A debunk of "How Does Wheel Size Affect Performance" post

Hey everybody! Time to get educational, so yeah, it’s gonna get boring. Today we are going to debunk most of Michael Fernie’s blog post on wheels and how they affect performance. If you haven’t read it yet - here’s a link https://www.carthrottle.com/post/how-does-wheel-size-affect-performance/
Read it if you must, but we’ll be covering most of it here anyways.

The reason I’m doing this is not because his post is wrong on multiple occasions, but because I’ve seen enough “thank you for informing us” posts. And while Im all for blind leading the blind, I’d rather not have people believe in pseudo physics and base their wheel choice on false info. It could be just bad and it could also be dangerous. So let’s dig in! Also here’s a picture of a wheel for you, just in case you got bored and wanted to see a picture. On the house

Now first we’re talking about the “smaller wheels are better for acceleration dynamics”. And we are starting with Michael’s picture. The statement being “bigger diameter wheel has a bigger circumference” which is very much true. And the smaller circumference of a wheel would get you better “gearing” so to speak and put more power to the ground. Which is also true. But let’s think about this for a second. Apart from donk guys, who are honestly not in any way modifiers with a single bit of sense in them, who would swap their wheels for a bigger size, and then put an even bigger tire on it, so that the diameter of the whole wheel becomes bigger? Somebody who knows nothing about cars, surely. Well that person would ask for wheels he can put on at a tire shop and they’d give him tires that would be the same diameter as his stock tires. Just lower profile, to fit the bigger rim. So all in all, sorry, that was a pointless bit of information that should not have been aimed at a community of car people. No educated tuner would change the diameter of the whole wheel, so the “gearing” (and Im using the term very losely, in layman’s terms, since it’s a wheel, not a gear) would remain the same.

The second point Michael makes is “bigger wheels are heavier, thus they’re reducing your dynamics, by adding to your rotating mass”. Which is a fair point, also very much true, IF we remove the “bigger wheels are heavier” statement.
Here is a simple example.
A simple alloy wheel from a BMW e46, called “Style 45” (yes, romantic name, I know) is a 7.5jx16, and it weighs in at 10.5 kilograms each. A wheel from a 5 series m5 e34, called “Style 21” (even more romantic) is a 9jx17, and weighs in at 8.8 kilograms, all that being 1 inch bigger and 1.5 inches wider. A Koenigsegg Agera R wheel is an R19 and weighs in at 5.95kg.
Im certain I did not discover America for a lot of you guys, but the story here is “bigger does not equal heavier”. It very much depends on the design, materials and the process of it’s creation. Alloy cast, flowform, forged, or carbon fiber, it all varies the weight significantly, so you may not state that bigger wheels equals worse dynamics, if you’re losing wheel weight in the process.

Another area affected by wheel diameter is the tyre that has to be mounted to it. Since the speedometer and gearing are finely tuned to the entire diameter of the alloy and tyre combined, as wheel size increases, the profile of the tyre must decrease to not mess this calibration around

This is a direct quote, and I am wondering, if Michael actually knew this all along, why even bother with the point we covered first. But “Ok! SOME ACTUAL FACTS!” I thought. Sadly it was promptly followed by “This therefore means that the sidewalls of the tyre have to be stronger to withstand the lateral forces applied through cornering, decreasing ride comfort and feedback to the driver.” (also a direct quote). I will agree on the ride comfort, to save time, but it’s debatable due to wheel weight playing a lot into it as well, but it seems Michael is not interested talking weight, but more interested in size. So let’s do this bit too. Feed back to the driver suffers as you get lower profile tires, and it is, very much false, sorry. Does not work like that, and I will explain why.

Here we have a prime example of sidewall flex under cornering. As the car with a high profile tire enters a corner at high speed, due to excessive force sidewall flex appears, and the rim stops being alligned with the actual contact patch. the tire will return to normal position, once the corner ends. What we see here is a difference between wheel position and contact patch position of about 4-6cm? I didn’t measure it but let’s call it that? Ok. So the whole car shifts side to side by 5cm and it is good for feedback? I am sorry, but no, this only makes your car float on the track and steering feedback sloppy, when you never know precisely where your wheels are pointed, you have a rough idea, sure, but not pin point precission. We’d go into the tire hopping phenomenon due to losing grip and bouncing the whole car up as the tire comes back to it’s original position, but it’s beside the point. A lower profile tire will have less sidewall flex and give you a crisper feedback to the steering wheel.
I am hoping I explain this properly enough, to bring the point across. If not, ask in comments, I’ll reply with a better explanation.

But it’s not all bad. Increasing wheel diameter normally means increasing its width as well making the contact patch between tyre and road larger, hence increasing grip. On that note, you should really stiffen your suspension along with increasing tyre width as body roll will increase substantially if the car is able to dig much harder into corners due to the added friction leading to corner stability.

I do not think I really need to explain that this is not the case, cause most of us have been visiting wheel websites and have seen the available sizes. You can have a 7j 20inch wheel if you wish so, but true, you can have a bigger contact patch. Which is great! But the next advice… By stiffening your suspension you get your sports springs and shocks, or coils, and you limit your suspension travel. Which should be great, right? Well not exactly, this is what a very stiff suspension results in

Front wheel drive cars tend to do this, RWD’s tend to lift the front inner tire on acceleration out of a corner, but it’s beside the point. This is exactly what overly stiff suspension gets you - tricycle cornering. It looks badass, you can show pics to your friends and brag! I actually wish to have a pic like that of my car, sadly it’s suspension is well setup. The reason behind why this is bad is simple - you now have half the contact with the tarmac on your rear axle, since only one tire is on the ground. If you wish to reduce body roll you should really look into “antiroll bars”, surprisingly enough, their main function is to … well you guessed it, to reduce body roll. What you will find surprising enough is that they don’t make your suspension stiffer.

But all this is also, beside the point of “Wheel size performance” thing we’re on about here, it’s a suspension setup and there’s alot more than stiffness to performance when it comes to suspension. Lets move on.

Smaller diameter wheels will make for more concise handling as each rotation of the wheel covers a shorter distance, meaning more acute adjustments can be made during cornering compared to a larger diameter tyre, which will try to push further on into the corner and provoke understeer. On the contrary, the increase in grip for the larger, wider wheels will culminate in faster and more composed handling as stated above.

Well… here we are guys. Now if he would have stated “smaller diameter wheel (as a whole) can improve your braking as well as your acceleration due to a smaller circumference”, that would be very much correct, but better adjustments? What adjustments are we talking about here? Steering? The amount of lock you’re applying to your wheels has absolutely nothing to do with amount of wheel RPM. Braking? You’re adjusting the pressure on the rotor by the pads, not on the distance travelled by the wheel. This is such a confusing statement I had to read it multiple times. And I am still confused as hell trying to make some sense of it, and it’s one of the most upvoted posts on CT this past week! HOW? Do people read gibberish and go “yap, I agree to that!” and upvote? Larger diameter tire will make a car understeer? Based on what? If you change your tires to a different diameter uniformly on both axles, you’ll change the handling uniformly. If it understeered it will do so, if it oversteered it will do so. How does a tire diameter on all 4 wheels affect lateral grip of the front axle exclusively? Wider wheels will result in a composed handling?

Well this car now has a wider wheel than it had when stock, it’s now also riding on a very low profile tires, unlike stock. So when pushing hard you do not get the warning of sidewall flex (I did not say all sidewall flex is bad), you get a loss of grip without warning. Now how severe it is depends on the tire’s ability to lose grip gradually, but there’s going to be a lot less warning of it reaching it’s limits before it does, so that’s not very composed.

Sigh… This is getting out of hand, cause the original post raises more questions than it does answers, but if you want a TL DR, then…

Choosing wheels and tires for your car is not a simple “big wheels are for show, small are for go” kind of an answer, the whole car setup is a ballancing act, finding tradeoffs, searching for compromises, there’s never a direct answer. You want no sidewall flex for feedback, but you want SOME sidewall flex as a warning to limits, you want wider tires for grip, but you don’t want tires so wide they stop fitting your car, increase your unsprung mass and let go at such high speeds you don’t have a millisecond to correct it, you want your car stiff to be more crisp in it’s feedback,but soft for all the wheels to be on the ground at all times.

There is no “which side are you on, show or go”, you’re on your own side, and you chose things that should fit your needs. And if you’re going to take away anything from this post is “do not listen to every “expert” on the internet, even me, go out, get a few racing driver’s and engineering books, read about car setup, draw your own conclussions”. All I want to do is to plant some doubt in your mind, so you’d go out and find answers for yourself. Ones that are based off physics and facts, presented by professional racing drivers and car mechanics, not a guy on the forum posting donk pics. For me it was “Michael Gorbachev’s (NO, not that one, other one, the racing driver) extreme conditions driving guide” book, for you it could be something different, but PLEASE, find answers in technical literature, guys, do not mindlessly agree to everything you read about on the web, after all its your car, your safety, your handling and your life at stake when it comes to driving fast.

Stay classy, CT.