 rpm? I wish I had a rev counter; all I know is that my car should do 24 mph per 1,000 rpm. Are you the man who has a 4.9 engine fitted to a MK VI?

I discovered a very interesting graph not long ago of axle ratio comparisons for R-Types produced by Norman Geeson. The 12/41 (= 3.42 : 1) gives, (I think,) a ton at 4,000 rpm and there is a 13/38 (= 2.92 : 1) available for the S1/S2 V8's which gives 115 mph at those revs.

As I don't usually go long-distance cruising in mine, I will stick with my good acceleration 3.73 : 1, (whereas in my youth I always tried to go as fast as possible,) but these days it takes longer to go, say, a couple of hundred miles on the M1 than it took me in a £40 old banger in the 1960's on A roads!

I do now also have a vehicle that does 40 mph per 1,000 rpm, but is apparently limited somehow to around 155 mph, (otherwise at the 4,500 rpm red line 180 mph should theoretically be possible?)

However, where can you go fast these days? I hear Le Gendarmerie are pretty hot on tracing speeding Brits now, whereas others get away with murder on our unpoliced highways!

[James O'Neil]

From Vintage Tyres website, re Avon Turbosteel tyres: "IMPORTANT INFORMATION:
If you are fitting this radial tyre as a replacement for a crossply tyre; pressures should be higher than those shown in the vehicle manufacturer handbook. Speak to Vintage Tyres (+ 44 1590 612261) about the correct tyre pressure for your vehicle, suggested pressure is around 36-38 psi (2.5 to 2.6 bar)."