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Question For Mechanical Engineers, Diameter or Width?

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It depends on what you mean by 'capitalize'. A wider wheel (and a wider tire on that wheel) will give you more rubber to grip the road with. All other things equal, this will let you transfer more power to the ground before you start loosing traction. It will also let you decelerate faster before locking up the brakes as well as carry more speed through corners.

A taller wheel (and a taller tire on that wheel) will increase the distance traveled per revolution, which is basically the same thing as lowering your final drive ratio. This will hurt acceleration, however some people prefer this in high horsepower applications because it will allow lower gears to be usable instead of just smoking the rear tires due to to much power. Another way to get this effect would be going from a 3.91:1 diff to a 3.64:1 or 3.23:1 diff. Same idea, but a larger overall effect. Another downside to larger wheels is that in most cases they will weight more, increasing rotational mass as well as unsprung weight. Both of these are generally frowned upon for performance.

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Wheel Tech, Part II: Width Matters

www.tuneruniversity.com

Last time, we talked about the various ways that wheel weight affects vehicle handling, ride quality and both acceleration and deceleration.

While wheel weight might affect the performance of your car, wheel size can affect both the performance and the safety of your car. This article could frankly save your life or the life of someone near to you.

Unfortunately, there are a number of enthusiasts out there that by following typical forum/magazine myths about wheel sizing, have put us all in danger. Many of these myths have been around for literally decades.

You see, what I’ve found is that if enough people say something long enough and loud enough, everyone begins to accept it as fact even if it’s not necessarily true.

This is certainly true of the general rules of wheel size selection that you get on forums and even in magazines.

Here are some of the myths you probably have been led to believe:

* Larger rims with lower profile tires handle better

* Wider tires give better grip and thus better handling

Both of these are (for almost all enthusiasts) entirely incorrect.

In this particular article, we’re going to discuss wheel width. Next time we will talk about wheel diameter.

“But how could this be? Of course wider wheels handle better! The tire is wider so you’ve got more rubber on the road!” I hear cried from a member of the peanut gallery.

Another member of the forum certified expert crowd shouts: “All supercars have super wide tires and low profile tires, if they’re doing it – of course it works!”

Unfortunately, both are only true in limited circumstances, and for most people reading this, they’re never true.

The point that both of these claims miss is that the cars they are looking to and learning from, were designed with those wheel sizes in mind, before hand. It’s not so much that wider tires do not give more grip – it’s that if you do not also have the suspension to match, you may actually be creating a very dangerous situation indeed.

Think about it, have you ever seen a factory tuned car that came with significantly wider wheels or significantly larger wheels? You may have seen an inch or just over added, but no factory tuner worth their salt has ever gone much more than that – without also altering the suspension.

The primary reason, is fitting a car with a wheel or tire combo that has a different offset or width will affect camber, toe, and caster angles in the suspension. Lowering a car will also have a similar effect. These changes have significant impact on the handling of the car and can even make it downright unstable.

Even where the alignment is corrected, there can be significant changes in the scrub radius of the vehicle and unfortunately, in almost all production cars, scrub radius cannot be directly adjusted.

In order to compensate for the change in the suspension geometry you introduced with the new size wheels, you would need to completely revisit the suspension geometry and this would require extensive modifications in most cases, far outside the resources of the average home mechanic. To correctly address these changes, you would literally need access to an alignment rack and the ability to fabricate custom control arms, and other suspension pieces.

Danger of Wheel Width

Which brings me to the first main point of this article. Given most people’s suspension tuning abilities and resources – straying far from the original tire and rim size will result in worsened handling and acceleration. In some cases, even downright dangerous handling characteristics at the limit , or in the event of a tire blow out or single wheel brake failure.

I’ll go ahead and brace myself for the “but Nathan!s”, but stick with me and I’ll explain why getting this point will put you miles ahead of everyone else on the street – and even save you a good bit of money and possibly even your life.

Wheel Width

Simply put, this is where everyone goes wrong. The actual tire width isn’t as important as the OFFSET of the wheel in this case however. Small changes in tire width are sometimes acceptable, though they still alter scrub radius, a key figure that determines how stable your car is, or isn’t at the limit.

You see, in order to fit wider wheels, typically folks use smaller offset wheels to accomplish this. The lower the offset, the closer the wheel face is to the hub. The opposite applies for those who fit larger offset wheels to push the tire edge out towards the fender.

In both cases, the scrub radius of the car is altered. It either becomes smaller or larger. Both can create dramatically different handling characteristics at the limit, and for the street especially, all of those are highly undesirable.

Symptoms of modified scrub radius include the tendency for the steering wheel to ‘rip’ out of the hands of the driver during hard braking or turning. The wheels will also tend to react violently to imperfections in the road surface, especially when being pushed.

Aside from safety, it should also be noted that given a stock suspension setup, or a simply lowered one, you will not gain any additional rubber on the road.

Think for a moment about an inflated balloon. When it is sitting on a table, there is no weight on the balloon and the air pressure inside the balloon is constant. If you let air out of the balloon, more of the balloon will touch the table. If you put more air in, less and less will touch the table. Think of that as tire pressure. Then, apply pressure with your hand. You’ll notice that you get more balloon on the table, the harder you press.

The point of this illustration is to illustrate that the rubber on the road is mostly a function of air pressure and weight on the tire. The actual tire size matters minimally.

Disclaimer (Added 7/25): Tires are infinitely more complex than a balloon. While the physics are at the very basic level the same, due to the complex construction of a tire, the basic physics are not a great model of what happens in the real world. It would take an extremely complicated simulator to even approach accurate predictions of tire width’s effect on a car especially at the limit. That said, the suspension geometry argument holds water. Contact patch is a little more debatable but from the data I’ve seen and from first hand experience, it seems that wider tires give more contact patch to a point, but then it goes backwards (less grip). It also is not easy to model or predict. The best method to find out if you’re competing is TRIAL AND ERROR, period. There’s no free lunch or general rule here. If you are not competing, then the advice given in this article is probably as close to accurate as it can be.

How can that be? Well, when a vehicle is sitting at rest (easiest to illustrate) , regardless of the size of the tire, the same amount of tire will be in touch with the ground, given the same vehicle weight and tire pressure. The only difference will be the SHAPE of the contact patch. This is an oversimplification and yes there are other factors such as tire construction that play a role – but stick with me.

Wider wheels will give you a more rectangular patch, while thinner wheels will give you a narrow yet longer more square patch. The thinner wheels will therefore be better (and of course there are extremes in both cases) for straight line acceleration and braking. The wider ones, because of the direction of the forces on the tire tread in the corners, will be better for cornering at the expense of some straight line acceleration. Unfortunately, it’s not quite that simple for say road racing as whatever gains you get in cornering, you’ll probably give up in the ability to get back on the throttle early. Something you would need to test and tune for sure, no guaranteed rule of thumb there.

In other words, wider wheels do not always net an increase in actual grip, but they can under the right circumstances with the correct suspension to go with.

As a matter of fact, the number one place you can pick up GRIP is in the tire tread and compound you choose. A 6″ wide tire, if it is appropriately sized and chosen for the application can handle over 1g of force if you use the right tire. You do not need a 14″ wide tire to achieve that kind of grip.

We’ll talk at another time about other ways that grip can be increased, just know for now that raw grip is not directly (and sometimes not at all) related to the width of the tire. Especially when we’re dealing with modified production cars.

As a bonus, thinner wheels tend to be lighter as well. You can gain a lot of wheel and tire weight by going wider.

In Summary and Some Recommendations

So, I’ve said a lot in this article. It’s quite long and there’s a lot to chew on here. Please feel free to post your questions in the comments section and I’ll see what I can do to answer them.

There’s a lot to say about this topic, and I’ll probably go in much deeper detail when I do my Braking & Handling online course coming up in I think May is when we’re going to do that.

I’ll be posting more information about that as we go along as I find out more. I’m thinking about doing it as a webcast event – where you can see me and hear me live or recorded (if you miss the live event), maybe even chat and ask questions in real time. Let me know what you think.

The big takeaway from this article is to stay as close to the stock width as you can. You will want to try to stick with the same offset wheel (possibly a few mm out or in, but try to stay at the same offset). Look at factory wheel sizes, especially optional sizes to get an idea of what offset and widths are appropriate for your car. Be sure when comparing wheel sizes and offsets from other vehicles to see if there are also suspension differences.

Next time we’ll wheel diameter and overall tire package diameter. It turns out that sidewall choice can not only change the reliability of your car on the street, but it can also significantly alter your handling in fairly surprising ways.

Last week I explained how changing wheel width too much on a street car can change your car’s handling at the limit and how it can actually make for some dangerous situations in the more extreme cases. We discovered that wider wheels do not always (and in most cases never) increase the car’s ability to grip the ground in the corners and how wider tires do not necessarily put more rubber on the ground.

Fortunately, while wheel width is limited by suspension geometry, wheel diameter can be whatever you like, as long as it fits and you’re willing to accept certain trade offs. But before you go thinking it’s a total free for all, let’s talk about the ways wheel diameter affects your car’s performance and the trade offs you make, especially when going larger.

The trade offs with bigger wheels are: (in no particular order) reliability (resistance to bending), slightly changed center of gravity, increases in power required to turn the wheels, harder steering effort in non-PS vehicles, possible decreases in cornering performance, added expense, ride quality suffers, and possible gearing change.

The upsides are fewer but pretty obvious and rather important to some enthusiasts: looks, better stability during spirited driving (sidewall temperatures do not soar) and the ability to fit over larger brake rotors.

Sizing

When selecting a wheel/tire combo, you should always select the size with the closest to factory overall wheel/tire 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. Within 0.8% is good, I like to stay with 0.3% or less whenever possible.

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

Tire sizes are usually given like this: 215/45/17. What this means is that the tire is 215mm wide, the sidewall is ~ 45% of the 215mm width and the last number is the rim size the tire is for. If you want more information on tire sizing and specifically decoding tire sizes on tires, check this link.

Sidewall & Overall Size Considerations

With wheel diameter increases come a decrease in sidewall size in order to maintain the same overall tire+wheel combo size. It’s important to stay as close to the stock tire+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 tire size. Increasing tire 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). There will also be changes in your speedometer reading in most cars with different sized wheels.

In any case, making the overall wheel/tire combo taller or shorter will again affect suspension geometry. You can change it a small amount without any real adverse effects but try to stay as close as possible when choosing a new size.

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. You can run lower profile tires with stronger wheels, but in general the stronger the wheel, the heavier it is as well.

I assume most of you are running fairly lightweight aluminum wheels that while strong, will not do well with less than a 45 section height against road imperfections. The sweet spot in my experience is to aim for a tire between a 40 and 55 section height. If you are running a 60 or higher section height, you’ll have a great ride but you’ll have odd handling at the limit due to excessive tire sidewall flex.

You’ll also probably experience premature failure on a road course due to excessive temperature in the tires from all that sidewall flex. If you’re running 40 and lower, you’ll have a very rough ride and be prone to bending rims.

From all the experts I’ve talked to, it seems that around the 45 sidewall size is the best all around. You’ll find this is a very popular size with modern cars (especially since the trend has been towards larger and larger wheels on factory cars). This means these tires will usually be reasonably priced and have a lot of available treads and compounds to choose from. Avoid “weird” sizes as they’re expensive and you’ll have few choices in performance rubber.

Small sidewalls are very stiff, great for cornering “feel” but terrible for ride quality and straight line stopping and acceleration (more on that later). Stiffer sidewalls do not give any additional grip.

As the sidewall size goes down, the weight of the tire stays the same or gets bigger.

However, as a result of an increase in wheel diameter, this makes the weight distribution of the wheel+tire combo worse – the heaviest part (the tire) is now further away from the center of the wheel – refer to Part I for why that’s bad. In short, 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 the tire weight and its effect on the weight distribution of the wheel.

Warning: Never make the mistake of choosing a TIRE by its weight. Usually a tire’s weight is directly proportional to how much reinforcement is inside the tire. Cheap tires are often lightweight, but only because they cheeped out on the sidewall reinforcement. I actually had a popular cheap tire go out of round because it was so cheaply made inside. Choose tires based on tread and compound – never by weight. DO consider the weight of the tire when picking the size of your wheel, however.

Larger sidewalls allow better ride quality, better straight line launch, braking and do not necessarily decrease grip but 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 tire 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.

You’ll 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 tire 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 tires off the line. Taller sidewall tires can generally bite harder both under braking and acceleration.

If you’re looking to fit wheels that weigh less than stock, the larger you go in diameter, the more difficult and expensive it will be to find a lighter than stock wheel.

For Project Lexus, which I’ll be posting about later on, I chose to stick with the factory diameter of 17 inches since it already had a 45 section height tire. The factory wheel weighed in at roughly 22 lbs and we were able to find a replacement wheel, the Kosei KS T1 weighing in at only 14 lbs. I replaced all 5 wheels including the spare, losing about 40lbs in the process. 32 lbs of that was unsprung, rotational weight as discussed in section 1 of this series. I noted a small gain in city gas mileage (1 mpg on average), faster acceleration and a better ride.

Acceleration actually was measured between 35-60mph and showed a repeatable and respectable improvement of just under 2 tenths of a second.

These wheels are just one of many lightweight wheels available in various sizes.

On some past Civic/Accord project cars, these cars tend to have very tall sidewalls. I have typically gone up to 15s or 16s on older Hondas to get to the 50ish sidewall height. For the new model Civic, a great upgrade for the non-Si models is to go for the Si or OEM optional sized wheels. The Si model actually already has the ideal size for the suspension.

In closing, check out the PDF at the end of the last part of this series (Part II). There’s some interesting tid bits in there about how running a lower profile tire actually affects center of gravity height. Granted, for most cars this change would be insignificant compared to an SUV. It’s interesting despite not being terribly relevant for most of my audience

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tink hit it -

..."those who want to capitalize increased

HP & Trq, which is better, wider wheel

rims or larger diameter wheel size?"

all depends on the road, use, event, suspension,

gearing, desired rpm 'sweet spot, weather, grip,.......

Racing02wheelliftpic.jpg

alpina02foto.jpg

6041053302_cd3c436c64_z.jpg

72Elbawarmbold.jpg

i'm sure all the above cars have 130+HP yet look at the different

rim width, Dia., ride height, road surface........

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