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MG MGB Technical - 15' or 14' ?

I am going to have to order new wheels and tires for the 74 CB GT project. I am planning on lowering the car an inch both front and back but was wondering if I would benifit by adding 15” wheels? Main goal is to increase the handling of the car…BTW the wheels will be replacing rostyle not wires….
Thanks, Tom
Thomas Strombeck

It's not the wheel - it's the tire.

The push for narrower aspect ratio tires is largely style driven, not performance. It is the 60, 50, 40, etc. aspect ratio which drives the need for larger wheels in order to maintain the rolling circumference.

Some say the skinnier the side wall, the more direct the steering input - but the fact is that tires NEED side wall in order to provide the grip, handling, ride, and braking performance we demand of them.

You'll find more grip (handling) in the width of the tire, and not its height. On an MGB the practical limit on width is usually given at 195 mm's without going to "Sebring" flares or some such. Some get 205's on their cars, but they might have to fiddle with offsets, which drive other suspension changes.

What you're going to find, though, is that there are few 14" tires in the 195 mm measurement (most now use 185/70R-14's, for example), and more in the 15". I'm not sure about the aspect ratio you'll need to match the rolling circumference, but I'd guess a 60 series tire (ie, 195/60R15) would be close enough.

Despite my rant from above, you'll find little practical difference between 60 and 70 percent aspect ratio tires in terms of ride or turn-in - I just had to get it off my chest. Also, the tread design and tire carcass construction determine grip, slip angles, etc. more that wheel diameter. You can get a 14" Yokohama semi-slick autocross tire which will give a ton more grip than any conventional tire you'll mount on a 15" wheel - just don't go out in the rain and keep plenty of room under your credit card's limit for lots of replacements.

What to do? Find a 14 or 15 inch wheel you'll like to see on your car, and then get the best tires to meet your driving style you can find that will fit in the wheel wheels (and not have to fiddle with speedometer gearing), IMHO.
John Z

Just mt opinion.............

15's look much better than 14's.

There is a better selection of tires in the 15" size.

The wheel size alone will play little to no part in your handling equation.

The tire size/aspect ratio relationship is not an exact science and varies greatly between tires.

David
David

I put Dunlop w-10 195 55 15 tires on my Gt on 6" wide alloy wheels. The difference in handling is dramatic. The ride is not that different. It makes the MGB Gt a car that is very contemporary in handling. The lateral support from the seats is now grossly indadequate.
I have a gm sports coupe that handles very well and has low profile "V" rated hi performance tires. I changed the tires to hi performance road tires - same size - and the improvement was substantial. The hi performance tires are noisier, ride hard and wear much more quickly, but they are far more responsive and stickier than the factory tires.
Moral: Modern wide wheels and low aspect ratio tires are "way mo betta". And modern super hi performance tires are a major improvement over "street" tires.
There are past threads where these issues have been discussed at great length. lots of good info there.
Barry
Barry Parkinson

One other thought:
If you go to 14" wide alloy wheels you will find the disk brakes are shrouded and out of the airstream and consequently more likely to overheat and fade. The 15" wheel gives more clearance for brake ventilation.
Barry
Barry Parkinson

My car has 14" with 185/70 tyres. A friends car has 15" with 195/65. We both have uprated front springs (around 550lbs) and uprated arb. His car felt a lot firmer than mine. MG garages suggested this was due to the tyre width and wall. It is personal. With his powerful 4.6 the 15" 195/65 felt like the right thing to have.
15" give more space to different brake setups. Maybe something to keep in mind. The Princess conversion for example will fit with difficulty in a 14", where there is enough space in a 15" wheel.

Frank
Frank de Groot

I will respectfully disagree with John Z's analysis of tire aspect ratio. Aspect ratio has a very direct impact on the handling of an automobile and it is not just for styling. Although I do agree with the rest of John's analysis that tire width, good tread design, and rubber compounding can have far greater impacts than aspect ratio and wheel size.

The biggest changes from then until now in terms of tire technology have come in three areas. First is rubber compounds that provide better grip, yet still provide reasonably long tire life. Second is tread designs that keep the tread blocks from rolling side to side in cornering and also allow good water removal. Thirdly is the much improved sidewall construction and new materials used in sidewalls.

70 series tires used to be the best one could get for performance street tires as it was prohibitively expensive to fabricate the lower profile sidewalls that race tires used. The tire companies knew that lower aspect ratio tires improved handling, they just couldn't build them for the street. Then came the kevlar fibers and improved tire construction techniques and now we have 35 aspect ratio tires for street cars and no one thinks twice about it.

John is correct in that a tire does need a certain amount of sidewall in order to corner, accelerate, and brake. Without this sidewall distance, the tire is too stiff and then you end up with essentially a solid wheel/tyre combination (think of the suspension on a solid axle cart - there is none). The reason for pneumatic tires is that they absorb a lot of the small bumps and allow the shock absorbers to take care of the bigger bumps. Pneumatic tires also roll over the tread block a little during cornering and this helps the suspension absorb the cornering load in a progressive manner, rather than as a sudden jolt. Sudden inputs to suspensions are what usually cause loss of control situations.

This being said, what you are looking for in a tire is a certain amount of sidewall height, relative to how the car's suspension was designed. There is a reason why the car manufacturer recommends certain tire/wheel sizes as they have designed their suspension to be optimum with those tires/wheels. Now this is where it gets tricky, since what is optimum to a car manufacturer may not be optimum to a car enthusiast. The car manufacturer may have been looking for a reasonably soft ride for driver comfort, and not ultimate cornering force. But this is where the aftermarket also comes in to play and provides alternate tire/wheel combinations that can adjust the handling and working of your suspension.

So, to make a long story short, tire aspect ratio has a direct impact on handling since the lower the aspect ratio, the stiffer the sidewall, and the less rolling of the sidewall during cornering. Taking this too far though is a bad thing as too little sidewall roll results in abrupt cornering load transitions on the tire and this can cause loss of adhesion (i.e., skidding) during hard cornering.

If you check the high performance tire sites on the Internet, you will find that the smallest tire width/lowest aspect ratio you can buy for a high performance street tire is a 185/50 series tire (and only a few manufacturers offer this size). Doing some quick math, this translates to a sidewall height of a little bit less than 4 inches (assuming that the tire is mounted on a rim of proper width). And if you check for greater tire widths, you keep seeing that a 4 inch sidewall is typically the smallest sidewall height that will be available (e.g., on a 235 width tire, the smallest aspect ratio offered for a street tire is about 45). Even some of the monster Corvette tires (e.g., the 325 by 35 tires) are still running about a 4 inch sidewall. Obviously, a lot of engineers have done their homework and figured that a 4 inch sidewall height is just about optimum for high performance street cars (actually, also just about optimum for race cars too, if you look at racing tire ratios).

Getting above this 4 inch sidewall height means that the tire has more opportunity to "roll" about the tread during cornering (look at film of a car with stock size tires going through a sudden cornering transition and you will see the tread roll under the tire) and this reduces the maximum cornering load and changes suspension geometry during cornering (if you have heard of bump steer, there is also roll steer). This is why high performance automobiles do not come from the factory with the same tire sizes as some luxo-barge that is looking for a pillow soft ride with minimal cornering ability. So, to sum up, aspect ratio is very important if you are going to use the car hard for cornering, but don't take it too far.

So, what does this mean for us MG owners? Doing the math, a 195 width by 50 aspect ratio is about as low as we can go and still maintain a tire that fits within the wheel well. A 195 tire is usually recommended to have a 6 to 7 inch wide wheel, in order for everything to work correctly. Therefore, you will need to check offsets and possibly even have the tire shop test mount some wheel/tire combinations for you. Don't go the way of the rice rocket racers and mount a smaller width tire (such as a 185 by 50) on a too wide wheel in order to produce an apparent lower aspect ratio. While this may look cool for the Saturday night cruising, this is very detrimental to both the tire (since the sidewall is now acting as tread and taking loads it was not designed for) and the suspension (without the sidewall to take the small loads, the shocks get over worked in aggressive driving).

The reason to increase wheel size, if you are going to a lower aspect ratio is to maintain the same rolling circumference/tire diameter. The stock MGB tire/wheel combination was a 145 by 78 (which, by the way, is a 4.5 inch sidewall height) which resulted in a an approximate 23 inch tire diameter. If you run a 195 by 50 tire on a 14 inch rim, then you end up with a 21.7 inch tire diameter, which if you want to lower the car is fine, but if you want a stock ride height for the car, then you need a larger wheel. Going to a 15 inch wheel size brings the tire diameter up to 22.7 inches, almost the same as original. You might also notice that if you run a 185 by 70 series tire for a 14 inch rim (a very popular combination), then you actually increase the tire diameter to 24 inches, which is 1 inch over the stock setting. In effect, by going to a lower aspect ratio, but not taking into account overall tire diameter, you end up raising your car's ride height (along with center of gravity) and can potentially decrease the cornering capability of the car.
Ron Kluwe

Thomas-
Lowering an MGB and then using tires meant for a family sedan is dangerous and simply asking for problems as such tires have hard compounds that are meant for high mileage at the expense of traction. You get what you pay for, and economy tires have no business being on any sports car. The steel (disc) wheels of the mgb roadster originally used 4Jx14" rims while the steel (disc) wheels of the GT model originally used 5Jx14" rims, and the wire spoke wheels of both models used 4.5Jx14" rims, both models being fitted with 5.60x14 bias-ply tires. Later, in 1965, radial tires became available as optional equipment, SR155/14 tires being used on Roadsters and SR165/14 tires on the GT. Interestingly, both of these tire sizes were used in conjunction with the same speedometer and speedometer angle drive unit on the transmission. Now obsolete, these tires have been largely superseded by tires of lower profile. Today, P175/70R14 tires equate closely to the SR155/14 in terms of overall rolling radius. Equating to the earlier 165/14, the P185/70R14 appears at this time to be the most popular size in use by MGB owners. This size also retains (as close as is needed) the rolling radius of the original SR165/14 tire so that a reasonable degree of speedometer accuracy is retained. These tires provide a larger 'footprint', thus allowing better grip. However, a larger footprint increases low speed steering effort. A lower profile increases relative sidewall stiffness and thus steering response becomes sharper. P195/70R14 tires have been mounted by some owners, but they will cause the speedometer to give an inaccurate reading and clearance of the rear wheel arch becomes a problem, especially when there is lateral rear axle movement during cornering. The P185/65R14 is now becoming an increasingly more common fit as it's more commonly available. The 65-series profile tire also has stiffer sidewalls than a 70-Series profile tire, so you may anticipate a further sharpening of the steering response and a corresponding increase in low speed steering effort as compared to that of the P185/70R14. If you go with a P195/60 tire you'll need to purchase a set of 15" wheels or the rolling radius of the assembled wheel will be smaller than stock and your speedometer will be optimistic. If you prefer to stay with 14" wheels, take a good look at P185/65R14 series tires. These have a rolling radius very close to that of the original tires (Ever so slightly smaller), will fit into the wheelwells a little better, and will actually be closer to the original rolling radius than P185/70R14 tires. Being a more modern (read: advanced) design, not only will you end up with a smoother ride, but also P185/65R14 tires offer a wider range of traction/handling/wear possibilities. Generally speaking, you should be able to come up with something that is superior in all categories to the old P185/70R14 designs. Why don't more people use them on their MGBs? Because when the original tires became harder and harder to get, people switched to the P185/70R14 because they were what was available at the time that was closest to the original rolling radius. Since then it's become something of an Urban Myth that they're the way to go. Actually, the P185/65R14 is superior on almost every count and is in no way inferior to the older design. Unless you're substantially uprating the power output of your engine or modifying the suspension so you can drive very, very hard on curves, you don't need to go much larger. For most people, the P185/65R14 will do fine. The P195/65R14 is also a popular choice since it offers the widest practical width that can be used inside the wheelwell. Rubbing of the inside of the fender during hard cornering is not unusual with such wide tires, so installation of a Panhard Rod is also advisable in this case. Some people try grinding away the lip of the arch that's rolled into the wheelwell, but this is a poor idea as the purpose of the lip is to reinforce the edge of the wheelwell aperture and prevent cracking. Be mindful of the fact that a 5" rim is too narrow for a 195-width tire. The lateral tread profile would be distorted at proper inflation pressure. You could underinflate the tire to get the lateral tread profile to the correct contour, but that would result in the sidewalls flexing beyond their intended design limits and the tread squirming, plus the tire would actually ride worse, not better, because you'd be forcing it to do something that it wasn't designed to do. The heat that would be caused by the flexure would ultimately ruin the tire and could possibly result in sidewall failure or delamination of the tread. The heat that warms up the tread compound is not the result of the tire's width, it is the result of flexure. Wider tires have greater surface area and thus dissipate heat better, therefore, if all other factors are equal, they build up heat more slowly. If you want a stickier tread compound, get a higher-performance tire with a higher speed rating.
I personally use the P195/60R15 Michelin Exalto high performance tire, but my car has had the aforementioned suspension improvements to make exploitation of the advantages of this tire possible. Without them, the use of such a tire would be pointless. However, the steering at low speeds is heavier, and the ride is as stiff as any performance-oriented driver would be willing to tolerate on long drives. The reduced Roll Moment coupled with the stiffer sidewalls of the high performance 60 Series tires makes the handling highly responsive, almost darty. The car requires that you pay attention to what you're doing, making for an involving driving experience, just as a true sports car should.
Steve S.

165 tires on 14" = 24.6" diameter.
185/70 on 14's = 24.2"
185/65 on 14's = 23.5"

But since most MGB speedos are optimistic the equation becomes one of personal preference or a recalibration of the speedo.

I like 185/70 on 14x6 alloy rims. Very good handling with Yoko's and closest to original diameter and over the last 30 years I've gotten used to the optimistic speedo, and drive using the tach to figure MPH (18 mph per 1000 rpm 4th gear; 22 per 1000 overdrive 4th).
Terry

Without reading this entire subject, I can offer you my experience when I went bigger and wider.

Old wheels: 14x4.5x60-spoke wires
Old Tires: Michelin MX4 185-70/14

New wheels: 15x5.5x72-spoke wires
New Tires: Michelin Pilot 195-60/15

The ride is slightly stiffer partly due to lower profile tires and partly due to heavier wheels. Steering response is improved noticably and low-speed steering effort is slightly harder.

The handling of the car has been transformed. I replaced all the rear bushings with superflex to prevent rubbing so I'm sure that helps as well, but the car handles a lot less like an "old car" and more like a modern one, but only in the good ways. :)
Steve Simmons



14' tyres are cheeper than the equivilant 15' ones, genrally and when a common size is chosen (eg- 185/70R-14's)
CB

Tom,

Lots of interesting info above which I will read at leisure.

My View

Wheel weight re unsprung weight and tyre width for cornering force are the two key areas, but check your rear wheel clearnce.

The lower aspect may slightly improve cornering force but a 15in is a noticable harder ride.

Larger footprint at same tyre pressure?

IMO Wide wheels run smaller slip angles due to patch being wider but shorter.

Paul
Paul

This thread was discussed between 01/02/2004 and 02/02/2004

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