Tire pressure again

[Randy Forbes]

Non-scientific data collection; I used to autocross my Healey__competitively__in the 1980s and I'd write the pressures on the fenders with a grease-pencil (China marker). Optimum pressures were arrived at by a dab of white shoe polish and then see how far it was worn off after a run. Ideally, the polish would be scrubbed off right to the edge of the tread wrapped to the sidewall. I had the best results at 42-43 psi front/ 38-39 psi rear. At that time I was running Pirelli P6 195/60 fr & 205/60 rr on 6" rims.

Now I'd drop those pressures to about 30 for street use (it was also my daily driver) or my teeth would've probably fallen out.

Fast forward to today__well, last week when I checked the tires for the first time in a few months__and I still run 30 psi (195/65 fr/rr on 6" rims). Earlier last week when my wife and I were out in the car, it didn't feel right, not really squirrelly, just not right, and when checked, all the tires were down evenly to 24 psi (it got "cold" here, you know, below 80...).

Back then

Sort of now

Really now (last Saturday)

 

[HealeyRick]

FWIW I just had new XAS's mounted by Allen Hendrix. He recommended 30 psi all around.

 

[Randy Forbes]

FWIW I just had new XAS's mounted by Allen Hendrix. He recommended 30 psi all around.

Best validation yet!

 

[4tecdog]

Had my Michelin XAS tyres set up by longstone tyres filled with Yorkshire air and sent down to Wales at 30psi perfect.
Lost all the scuttle shake I had.

 

[Michael Oritt]

"..set up by longstone tyres filled with Yorkshire air..."

There's a fortune to be made for someone who can get that air over this side of the pond.

 

[HealeyRick]

"..set up by longstone tyres filled with Yorkshire air..."

There's a fortune to be made for someone who can get that air over this side of the pond.

We've been selling this stuff around here since I was a kid:

 

[Editor_Reid]

We've been selling this stuff around here since I was a kid:

View attachment 56114

Most often served "hot" I believe.

 

[vette]

https://www.britishcarforum.com/bcf/attachment.php?attachmentid=56114

Most often served "hot" I believe.

Probably hot compared to what you are use to.

 

[RAC68]

Hi All,

This is a Thread I have enjoyed but found like to see in greater context of the elements addressing better road stability. Although Air Pressure can change the tracking of a tire, we have had other discussions addressing caster/camber, and rear wheel tracking that can have a greater effect on high speed stability ... especially when conbined with tire pressure. So, when considering the air pressure of front and rear tires, how would camber and caster affect this selection in a non-modified (suspension) car? Also, how would you measure and address improvements in the adjustment of wheel tracking?

Ray(64BJ8P1)

 

[vette]

Hello Ray, I too have had a passing interest in various suspension set ups for a variety of types of driving. Although i doubt that I have paid as much attention to it as many of our friends who actually race their cars, (as I am not a racer). But in feeding my curiousity I have found a book by William Milliken, titled Equations of Motion. This book tries to address many forms of motion, such as airplanes and cars. He has a chapter on camber and had built a car called the "MX-1 Camber Car" or Maximum Camber Vehicle. They experimented with racing this vehicle with as much as -25* of camber. In one of the listing of their specifications on page 499, he list the tires and tire pressures. In one spec they ran Dunlop K591 motorcycle rear tires 150/80 VB16TL directional tread with a maximum load rating of 761lbs at 42 psi. But while in use on the MX-1 they operated the tire pressures at 20-30 psi. Since I tend to fall asleep shortly after starting to read a book like this I have come to the conclusion that the most optimum tire pressure is directly related to the load being carried. You might enjoy the book.

 

[RAC68]

Hi Vette and thanks for your reply.

Although I have always perceived the Healey's suspension as a system, I seem to have addressed each as a collection of discrete components. So, now I am at a stage of learning to want to a more complete perspective when making changes to any part of the suspension. What is the effect of changing the camber/caster and how does tire pressure relate to this change? How does tire pressure relate to high speed road stability or what would moving the rear spring anchor in 1/2" do to overall road speed stability? We, on this Forum, seem to address the HOWs but infrequently speak of the basic WHYs of the issue. Keep in mind that it is not because there is a lack of knowledge on this Forum.

So, thanks for the book reference and I hope to gain a better understanding from reading it with the support of my fellow Forum participants.

All the best,
Ray(64BJ8P1)

 

[vette]

One of the things that I found interesting is that while ranging the camber from a more conventional setting all the way out to -25* he only varied the tire pressure within a range that we are all quite used to. That being 20 to 30 psi. I would have thought that with such extreme camber on the wheels that he would have needed a lot more pressure to keep the beads from breaking lose. The car was quite ridiculous looking with its wheels all splade out to an extreme. Having just said this, it has occurred to me that the genre of extreme low riders might have discovered some tid bits of specification along this line.

 

[glemon]

I have been following this topic and find it very interesting. Also interesting that factory specs were more pressure in the rears than the fronts. I have spent a lot of time driving old British cars both in a spirited manner on the street and in autocross. I consider the big Healey, little Healey, MGB, and 4 cylinder TRs to be fairly similar (though certainly not the same). They all have a front weight bias to a lesser or greater extent, a stiffer rear suspension than front, and also less travel in the rear suspension than front, and a lot of unsprung weight with the live rear axle.

They can get skittish on a bumpy turn with the rears ocassionly losing traction and stepping out.

From a ride and handling perspective I like to run about 32 pounds in the front and 28 or so in the rear. The lower rear pressure helps reduce rear axle hop (think underinflated v. overinflated basketball) and helps the ride a little bit, the higher front pressure reduces oversteer and also steering effort slightly.

I will admit I have not recorded tire pressures and compared autocross times, but seat of the pants it has worked for me for touring, street and performance driving.

All the shade tree mechanic theory aside about front and rear pressure bias, I am in general agreement with Keoke and others that you run your car with modern radials somewhere in the high 20s to low 30s.

 

[RAC68]

Hi Glemon,

I like your logic and approach in evaluating the component (tire pressure) within the context of the total suspension system's operation. By having softer rear tire pressure and reducing rear wheel hop, you would secure even greater road adherence but also increase lean. On heavy turns, wouldn't a stiff McPherson strut suspension with the axel above the frame increased lean and raise the inside wheel … placing most road adherence on the outside wheel. In this condition, should I expect a reduction in road adherence when depending on a softer outside tire?

Ray(64BJ8P1)

 

[glemon]

If you watch a video of the inside rear wheel of a car lifting in a corner it is usually because the outside front suspension is compressed more than the outside rear wheel, if you think of an elongated x drawn between front and rear wheels you push down on the right front and the left rear will go up, suspension travel, roll center, spring rates and sway bars have a lot more effect on rear wheel lift than tire pressure I imagine

I didn't modify my 100 much beyond some poly bushings and lowered spring pans, but I did autocross it a few years on 165 width radials, it was reasonable well balanced and agile, but certainly had a bit of a vintage feel to it. I don't have as much experience with the six cylinder cars, but have also autocrossed with an MGB (a 79 on cheap radials, surprisingly well balanced and fun to chuck around) Porsche 924 (on summer tires, lots of stick, but no go) Saab Sonett (an abandoned experiment that didn't turn out as well as I hoped, partly because FWD dynamics approaching the limit are somewhat of a mystery to this old dog), and my current TR250, which got vastly better with front and rear sway bars, but it has a much softer and different rear suspension set up than a big Healey.

 

[steveg]

I spent a lot of time on the internet trying to find any authoritative recommendation for tire pressure in the low 20s. Zilch.

Not lending any weight to old road tests - IIRC everyone used lower pressure in those days.

 

[Randy Forbes]

If you watch a video of the inside rear wheel of a car lifting in a corner it is usually because the outside front suspension is compressed more than the outside rear wheel, if you think of an elongated x drawn between front and rear wheels you push down on the right front and the left rear will go up, suspension travel, roll center, spring rates and sway bars have a lot more effect on rear wheel lift than tire pressure I imagine

I didn't modify my 100 much beyond some poly bushings and lowered spring pans, but I did autocross it a few years on 165 width radials, it was reasonable well balanced and agile, but certainly had a bit of a vintage feel to it. I don't have as much experience with the six cylinder cars, but have also autocrossed with an MGB (a 79 on cheap radials, surprisingly well balanced and fun to chuck around) Porsche 924 (on summer tires, lots of stick, but no go) Saab Sonett (an abandoned experiment that didn't turn out as well as I hoped, partly because FWD dynamics approaching the limit are somewhat of a mystery to this old dog), and my current TR250, which got vastly better with front and rear sway bars, but it has a much softer and different rear suspension set up than a big Healey.

Course workers would tell me that I'd pick up an inside front wheel in big steady-state sweepers with my Healey (and similarly, with the M Rdstr too). It didn't really carry the wheel, as described to me, it was more of a touch, skip/glide, touch scenario.

Still though, getting daylight under a Healey's front tire is pretty impressive

 

[Lin]

I wonder what his tire pressure was? Tee hee.

 

[Keoke]

Close--LOL

 

[Keoke]

Reset the game clock picture out for review-Keoke--LOL


OR he probably got balloon tyres