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MG MGB GT V8 Factory Originals Technical - Roadholding/Handling with fat tyres

Wheel tyre size is a recurring theme on this BBS - we seldom look at roadholding with fat tyres.

Would the racing fraternity like to comment on the 'wheel/Tyresize' thread.

thanks RW
Roger

Touchy subject. It's a balance between the amount of tread on the road, unsprung weight, and tire composition mainly, along with tire life and a number of other practical considerations, and finally personal preference. Many would say that the 265/50-14 BFG's that I use are gross overkill but when you consider the other factors it appears less so, for instance extremely light and rigid wheels (Centerline) making unsprung weight certainly less than it was with the wire wheels and competitive with the steel disc wheels. The rubber is a somewhat harder composition which means longer tire life but it means more tread is needed for the same amount of grip. Likewise the overall diameter is less meaning the tire has to be wider for the same contact patch size. However, these certainly will not fit under the stock bodywork, so for most it is out of the question, but with the bodywork I've done they look just right.
Jim Blackwood

Roger,

The benefit of fat tyres on rear is that they run smaller slip angles with drive being transmitted through them. The more power the fatter the tyre, too fat will kill power oversteer. As Jim says the fatter the tyre the more unsprung weight being the downside, and finding weights of various wheels is not easy. With standard bodywork you are limited to 195-215 and any wider at front gives interference problems. At the same pressures I'm not convinced about bigger footprints with wide tyres, bigger diameter yes, with a lower profile the sidewall should be stiffer and probably can be run at lower pressures.

All above IMO, but definately a very good question.

Paul
Paul

Footprint is largely a function of vehicle weight/ tire pressure, and with this tire can go as low as 22 psi, although around 23-24 gives the best tread life. Rolling over on the sidewall under cornering forces really isn't a factor and I don't know the exact mechanism at work here, but real life testing demonstrates no scuffing of the sidewalls and not overmuch on the edge of the tread. I suspect it has a good bit to do with the ratio of width to height and tread or belting stiffness though, and probably rim width to tread width as well. I tend to run narrower rims for a number of reasons such as to help control tread rollover, to protect the rims from curb damage, to make the tires easier to seat on the rims, and for a lighter unsprung weight, although I do stay within the manufacturer's recommendations. The tread control issue is unproven though in my personal experience, just a hunch. Interestingly enough, my 7,000lb truck with BFG 35/12.5-16.5 tires (considered low profile in comparison to the 15" size) runs nearly the same pressure at 25psi. In both cases this was the result of a rough calculation from the desired contact pattern, manufacturer's specs on same, and vehicle weight, verified by appearance, and refined by usage and tread longevity, as I'm into the 2nd set on the truck and the 3rd set on the MG. Tread rollover is not a problem on the truck either and I have cornered both vehicles extremely hard, beyond the limits of tread adhesion.
I don't know that this is supposed to prove anything, but in these two specific cases it illustrates that a higher tire pressure is undesireable as it adds nothing to cornering ability (actually can hurt it) and reduces tread life significantly. It also illustrates that as tire size and footprint goes up, tire pressure must go down unless vehicle weight goes up proportionally. In both of these cases the tires are considerably oversized and understressed, which also reduces tread heating to a dramatic degree. Either vehicle driven at 100mph for 1/2 hour or so will have tread that is warm to the touch, but definitely not uncomfortably warm.(a racing buddy says"cold") This is another factor to be taken into account, as the grip does not vary significantly regardless of the driving conditions. That is to say, unless it rains!
Jim Blackwood

Paul / Jim - I guess my concern is about predictability - the unmodified MG is a very forgiving animal and if one changes the characteristics it is important (for me at least) to know how and what will happen.

The MG was designed with radial tyres - they weren't very tight but there wasn't a lot of what Jim terms 'roll over'. The handling was very predictable - the tail went sideways relatively slowly in the dry and rather faster in the wet. Driving habits were more ambitious and most of us had a go at drifting round corners.

Radials changed the handling markedly - much more grippy - we could all get round corners more quickly but when the slip came, it was less predictable. Now we have options for improving the grip further but the characteristics are not totally clear - Jim - yr point about weight is irrefutable - too much weight will not have the desired effect however I am not so sure about roll over - you don't mention wheel size but unless I misunderstand the 'roll over' expression, the tyre must be the right size for the wheel - the right tyre must then be inflated enough to give appropriate tread and 'roll' characteristics. The wrong pressure will lead to high wear shorter life - low pressure can also lead to unpredictable handling even to tyres coming off their rims.

Paul - reading between your words, you favour the 'limit the roll-over' train of thought which points toward larger wheels (15 or 16 ") - shorter sidewalls (50 or 55 instead of 60 or 70) and to slimmer tyres 185/195/205 perhaps ?

The bottom line for those of us who have brought our MGs mildly up-to-date and who watch the amazing calls for power and extraordinary questions about anything from wheelsize to rear suspension geometry - there must now be a lot of handling variations in the cars that are around. Advice on the BBS is very useful indeed but much of it is anecdotal - with 200 BHP quite commonplace, we need some scientific advice - perhaps its time one of the clubs did some tests and set some standards ?

Safety fast.

RW

There is the front steering geometry - i believe the negative camber wishbone mod (there are several degrees) will improve adhesion as will ride height and the stiffer (3/4 or 1") sway bars.
Roger

Taking the points raised in order and all in IMO

In the MG production era a minimum narrow rim was used to save weight and cost - with a 195 you could use a 5.5 - 7inch rim. As Jim points out the narrow rim gives a lighter unsprung weight but the wide rim may give better tread control.

The advice I've seen from Goodyear and Michelin is that with a slightly wider tyre the handbook pressures should be used. Worth checking if using 215 and above.
Higher tyre pressure should stabilise tyre walls but general advise for track days is to try 0-10psi above recommended pressures. Front to Rear differences can be used to adjust handling.
Road tyres can handle wide temperature differences. I've never seen any numbers for optimum tyre temperature to grip.
Tyre companies I assume are constantly working at making radials more predictable - Michelin being the first to achieve this back in the days of crossplys.

Porsche handling has become predictable because of the new generation of wide tyres and the F1 rear has to handle power and weight at rear.

I am limited by choice to the standard shell without flares but with additional power larger and wider wheels are an option the keys are probably 15inch at least to fit brakes and 60 aspect ratio to give some degree of comfort.

Wide tyres are a safe option but on an unmodified B will kill the handling fun. Tuned V8's need wider tyres and if you are limited on width fitting a rear bar may not be sensible as it takes away grip, the Frontline TCL may be useful here as you can adjust roll centre.

I agree with your summation and a few tests with the MG Owners Supersports would be useful.

I already use neg camber a lower car and a 3/4 front anti roll bar, which all gives perfect wear with Goodyear Eagle F1's.

A last thought is going really wide with additional unsprung weight may require a change from uprated levers to tubes.

Paul

Paul

First, you absolutely must differentiate between CB and RB cars as the suspension setup has to be significantly different. To illustrate, the RB needs a rear swaybar to maintain some semblance of neutral handling whereas on a well set up CB it is likely to be completely unnecessary, although going too big on the front bar may make it useful. However the purpose and action of a swaybar is to loosen up the opposite end of the car to balance the handling so in this case less is more. As long as neutral steady state attitude is maintained, which as Paul said can be fine adjusted with tire pressure to balance between front and rear, in my experience the breakaway characteristics of the CB remain astonishingly predictable regardless of the state of suspension tune and width of the tires, even using bias ply tires. YMMV This characteristic of the MGB is so exceptional and controllable that I strongly resist any urge to monkey with things like the steering rack mounts or fit an IRS as the gain could be more than offset by the loss. I have also found that if the car is low enough the GT swaybar on the front of the CB roadster is all that is needed, although for this it must be very low indeed. Although I considered at one time the negative camber devices which are supposed to increase grip at the front, once I realized I was getting even tread wear across the tire and had neutral handling with the wide tires I decided against it, although before I reached that point I had to recognize the need to reduce toe-in with the wide tires. This didn't take long as with the stock setting tire wear was greatly accellerated, and I now set it by feel using the lightest setting that does not give too much dartiness. This turns out to be very close to 0.

Back to tire pressures, bear in mind that extreme examples of vehicles with oversize tires often run extremely low pressures. Sometimes as low as 2-3psi, with unseating the bead being as rare an occurrence as with other more highly pressurized tires, but again much has to do with the configuration. Since I didn't mention it, I run 7" wheels with the 265/50-14's which have a tread width of a little over 9", and this gives me a little triangulation in the sidewall/tread/bead area, meaning that in order to pop the inside bead loose it has to wrap a considerable amount of the tread up towards the outside sidewall. I don't have my datasheet but I think this tire can be run on up to a 10" rim, but would not have the same triangulation effect and you would be working against the bead rather than with it. It's easy enough to understand that this same effect would be even more pronounced with lower profile tires and less so with taller ones.

Racing being what it is, recommendations on tire pressures should be taken with a large grain of salt. Track promoters would prefer to err on the side of keeping the tire on the rim, while competitors are all too ready to officially agree even if their actions are otherwise. If increasing the tire pressure reduces rollover without reducing contact patch area it's a good idea, but suggests that the starting pressure may have been too low. OTOH, if it decreases the contact patch area without significantly affecting tread rollover that may be an indication of going the wrong way, and can reduce cornering force. Usually a good visual indication of this is the near elimination of the characteristic radial sidewall bulge.
Addressing tire temperature, I would think that between NASCAR, drag racers, F1, and all the other competitive racing organizations that the relationship between tire temp and grip should be extremely well quantified, at least by the tire makers, but in street driving it's not often that you even have a chance to get the tires hot. So to me consistency here is another good idea.
Again as Paul noted, all this is really only useful if you have enough power to overcome the grip at the rear at least on occasion, such as enthusiastic cornering when you want to break it loose a bit. With a V-8 the MG is more than capable of being steered around corners with the throttle, and gives a very respectable performance when called upon for a 4 wheel drift. It's supreme strong point under these conditions is it's predictability and controllability. In that regard I think it'd even do a dirt tracker proud.
Jim Blackwood

This thread was discussed between 05/12/2001 and 06/12/2001

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