tyreprofile

When selecting a wheel/tyre combo with any existing car you have, you should always select the size with the closest to factory overall wheel/tyre diameter, regardless of the rim size. You can use a calculator like this one to calculate the difference in overall diameter from your stock wheel to the new one. I have found that staying within .8% of the original is good. I like to stay with 0.3% or less whenever possible. For Morgan Classic owners, this calculation is made easy by simply looking to earlier years with their smaller rims and adopting their tyres specs as well.

The tyre size you use with whatever wheel diameter you pick can be juggled with a little bit to achieve the right overall diameter.

With wheel diameter increases come a decrease in sidewall size in order to maintain the same overall tyre+wheel combo size. It's important to stay as close to the stock tyre+wheel combo size as the overall wheel size affects gearing.

Smaller wheels improve acceleration at the expense of top end, and taller wheels decrease acceleration while theoretically increasing top speed (though in practice top speed is limited by power and drag long before tyre size..so for most cars, and definitely for Morgans, this factor is irrelevant. Increasing tyre size usually also hurts top end acceleration and top speed as it decreases the engine's ability to overcome the air drag holding the car back).

In any case, making the overall wheel/tyre combo taller or shorter will again affect suspension geometry. I don't advise this, especially for a road car. You can change it a small amount without any real adverse effects but try to stay as close as possible when choosing a new combined size.

Additionally, the big "gotcha" with sidewall size is that the smaller the sidewall, the less protection your probably expensive rims will have from the inevitable pothole or road debris. If you keep the same wheels with lower profile tyres, the only route to more protection for your rims is increased pressure, which will prejudice safety, handling and comfortt, thusly prioritising the rims over yourself.

However, as a result of an increase in wheel diameter, the weight distribution of the wheel+tyre combo worsens - as the heaviest part (the tyre) is now further away from the center of the wheel - a no-no as it robs precious power from getting to the road. Many guys spend so much time focusing on the wheel weight but never even think about thedistribution of the tyre weight and its effect on the entire weight distribution of the wheel.

Larger sidewalls allow better ride quality, better straight line launch, braking and do not necessarily decrease grip though they do work to prevent a sudden (aka losing it!) rather than gradual break from the road. But they may make the car handle less precisely. In other words, the sidewall flexes so the steering response will be slower and you get a lot more "roll" in the bends. Too big of a sidewall can cause excessive tyre temperatures due to a lot of flexing about and possible failure in race conditions so it still is ideal to keep relatively low profile sidewalls on performance applications as we mentioned earlier. As mentioned, Morgans successfully used 14 and 15" rims for decades. Still do.

You will notice that drag racing slicks always have very tall sidewalls. You never see low profile drag slicks. This is because when a drag car launches, the tyre actually deforms and ripples up to create an even fatter patch of rubber on the ground and to generate 'bite' into the ground. The smaller the sidewall you use, the more likely you will spin your tyres off the line. Taller sidewall tyres can generally bite harder both under braking and acceleration. Yet as modern Morgans become more powerful, their sidewalls are shrinking. So again, this doesn't make sense.

Yes. These market factors have reduced the neighborhood availability of 14/15 inch tyres. But Morgan Classics are a vintage design and the offerings from vintage tyre suppliers have increased, rather than diminished. It makes no sense to own a Morgan unless you treat it right.

The Problem With Large Wheels by David Finlay for MSN February 19, 2018

Not that he was complaining about it. There’s a lot of profit in persuading people to pick the large-wheel option. It usually adds several hundred pounds to the price, in some cases a few thousand, and you can bet your life that the manufacturers are paying the wheel and tyre makers only a small fraction of that.

This isn’t always true. Back in 1997, Subaru sold a car called the Impreza Sport. The only technical difference between the Sport and the GL was that the Sport’s wheels were one inch larger. You might not think this would make much difference, but it did. The GL was a pleasant car to drive. The Sport was glorious. That small change in wheel size sharpened up the handling almost unbelievably. More commonly, though, the optional wheels and tyres make little difference to the handling and have a devastating effect on the ride quality.

It’s not the wheels themselves that cause this. It’s the fact that you have to fit lower-profile tyres to them so that the circumference remains the same. If you didn’t, there would be space issues and the gearing would go to pot. Lower-profile tyres have smaller sidewalls (the sidewall being the part of the tyre you can see around the wheel when you’re looking at it side-on). Since there is less rubber, it can’t be deformed as easily, so shocks which would otherwise be absorbed by it are passed through to the suspension instead. Suspension is set up to deal with a particular wheel/tyre size. (As Morgan never changed their suspension when they changed wheel size), the suspension is no longer able to cope as well as it did before. It does not damp the shocks coming from the road as effectively, so those shocks are transferred to the rest of the car and, ultimately, to you.

This is noticeable in all cars, but in some more than others. The first-generation Audi Q5 and its close relative, the Porsche Macan, were set up on the assumption that they would be fitted with 18-inch wheels, and were very comfortable when that happened. In each case, fitting 20-inch wheels instead was a disaster. An obvious contrast was apparent within the Porsche range. By the time the Macan was launched, all 911s came with 20-inch wheels as standard, and their suspension were set up accordingly. It may seem strange that a company could make a sports car with a more comfortable ride than an SUV, but that’s what Porsche did.

There’s little sign that other mainstream manufacturers will follow Renault’s example, but perhaps some day they will. Until then, I’ll always choose the smallest wheels available. My car may not look as good as one whose owner went for the large-wheel option, but it will be far more comfortable to drive.

Newer owners are not going to like the following comments. When choosing tires for your vehicle, you might believe that picking the widest tire is best. However, that may not be the case, especially with Morgans. There are advantages and disadvantages to both wide and narrow tyres. When determining which kind is best for your vehicle, it’s important to think about what you want out of the tire and also about your car itself.

CONSIDERATIONS

Car weight — For some vehicles, buying a wider tire is not realistic because the weight of the vehicle will put insufficient pressure on the tire. If you were to mount a wide tire on a car that was too light, you run the risk of loosing traction and slowing the car. If you mount tires that are too wide on a vehicle that is very light, you will experience wheelspin, which can damage other internal systems in your car. So make sure you purchase the appropriate tires based on the weight of your vehicle. Morgans are close to the the lightest cars in the world.

Traction — Having more pressure per square inch or a wider contact area are two options for creating greater traction for your car’s tires. With wide tires, you have a broader contact area, while skinny tires create more pressure per square cm. There’s no way you can have both options, so it’s best to find a balance based on what kind of roads you will be driving on and other conditions that will affect your vehicle. Always tune and set up your Morgan for the roads you enjoy the most.

Wet condition driving — Wide tires are better for driving in wet weather since they have sipes, which help to trap and remove water from the contact surface. Narrow tires have sipes as well, but since they have a smaller surface area, they have less of them. One positive aspect of narrow tires in the rain is that when going through puddles, the higher contact pressure moves water out of the way. However, tyree technology has moved ahead. You can buy narrow tyres that give amazing control on wet roads and puddles.

Dry road racing — If you’re mostly going to drive on streets with dry asphalt, you should consider going with wide tires. For this type of road material, the increased surface area provides better traction than having more pressure at a sacrifice to speed..

Fuel consumption — The less friction your vehicle produces, the lower your steering effort, which also means less fuel consumption. Wide tires provide for more friction, making your vehicle consume more fuel, whereas narrow tires cause your car to consume less fuel because they have lower amounts of friction.