MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGF Technical - Low profile tyres

My MGF (Millenium edition) has factory fitted low profile tyres (Goodyear Eagle F1 215/40ZR16 82W)which seem to be wearing badly after only 7000 miles. I have not been impressed with the low profiles and when changing the tyres I would like to fit normal profile tyres. Is this
a) possible on the existing rims
b) acceptable to most insurers who seem to want to know about every detail on the car - especially replacement rubber for insurance claims!

Has anyone else made this transition?

Any help advice appreciated.

Ta
Ian

Ian

Beware - if you change profiles too much you will ruin handling, ruin steering, ruin speedometer readings, ruin odometer readings.

Beware - insurance companies if you fit tyres that are dramatically different to manufacturers recommendations.
John Ponting

Ian,

Elabourate more on the problems you've been experiencing. Why are you not impressed with the low profile tyres? It may well be something that can be rectified by keeping the same tyre profile, but changing the manufacture on the car.

If we knew more about the problem, it's easier to suggest a suitable alternative.

Leigh
Leigh

Ian, on 16" wheels, you cannot alter the profile significantly. Perhaps narrower 205/45 R16s, but that would be about it.

However, what you are interested in is how the tyres wear. Tyre profile does not determine the rate of tyre wear at all. To help you in respect to get an idea on how a tyre will wear, all tyres have a Treadware index (200 on GS-D2 F1 tyres) - which is one indication of the tyre hardness. The higher the index, the longer the tyre will last. This isn't the only factor of course, but it is the one that is easiest to understand.

HOWEVER there are significant risks involved in changing tyres. When changing tyre makes/brand/type, it is always best to change all four tyres at the same time.

Very importantly, some tyres simply do not work well with the MGF's chassis, giving potentially dangerous handling (witness the recent controversy involving the GS-D3 F1 tyres as one example). So swapping tyres can have very detrimental effects on your cars handling: something to be wary of.

A good tyre that works well on the MGF, and wears well, is the Bridgestone RE720. Not sure whether this is available in the suitable 16" profiles? Probably more information on their website. Given the positive feedback that these tyres have had (http://www.mgf.ultimatemg.com/tyres.htm), these would be my choice if I had similar priorities to you.

Hope this helps
Rob Bell

Let me get this straight, are you saying you want a higher profile to the side wall?

Not sure i have ever come accross this before, but the advice i normally give to people wanting to go the other way should be just as relevant in reverse i suppose.

The important dimension is the rolling radius. This is the radius of the wheel added to the profile of the tyre. If you keep this figue within say 6mm, or the wear of the tread, then you should be OK regarding speedo calibration, milometer accuracy etc.

Some information on tyre sizes:

Tyre sizes are made up of three dimensions and take the form "tread width" x "profile" x "wheel size" where the:

"Tread width" is in mm and represents the contact area of the tyre on the road, measured perpendicular to the side wall.

"Profile" is a percentage of the tread width (note this is NOT a figure in mm).

"Wheel size" is in inches (this is i presume a throwback to pre-metric days). The letter are a speed rating by the way, they represent the maximum speed the tyre has been tested at.

So, using your tyre sizes - 215/40 ZR16 you have a 215mm wide tread, with an 86mm (40% of 215mm) profile height on 16" (16" x 25.4mm = 406.4mm) diameter wheels.

To calculate the rolling radius, simply divide the wheel diameter (in mm) by two, then add the tyre profile, thus:

406.4 / 2 = 203.2mm + 86mm = 289.2mm

This is the critical figure.

To fit higher profile tyres, and retain this rolling radius figure, you need to change the wheel diameter (smaller). This can be done using MG official wheels, namely the 15" wheels that originally were specified for the F.

15" x 25.4 = 381mm / 2 = 190.5mm (radius of a 15" wheel).

289.2mm - 190.5mm = 98.7mm (required profile of tyre)

205x45 = 45% of 205mm = 92.25mm - (within 6mm of 98.7mm, therefore) this is a suitable tyre size (205x45x15)

Also, original spec MGFs had narrower tyres on the front, so:

195x50 = 50% of 195mm = 97.5mm - (within 6mm of 98.7mm, therefore) this is a suitable tyre size (205x45x15)

If you want a higher profile tyre than this, then you are moving outside MG specs, but it should be possible providing the rolling radius is kept close to the 289.2mm. 13" and 14" wheels should be available second hand (Metro wheels), these will have the same stud pattern as the F and will fit no problem.

But, is this a good idea?

Well, the height of the side wall affects the handling and the ride comfort. A high side wall gives a good cushion of air to smooth out the ride, but during cornering when the weight of the vehicle and centrifugal force is deforming the side wall, a lower profile will have far less flex in it and thus gives a truer line through the bend and more driver feedback. Thus it is a compromise situation, higher profiles give greater ride comfort, lower profiles give better handling. If you go outside MG spec with higher sidewalls, i would presume that this will have a noticable (negative) affect on the handling of the car and therefore could be dangerous.

I therefore wouldn't advise going beyond MG specs in this regard, if you are still intent on going down this route, source some second hand MGF 15" wheels from somewhere like the MGF Centre and stick to MG spec tyre sizes.

Note that the profile of the tyre will have no impact on the tyre wear, this is caused by suspension geometry and/or tyre pressures is an entirely different ball game.

HTH

SF
Scarlet Fever

D'OH! (_8(I)

>> 205x45 = 45% of 205mm = 92.25mm - (within 6mm of 98.7mm, therefore) this is a suitable tyre size (205x45x15) <<

This is (give or take) a 6mm difference - still OK though.

D'OH! (_8(I)

>> 195x50 = 50% of 195mm = 97.5mm - (within 6mm of 98.7mm, therefore) this is a suitable tyre size (205x45x15) <<

The last tyre size should read 195x50x15 (bleeding cut and paste!)

:-)

SF
Scarlet Fever

Thanks guys, I am overwhelmed and will now stay with low profiles if chnaging them is such a hassle!! The only real reason I dont like low profile tyres is the relatively rough ride you get. What I find difficult to accept is the low mileage ie 7000 miles only. Is this normal on the tyres specified and could this be improved upon by using, say, Bridgestones? Any more advice please?

Cheers
Ian Beaton

Ian, your problems (7000 miles wear) are likely to be from incorrect wheel alignment and or suspension wear.
Correctly set up you will see in xs of 14k.
The rough ride you are experiencing is very unusual for a hydrogas car, get it checked.
Regarding tyre profiles I had the opportunity to test out Gerry's gold wheels 205x55 16 continentals on the front of my F (don't ask why it's a long storey) and even in torrential rain couldn't unstick 'em nor did it up set the handling;-)
Mike.
mike

Ian

I myself need new tyres as well i have done 10000 miles.

I was also dissapointed in this as i thought it was pretty poor was reassured my fellow members on here that this is about average for Goodyear F1s.
My problem now is that i cant get any GS D2s and here that the D3s are not so good so i am trying to decide which tyre to change to.
s lammie

Stuart,

I am running on Goodyear F1's, after 20,000 miles in February replaced my rears, only as i'd curbed a tyre, and it had ballooned up on the inside.

The fronts are still on the car, although will need replaced soon.

These tyres are running on 17" rims.

I was chasing Mike thru the afore mentioned torrential rain, and yup never lost traction once.

paul weatherill

Stuart, they are also the tyres I was running on during our spirited run back from Oban.
paul weatherill

>>What I find difficult to accept is the low mileage ie 7000 miles only.<<

I agree with Mike - this does smack somewhat of a tracking problem. Is the pattern of wear equal, or are the inner edges wearing out faster than the outer?

Regarding ride, the best way to put a higher profile tyre on your car (and in reality the safest), is to fit smaller 15" wheels. These came with 185/55 R15 front and 205/50 R15 rear.

Another advantage: the tyres are cheaper too.

Depends on whether your desire for a smoother ride over rides your sense of aesthetics! ;o)
Rob Bell

<<Another advantage: the tyres are cheaper too>>

Very true Rob.

I find the main advantage of having lower profile tyres, is not more grip, but more predictablity.

Especially on my last track night, I found that the back end came out in a very gradual and predictable way, and was very easy to control.

I found on my 15" rims, they may have had more grip (debatable), but they seemed to suddenly lose grip, and I ended up in the usual manic over correction scenario.

The ride is very hard though, and definately not to some people's liking, me however I love being able to feel every bump, stone and piece of gravel i go over ;-)

paul weatherill

Paul,

I find it completely the reverse of what you say:
15" rims -> less physical grip, more predictability, smoother ride.
16" rims -> more physical grip, less predictability, harsher ride.

Physics explains the more grib of the wider footprint tyre.
The lower profile explains the harsher ride.
The predictibility is more subjective but I'd assert that if the forces are greater on the edge (ie. more grip) then when the tyre lets go then it will do so more quickly. Also, with a lower profile tyre there is less sidewall and therefore less give when the tyre is absorbing horizontal bumps when on the edge.
(Of course, this comparison assumes the same compound rubber.)

But to the original question...
...get the tracking and pressures checked Ian. That'll most probably cure the tyre wear problems.
Then, if you want a less harsh ride you could easily fit 15" rims... or get another car. This is a sports car after all. ;-) ;-) ;-)

P.
Paul Nothard

<<Physics explains the more grib of the wider footprint tyre>>

except in the wet/dirt where the same weight over a greater surface area means less pressure and therefore less traction.

Also I was running 205's on both the 15" rims and the 17" rims.

paul weatherill

I guess it comes down to personal preference in the end.

I don't think the 17" rims are necessarily better than 15" rims, but I can drive on the 17" rims with confidence, something I couldn't do with the 15" rims.

paul weatherill

predictability is one thing, but progressiveness is another !
If you can predict the back end will come out, but in a very sudden manner, it's not very useful.
I've always found that bigger rims and wider tyres will definitely provide more grip, but at the depends of less progressiveness.

Why is a low profile tyre less progressive ? simply because it deforms less than a normal tyre, and thus gives less feedback before it looses grip.
Fabrice

totally agree, Fabrice.

however, just a thought.

Because lower profile tyres have less flex in their sidewalls, and therefore distort less could this mean that on higher profile tyres with sidewall flex and a distorted tyre be more prone to suddenly losing all traction?


The discussions about tyre behaviour seems to be a huge topic, would be interesting to see if people's experiences support or contradict the physics ;-)

paul weatherill

<<except in the wet/dirt where the same weight over a greater surface area means less pressure and therefore less traction.>>

In snow, yes. In wet/dirt, not really. In dry, def no.
Tyre manufacturers have made this bit of physics somewhat insignificant compared to the main forces. But having said that, it is exactly why wide tyred sports cars are a complete b*tch to drive in the wet!! :-)
Also why all the ski instructors in the Alps use their beaten up Panda 4x4s with skinny skinny tyres. Bleurgh! Horrid car! :-)

<<I guess it comes down to personal preference in the end.>>

Yup! :-)
That's why I'm holding onto my technically inferior 15" rims. Maybe it's in the head, but I feel I can catch the car more on track.

<<predictability is one thing, but progressiveness is another!>>
I was (wrongly?) using the words to mean basically the same Fabrice. :-)
I totally agree with your post. I'd buy a progressive tyre every time.

Just to confuse people (and go more off topic!) you could throw in the different tyre compounds into the equation. :-)
Have had a number of laps in the soft soft Dunlops that the racers use. Wow...! Now that's predicatable!!! Would love to have it on my car - except for the fact it lasts no more than 1000 miles! Eeek! :-)

P.
Paul Nothard

Yes Paul, it's an interesting discussion ;o)
I just miss some English vocabulary ...

The friction coefficient ("grip") decreases dramatically when the relative speed at the contact area increases. For example, (without ABS of course) when you brake and lock the wheels, you feel the tyres "sliding" and as a result, the stopping distance is greatly increased compared to braking at the adherence limit (or the "pumping" technique).

A high profile tyre will deform and thus provide an imperfect contact surface, but at least the friction coefficient will be OK (where the tyre touches the ground). In the end, it's a combination of more deformation and a decrease of the friction coefficient that will cause the tyre to suddenly loose its grip.

I got the oppotunity to confirm this with my F (from 15 to 16") and a long time ago with an AXGT (from 13 to 14")
Fabrice

Tyre profile has an impact on tyre wall stiffness.

Tyre wall stiffness has an impact upon tyre slip angles.

Tyre slip angles are a determinant of tyre adhesion.

When the optimum tyre slip angle exceeds it's maximum - say 8º - then no more grip is generated by the tyre.

In fact many tyres start to loose grip beyond the maximum slip angle - and it is the rate of this decline that defines who predictable a tyre is: the faster the rate that adhesion drops away as slip angle increases, the more likely that the tyre is to slide, and break-away. If however, the tyre does not loose adhesion too quickly, then you can more readily regain control of those sliding tyres. This is regarded as more predictable.

So it is the tyre, first and foremost, that determines predictability.

The taller the profile of the tyre, the more rapidly a slip angle is generated. Similarly with soft compound tyres (the tyre blocks on the tread contribute to the slip angle of the tyre). So initial grip tends to be greater (although the lost motion in the tyre side wall means that the steering can feel more sluggish).

Lower profile tyres tend to have stiffer side walls, so slip angles tend to develop more slowly, but it will take longer for you to exceed the optimal slip angle. Similarly, so some extent, with firmer compound tyres (although these will generate less low-speed grip).

So you see this is indeed quite a complex subject - because a high-profile tyre could generate more lateral cornering force than a lower profile tyre if the tyre side walls were stiffer or compound harder.

It is therefore, completely understandable that you are all correct given your individual experiences with specific types of tyre on different wheel rims.

Ah. Chassis dynamics. You got to laugh... ;o)

More at http://www.mgf.ultimatemg.com/tracking.htm and http://www.mgf.ultimatemg.com/chassis_and_handling/slip_angle.htm
Rob Bell

<<That's why I'm holding onto my technically inferior 15" rims>>

Noone is saying 15" rims are technically inferior.

I don't see any Cup Cars running on 17" rims either.
Techspeed also advised me I'd get better performance from 15" or 16" rims.

Just saying that since I've moved to 17" rims, once the back breaks free, it does so in a gradual manner, and is easy to get back under control.

Just trying to work out why the change in behaviour.

Both sets of wheels are 205's so we aren't talking about wider tyres.

There is a difference in rubber, was using Bridgestone S03's on 15" rims, and now using Goodyear F1's on the 17" rims.

Agree, would be nice to have a set of slicks for the track, only problem is they won't fit in the boot.



paul weatherill

<<Noone is saying 15" rims are technically inferior.>>

I am! <grin>

I'd be able to go faster on 16" rims... but I think I'd fall off the track more - which can be problematic. ;-)
Paul Nothard

low profile: harsher ride, better handling

high profile: softer ride, average handling

It really depends on the roads you drive on. I do a lot of country driving, handling is essential to me. And Swiss roads are so smooth and well kept that the harsher ride is quite bearable. Hydragas helps too, I'm not sure I'd like it as much in a TF.

As for wear, the better the grip on dry and especially wet, the shorter their life. NCT3s last for ages but are bad on the wet...

So in the end the choice depends on:
- your driving style and environment
- your disposable income ;-)
Anthony Braham

This thread was discussed between 05/07/2003 and 09/07/2003

MG MGF Technical index

This thread is from the archives. Join the live MG MGF Technical BBS now