|The MGA With An Attitude
Adjusting your TIRE PRESSURE for Street Cruising - TI-102
>.... I have a '62 Mark II. I have bias-ply Dunlop gold seal (4 ply) size 15-6.00. ....
>Max PSI is 32,
Nearly all bias ply tires have a max rating of 32 psi for max load capacity. Also, nearly all car tires are sized to have some significant reserve load capacity, and so will generally perform best at less than the max spec'd pressure (except in the most severe competition use).
>"and the PO has it at 22psi front and 20 rear, but that may be from neglect and ...."
Original tire pressure was spec'd at 17 front and 20 rear. For fast motoring add 4 psi all around. For competition work and sustained high speed motoring, add another 2 psi all around. But that means absolutely nothing unless you happen to use tires of the original size and design (not very likely), and your suspension is still dead stock.
For a short time I had 5.60-15 2-ply nylon tires on the car, and it was absolute mush to drive. The tires would look half flat and handling was commensurate with appearances. I had to use 30 psi just to keep those tires from running on the sidewalls in hard cornering, and then they had very little grip, rather like driving on sand or pea gravel. I promptly pitched those tires, bought a set of Michelin X radials (1988), and have been wearing bigger grins ever since. But then my car likes spirited driving and is generally allergic to auto shows. And I have used many other types of tires since.
My best guess for the tires you have now is that the 22 psi in front may be pretty good for ordinary street use, but it should have a bit more in the back. Suppose you push it fairly hard through a tight turn, like 20 mph around a square street corner. If the front tires skid sideways and the front end wants to continue more straight ahead rather than making the turn, you have sever understeer, because the front tires have less grip than the rear tires. Here you may try a little more pressure in the rear tires to reduce the footprint a bit, which in turn will reduce the grip a little and put it more in line with the grip at the front. This may sound counter intuitive, like less grip can lead to faster cornering, but it goes much better if the steering is closer to neutral, not oversteer or understeer. Above all else, you want the front wheels to go where you point them, and the rear will eventually follow along, after some fashion or another.
It is significant to notice however that it was spec'd for more pressure in the rear than in the front. The stock rear suspension of the MGA is quite stiff with 6 leafs (plus the short bottom plate) in the rear springs. The stock front suspension is much softer by comparison. Even though the front carries about the same weight as the rear, the front springs have a much lower spring rate, so more vertical travel at the wheels to pick up the load. The end result is that in moderately hard cornering the car has some body roll, and the front tires pretty much stay planted on the ground while the car will lift the inside rear tire off the surface. This puts a higher temporary load on the outside rear tire during cornering, so that tire tends to mush over and squirm more and loses some of its grip. This causes the rear to get loose and slide out a bit, causing oversteer where it wants to fish tail or take the turns sideways. If you lift off the throttle in a high speed turn you may find yourself traveling tail first down the road. To avoid this somewhat wild and hard to drive handling characteristic, they specified more pressure in the rear tires to carry that higher momentary load.
I would guess that with lower front tire pressure the car would have a mushy feeling in normal street driving. A little more pressure improves the steering response because the tires flex less allowing the suspension to set up quicker to the cornering loads. This does not necessarily mean that you can get around the corners at a higher top speed, and it may in fact result in a lesser maximum cornering grip. If more pressure in the front gave it a little more lively feel in normal daily driving, maybe the DPO liked it that way. And maybe he never pushed it hard enough to notice the understeer effect, and lower tires at the rear make it ride a little softer.
Each to his own. The point being that you should use tire pressures that best tickle your fancy, best suit your personality, and best handle your driving habits. Small changes in tire pressure can have a rather dramatic effect on handling characteristics. Being a regular autocrosser, I happen to like a little oversteer, but when some other unsuspecting soul takes a drive in my car they may find the handling a bit too wild for their experience and tastes.
If you install a front anti-sway bar to increase the front roll stiffness and reduce the body roll, the rear pressure can be much closer to the front pressure. This will also improve steering response because with less body roll it takes less time for the suspension to take up the load in a quick turn. This makes the handling more responsive to steering input, gives the car a lighter and more lively feeling, more fun to drive, and incidentally somewhat quicker getting around an autocross track.
At any rate, newer tires have different pressure requirements. Even the tubeless bias ply tires you have now will like a little higher pressure than the original tube type tires.
So here's how you go about getting it right. Find some open stretch of nicely winding road without much traffic that you can drive on a regular basis. Push the car a little faster, like up to the point where you hear the tires squeal a little bit in the turns. And those bias ply tires can get up a really nice squeal before get too near the edge.
If you have understeer, the front end will sometimes want to continue straight ahead rather than make the turn. The immediate reaction is very natural, just slow down a bit to get the grip back. This is the very reason that literally all production cars have at least a little built in understeer.
If you have oversteer, the rear end will want to run out wider than the front end, maybe to the point that the car wants to get sideways and you end up steering right a bit in a left turn with the tail hanging out like the dirt track racers. This is not a good setup for road use, because if you ever have to lift off the throttle or hit the brakes when you're near the edge, the car could change ends very quickly, maybe more than once. I just happen to prefer a little oversteer for autocrossing, but not a lot, not ever.
Now after running this same road a few times you will be getting used to the feel of the car so you know what "normal" is. Then you can start changing the tire pressures a little with each successive run. Make only small changes, maybe 2 or 3 psi at a time, and do only one end of the car at a time. Run the pressure progressively higher until it is obviously worse handling, then back off a few psi, and start again with the other end of the car. At some point you will have fairly high pressure at both ends, and you will have determined the upper limit. At this point the car may feel a little light in the steering, may have started to lose its grip a bit, and may even skip or shudder a little on small bumps in a turn. If you drop the pressure just enough to get rid of the judders, you may have a fairly decent autocross setup (but not necessarily the fastest one).
Then you go the other way, back in the mid range where you started with the pressure, and start lowering the pressure a bit with each run, just one end of the car at a time. When it feels obviously worse to drive you have undershot a bit, so put back a few psi and then go to work on the other end. At some point you will have fairly low pressure at both ends, and you will have determined the lower limits. The car will then have a relatively soft ride, very docile steering characteristics, and maybe a slightly mushy feeling in hard cornering. This is a very reasonable set up for casual touring.
Now in general, lower pressure puts down a larger footprint and gives a better grip, up to the point that the tire may start to roll over on the sidewall and pick part of the tread off the ground, when it loses grip and gets really mushy. At just slightly higher pressure where it is doing none of these nasty things, you get maximum grip. You may also find that it wears off the outside shoulder of the tread rather quickly. Some years ago I decided to treat tires as an expendable and replaceable expense item. Since then I do what's best for my pleasure and not necessarily what's best for the tires.
Also in general, higher pressure improves steering response and gives that lively feeling of a light car and fun driving. It will probably also reduce tire wear and increase fuel mileage a bit, but you may not be able to properly quantify that last effect. For smoother roads on long runs this is a good idea.
If you load up the boot, add a couple of psi in the back to carry the extra load. If you hook up a trailer, add a couple of psi in the back to accommodate slightly higher lateral loading from the trailer and to avoid that twitchy feeling. If you will be driving on rough roads at moderate speeds, lower the pressure all around to give you more comfortable ride.
If you really want to blow the socks off your competitors at an autocross, prepare to spend $500 on a set of soft rubber compound road racing tires that will wear out with 5000 miles of road use or 3 hours of track time, whichever comes first (or a combination thereof). Then we can talk again about what goes into safety faster. In the mean time, be happy that you have really neat little car with lots of unique character, and one that makes no airs about being a stop light drag racer. Enjoy your car.
Shoe Polish 101, is an introduction to the MAGIC SHOE POLISH and how to get started on tire pressure tuning for maximum grip.
1958 MGA with an attitude