0-60 simplified wheel physics and garfield's wheel test

[kurzhalse]

To reopen the dicussion a bit (why not - I got tired of waiting for my S and ordered up a set of SSR's for it). I noticed the post about improved braking with lighter wheels.

I would say wheel weight reduction is actually not a benefit, except in reducing brake fade after repeated stops. Braking speed after the calipers are closed is really a factor of tire grip. Other improvements such as calipers, pads, wheel weight don't matter, except when you're heading for Turn 1 at 130mph after repeated stops.

[Grinder]

Quote:

Originally posted by kurzhalse
I would say wheel weight reduction is actually not a benefit, except in reducing brake fade after repeated stops. Braking speed after the calipers are closed is really a factor of tire grip. Other improvements such as calipers, pads, wheel weight don't matter, except when you're heading for Turn 1 at 130mph after repeated stops.

You seem to be assuming that you're going to lock-up your wheels. That's *not* the way to come to a stop as quickly as possible (nor even possible with ABS).

[kurzhalse]

I see where my message was confusing. I did not mean brake lock-up at all, but rather threshold braking. The bottom line is that mechanically most any calipers/pads can stop a any weight wheel as fast as the driver likes. The determining factor in braking distance is the stickiness of the contact point with the road, rather than any of the mechanical bits. By saying, when the calipers close, I was just trying to eliminate the other factors that affect a driver's ability to get to threshold or ABS level...e.g. pedal travel, brake line flex, etc.

[Keith]

Here is a different (and maybe simpler) way of getting to the same answer...

If the weight of the wheel acts as though it was all lumped at a radius of 9", then the rotational energy is the same as if the wheel was moving in a straight line at the speed of a point 9" from the center.

If the wheel radius is 12", then this speed is 9/12 = 75% of the speed of the vehicle.

As energy is proportial to speed squared, the rotational energy of the wheel is 56% (75% x 75%) of the linear energy. i.e. A 1 lb saving from the wheel is equivalent to a 1.56 lb saving from the car.

[BrantV]

Quote:

Originally posted by kurzhalse
The bottom line is that mechanically most any calipers/pads can stop a any weight wheel as fast as the driver likes. The determining factor in braking distance is the stickiness of the contact point with the road, rather than any of the mechanical bits.

It's much more than just the contact patch. It's needs to be compared over the length of the stop.

A lighter wheel has some advantages when it comes to ABS, and the texture of your average road.

In an ABS condition, over slippery road surfaces, where the wheels lock, a lighter wheel will be able to spin back up faster and regain control quicker.

On a rough surface, when either stopping, accelerating, or just steering at the limits, a lighter wheel can be pushed back to the ground by the coils much faster than a heavier one. This increases the percentage of time that the wheels are in contact with the road and the average contact patch size and pressure throughout the length of the maneuver.

I don't know how effective this last one is, but here goes. In ABS a lighter wheel should be more prone to lock up if you applied the same pressures that you used with a heavier wheel. Yet it should also require less brake pressure to stop compared to the heavier wheel. That means that the ABS system is dealing with lighter pressures and might last longer.

Whether the small gains are meaningful to you or not is entirely subjective.

[tomstork]

Your point on maintaining contact with the road (i.e., a heavier wheel will have more momentum if it bounces upward on a bump) is a completely legitimate point; it seems to me that this (and the related ease of turning the wheel to steer at speed)--and not relative braking or accelerating performance--are (IMHO) the most valid reasons for having light wheels.

The rest of it--like the advantage in 0-60 runs--seems like a bit of a reach to me. A lighter wheel would indeed require less brake pressure to stop than a heavier one--but almost solely to the degree that the total momentum riding on the wheel is reduced, since this is by far the greatest force that the brakes have to overcome (i.e., not stopping the spinning wheels); given that the difference we're talking about here is only at most 15-20 pounds, relative to a fourth or so of the car's total mass (the length of the moment arm described by the radius of the brake disc to the caliper--that this weight is working against--is constant), it probably goes beyond "entirely subjective;" it simply isn't going to amount to much. I'd be surprised if it could be demonstrated to amount to as much variation between stops as the differences between relatively good/bad stretches of any normal road.

And, of course, a light tire and 15lb wheel that have just crumpled in a bad pothole in DC or some other real-world city probably won't accelerate or corner very well at all...

YMMV.
- Tom
'02 MCS DS/W
with ponderous 17" R90s

[Garfield]

Quote:

Originally posted by tomstork
And, of course, a light tire and 15lb wheel that have just crumpled in a bad pothole in DC or some other real-world city probably won't accelerate or corner very well at all...

Ah, and that 25lb wheel will crumple just as easily as the 15lb wheel. A big piece of swiss cheese is no stronger than the small piece.

Brian

[kurzhalse]

Very nice points about the wheel responsiveness to road irregularities!

I think its a great forum to emphasize again one of the easiest points to miss about braking. Its not the hardware that brings you to a stop...rather, its the grip of that tire contact patch and how well it sticks to the road surface.

A little more elaboration...if you think about it, any stock setup will allow you to lock up the brakes (at least for the first few stops.) So, that means you'll be able to reach a good threshold stop with stock hardware (and any wheels, light or heavy.)

So, the heavy stock wheels will be stopped just as quickly as the lighter wheels. But with repeated stops there will be less heat, and therefore many more quick stops with the lighter wheel.

[tomstork]

Well, a reply to each of the previous posts:
1) the idea that an object made with more of a material isn't any stronger than an object with less of it is just silly. That's why you use bigger structural girders when making a bigger building. Or, for that matter, why it's safe to jump on 8" thick ice on a lake, but not on 1" thick ice.

2) I have no real argument, in principle, with the idea that lighter wheels' brakes will be cooler, ceteris paribus, but again, how much cooler? The bulk of the work those brakes are doing is hauling the 2,600lb car down from speed, not in stopping the rotational momentum of the wheel/tire combination per se. This difference probably is not as big a factor as the degree to which the wheel design forces air across the rotors. Someone could calculate difference in rotational momentums (momenta?) between a 40 lb wheel at 60mph and a 30lb wheel at the same speed, and compare that to the momentum of a 2600lb body moving through space. Momentum is mv2, not mv or m2v--the speed is the bigger factor.

Enough. I'm sure everyone's sick of hearing about it; I'm going to go see if I'll be able to drive my MCS to work tomorrow. Maybe the snow has made me cranky...
- Tom

[swamos]

Hooray for science is all I can say. I seem to remember saying exactly this (that heavier wheels make a very very small difference to acceleration and deceleration), and that the only difference would be in ride and handling, and I got soundly howled down by all. About time we had some sense in here.

Some of the people in here actually said they could pick the difference in acceleration

And in the real world, having a wider tire offering more grip might well make more difference to the times (particularly in decelleration) than a lighter wheel with a smaller tire (as for MINI stock).

Ride in theory won't be as good with a heavier wheel though. However choosing the stock 17's over the sixteens may give you a car that corners better, and brakes quicker on smoothish road.

[BrantV]

Quote:

Originally posted by tomstork
1) the idea that an object made with more of a material isn't any stronger than an object with less of it is just silly. That's why you use bigger structural girders when making a bigger building. Or, for that matter, why it's safe to jump on 8" thick ice on a lake, but not on 1" thick ice.

Well in some cases, astonishing things really do happen. In the case of forged vs. cast with aluminium and many other metals, lighter forged parts are in many cases actually stronger than cast parts.

When the material is forged, it results in a better alignment in the crystalline structure. The actual atoms, are packed more efficiently, and the chemical bonds between the individual atoms become stronger. Casting on the other hand can result in bubbles. Aluminum has always been a difficult metal to cast without introduction of gaseous contaminants. Ironically enough, after decades of trying to solve the problem, there's now a huge growth in R&D trying to do just the opposite with aluminium foam.

Have you ever noticed how brittle cast iron is? It's the reason, column, girders, plates, and the connecting bolts are made from pressed materials and not cast iron.

Depending on the material, forging also gives it other attractive properties. It makes the metal more malleable and allows it to better withstand shock.

[BrantV]

Quote:

Originally posted by swamos
Some of the people in here actually said they could pick the difference in acceleration
...

Ride in theory won't be as good with a heavier wheel though. However choosing the stock 17's over the sixteens may give you a car that corners better, and brakes quicker on smoothish road.

I remember saying last year, that I would be able to feel the difference in the way the wheels performed, not their acceleration. Still waiting for someone to let me drive their MCS while blind folded.

The problem around here is that we don't have smoothish roads. So a lighter more responsive wheel becomes advantageous in many respects. A real nice benefit, is the confidence it gives you. Another is that while the 17" wheels can give faster turn-in, they are less forgiving.

A bit of exaggeration but when a 15" wheel approaches the limits of adhesion, it gives much earlier feedback that it is approaching its limits. It's like;

Driver: Can we go faster?
Tires: Yes!
Driver: Can we go faster still?
Tires: Yes.
Driver: Can you take any more?
Tires: Yes? Probably.
Driver: How about now?
Tires: Not likely.
Driver: Are you sure?
Tires: YES!

The lower the side-wall and wider you go, the more the conversation starts to look like:

Driver: Can we go faster?
Tires: Yes.
Driver: Can we go faster still?
Tires: Yes.
Driver: Can you take any more?
Tires: Yes.
Driver: How about now?
Tires: Yes.
Driver: Are you sure?
Tires: Yes.
Driver: What the @#$% happened?
Tires: Oops. I guess not.

[swamos]

Quote:

Originally posted by BrantV
A bit of exaggeration but when a 15" wheel approaches the limits of adhesion, it gives much earlier feedback that it is approaching its limits. It's like;

I do agree with you on this point. My other car has very small high profile tyres, and is very forgiving. However, in the 6 or so months we've had the MINI on 17's, it's never bitten me, and never really threatened either. Maybe if we were talking about a mid or rear engined car like an Elise I would be more likely to be swayed by these arguments.

As for smoothish roads, the MINI is more of a weekend car for us, so if the roads I'm travelling are very rough, I'll take our Toyota and leave the MINI at home When I take the MINI out, I seek good windy roads

I still stand by my original point though, and that is that the differences between the stock 16" 5 stars, and the 17"s wheels are very small.

[Garfield]

As well, the MINI will never bite anyone, it's a very simple and user friendly car to drive. It will only bite when made to, through rear toe out and high rear pressures. Even then, you just step on the gas to come out of it.


I am not going to argue that the wheels make such an incredible difference to everyone. But to those who do feel the difference in cars, the less unsprung weight the better.
I have nothing to gain from telling people my G-Tech results except giving people more feedback on the given situation. I'm not selling anything, nor am I gaining fame from giving out information (I wish I worked for Supertrapp).
I can still firmly say that I can feel the difference in both acceleration and handling from the lighter weight wheels. To argue with me is to not have driven both back-to-back, as many autocrossers/racers do because of wheel and tire changes before and after races.

In more cheaper terms, I'll admit that the 15's and all-seasons even feel BETTER. They made not hold a turn as long because of tire grip, but the turn-in, suspension response, and tossability are all better with the skinny little stock 15's. The only reason I'm not running 15's is because I bought wheels/tires for the Cooper S to run in Stock.

Like I've said before, I didn't do the math and then try to prove the math correct. I did a real world test, and others are saying the math proves it's correct.

I think the 17's look great, I love seeing them on my father's car. But, I believe the driving feel of them is not nearly as good as the much lighter 16's, hence the change. Something to keep in mind is that my MINI is very rarely driven "soft". The 15k miles I have on the car are all hard driven miles, same as most of the 40k on my Miata previous. This is my fun car and it's driven at the limit constantly, around every safe turn.

As it's been said before, to each their own.

Good Luck and Have Fun!

Brian

[Andy@Ross-Tech.com]

You guys are arguing about the dumbest things. What really matters in terms of wheel weight is how quickly they can stop when you drive into the back of a speeding semi trailer: