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ChrisTropea
ChrisTropea Associate Editor
3/31/23 5:00 p.m.

https://www.youtube.com/embed/1aEX5dD1IT0

Bigger might be better when it comes to fast food combo sizes, but can the same be said of wheels?

Here’s why we downsized the wheels on our 997 Porsche 911 project car from 19 inches to 18 inches.

Presented by CRC Industries.

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MadScientistMatt
MadScientistMatt UltimaDork
4/2/23 7:58 p.m.

This seems like a category where there should be a just right size, and either bigger or smaller would slow you down. Older cars often had wheels that were too small for maximum performance, but it wouldn't surprise me if a lot of newer cars made their wheels too big.

LanEvo
LanEvo GRM+ Memberand Dork
4/3/23 12:42 a.m.

I've tested 15-, 16-, and 17-inch wheels on my racecar (Mercedes 190E 2.3-16) and found 15" was clearly faster. This was mainly down to a smaller overall tire diameter, which was a better match to the trans/diff ratios. There's also a significant savings in weight and tire costs. 

wspohn
wspohn SuperDork
4/3/23 10:14 a.m.

Adding unsprung weight is never the best way to go in terms of handling. The big wheel movement is style driven, not performance oriented.

Older cars had some odd choices - V8 powered cars with 13" wheels that didn't have room for adequate brakes and such.  Today we go overboard the other way .  Is there an ideal compromise size? Maybe 15", maybe a bit larger, but no one needs some of the huge wheels chosen for fashion rather than performance that we see today.

AngryCorvair (Forum Supporter)
AngryCorvair (Forum Supporter) GRM+ Memberand MegaDork
4/3/23 10:27 a.m.

brakes should be sized for vehicle weight, intended usage, and maximum speed.

wheels should be just big enough to clear the brakes.

fight me.

MauryH
MauryH GRM+ Memberand New Reader
8/11/23 1:55 p.m.

Another advantage of 18s vs 19s I found on my Cayman is the car is at least a half inch lower with the 18s which would lower the center of gravity a bit and thus seems to improve cornering. Partly tires since the 19s are 40 and the 18s 35 aspect ratio, but mainly wheel diameter...I think.

PT_SHO
PT_SHO New Reader
8/11/23 3:05 p.m.

If you aren't tied up in PCA events where they demand exact rim and tire of the as-delivered vehicle, then changing the tire / wheel size to improve your final drive gearing is one way to go. But your car may seem faster only because a smaller total height improves torque while it increases highway RPM. 

Changing total tire+wheel height and/or final drive ratio can actually muck with the car's computer as it senses the speed isn't "right" from comparing GPS speed with wheel RPM's, that can be a nasty surprise.

The rim protectors being sold as tires in 20, 22, etc are solely style driven.  And the tires are so short that they don't even do a good job of rim protecting.  C&D long term tests regularly show wheel replacements due to road damage.  I could tell the ride difference when I did back to back tests of an SUV with 17 vs. 18.  Now they are mostly 19 or larger. frown

A shorter tire height, all else equal, will usually mean quicker response though not necessarily better cornering.  That depends on camber control not allowing the shorter, stiffer side wall to roll so far that the tire footprint is compromised.  If a competitive tire in the same model is sized to retain the same total height and tread width, then compliance will be improved, weight of total assembly will be reduced, and results will usually improve.  But as tested here people usually _also_ change wheel widths so that's not an apples to apples comparison.

84FSP
84FSP UberDork
8/11/23 4:47 p.m.

Relevant factors are always weight, rolling diameter to move gearing or maintain, and width.  A smaller heavier wheel is not better in general.  A wider, taller, and lighter wheel could actually be better.  Generally the lightest, widest, smallest setup is best if this is autox.

VolvoHeretic
VolvoHeretic GRM+ Memberand Dork
8/11/23 5:55 p.m.

Taller no, wider YES!laugh

CrustyRedXpress
CrustyRedXpress GRM+ Memberand Dork
8/12/23 8:09 p.m.
LanEvo said:

I've tested 15-, 16-, and 17-inch wheels on my racecar (Mercedes 190E 2.3-16) and found 15" was clearly faster. This was mainly down to a smaller overall tire diameter, which was a better match to the trans/diff ratios. There's also a significant savings in weight and tire costs. 

Not surprised at all, but awesome to see it tested.

I remember the 2.3-16v's power band being relatively high in the revs and peaky...but also that the stock gearing allowed something like 145mph top speed.

Did you ever test a 14" wheel? Alternatively, can you swap in "shorter" gearing either for the Getrag or the rear diff?

DavyZ
DavyZ New Reader
11/16/23 7:17 p.m.
wspohn said:

Adding unsprung weight is never the best way to go in terms of handling. The big wheel movement is style driven, not performance oriented.

Older cars had some odd choices - V8 powered cars with 13" wheels that didn't have room for adequate brakes and such.  Today we go overboard the other way .  Is there an ideal compromise size? Maybe 15", maybe a bit larger, but no one needs some of the huge wheels chosen for fashion rather than performance that we see today.

Well said. Agreed.

malibuguy
malibuguy GRM+ Memberand HalfDork
2/13/24 8:44 a.m.

It looks good and makes sense.

In the 15" world tire choices are limited.  Its basically either race tires or basic tires.  Very little good stuff inbetween.

On my street car Id love to have a 300-400tw sporty all season tire because that car sees all temps.  I am stuck with some 500tw Kumho pa31s cuz sadly thats about the best option.  Going to 16s or 17s is not practical plus that car has classic 3spokes so I aint changing it.

On the race car Ive considered going to 17s for more tire options but I think having thicker sidewalls is better for dealing typical autoX parking lot surfaces.  If all it saw was glass smooth surfaces then the 17s may be an advantage over 15s.

 

theruleslawyer
theruleslawyer New Reader
2/13/24 9:16 a.m.
wspohn said:

Adding unsprung weight is never the best way to go in terms of handling. The big wheel movement is style driven, not performance oriented.

Older cars had some odd choices - V8 powered cars with 13" wheels that didn't have room for adequate brakes and such.  Today we go overboard the other way .  Is there an ideal compromise size? Maybe 15", maybe a bit larger, but no one needs some of the huge wheels chosen for fashion rather than performance that we see today.

A lot of performance cars today need 18-19" wheels just to clear the brakes. Non performance cars could probably get by with 17" or smaller. Of course brake size also has to do with increasing mass. Given that we're moving to even heavier EVs I don't know that brake size will decrease a lot. With regenetive braking maybe? Still I wouldn't expect 15" wheels to become a thing again.

FWIW when does tire size become  a problem for speed rating? Smaller tires have to spin faster. There is probably a compromise in there in terms of tire contruction vs rpm.

aircooled
aircooled MegaDork
2/13/24 9:30 a.m.

Yes, please bring back 15's.  As far as I can tell there is only one decent street performance 15" tire available with decently wide widths.  I really don't want to dump a bunch of money into new rims and tires hoping that one will still be made!

With the proliferation of electric cars though (much heavier = more brakes = bigger rims) I suspect ridicolarge rims are here to stay.

aircooled
aircooled MegaDork
2/13/24 9:30 a.m.

Yes, please bring back 15's.  As far as I can tell there is only one decent street performance 15" tire available with decently wide widths.  I really don't want to dump a bunch of money into new rims and tires hoping that one will still be made!

With the proliferation of electric cars though (much heavier = more brakes = bigger rims) I suspect ridicolarge rims are here to stay.

DirtyBird222
DirtyBird222 PowerDork
2/13/24 10:56 a.m.

Roll Center - we went up to 16s from 15s during a test, threw the whole feel of the car off. We did it because 15in tires are becoming increasingly harder to find. We could drop the car more to get the roll center back but seems unneccsary and then we would have to change out even more components and take points in Champcar to do so. 

gencollon
gencollon Reader
2/14/24 3:30 a.m.

In reply to aircooled :

Ah, but they can just run the motors in reverse to brake too. 1000 hp of go and 1000 hp of stop. It would cut down on the range a bit, and you'd have less brakes when the battery gets low, but how often do you need 1000hp of stop anyway?

Nockenwelle
Nockenwelle Reader
2/14/24 11:02 a.m.
AngryCorvair (Forum Supporter) said:

brakes should be sized for vehicle weight, intended usage, and maximum speed.

wheels should be just big enough to clear the brakes.

fight me.

15 x 8 on the dirte30. I think it will wear 15s for life, have not found a convincing reason to go bigger though I might try to fit a 9" wide wheel. The 23" tire works well.

.080" clearance, all homebrew BBK with 4-pot FSLI. The wheels are just big enough to smear clag on the calipers.

 

codrus (Forum Supporter)
codrus (Forum Supporter) GRM+ Memberand PowerDork
2/14/24 1:31 p.m.
gencollon said:

Ah, but they can just run the motors in reverse to brake too. 1000 hp of go and 1000 hp of stop. It would cut down on the range a bit, and you'd have less brakes when the battery gets low, but how often do you need 1000hp of stop anyway?

Pretty much every time you stomp on the brakes at the track?  You're using 90-95% of the length of a straight accelerating and only a couple hundred feet to brake.  There is WAY more horsepower in the brakes than the motor.

 

j_tso
j_tso Dork
2/14/24 1:57 p.m.
DirtyBird222 said:

Roll Center - we went up to 16s from 15s during a test, threw the whole feel of the car off. We did it because 15in tires are becoming increasingly harder to find. We could drop the car more to get the roll center back but seems unneccsary and then we would have to change out even more components and take points in Champcar to do so. 

were you unable to find lower profile tires to make up for the change in diameter?

 

aircooled
aircooled MegaDork
2/14/24 2:47 p.m.
codrus (Forum Supporter) said:
 

Pretty much every time you stomp on the brakes at the track?  You're using 90-95% of the length of a straight accelerating and only a couple hundred feet to brake.  There is WAY more horsepower in the brakes than the motor.

 

That's an interesting concept.  How much "horsepower" do the brakes generate stopping a car?  Pretty easy to calculate (I think) with just the distance and the weight of the car.

Here is a link to some MT best of's:  https://www.motortrend.com/features/20-best-60-to-0-distances-recorded/

2014 Corvette: 60 to 0 = 90ft
Curb Weight: 3360 lbs.

Bing AI says that represents 1.33 g's and 2.5 seconds.

It also says that will take 962.67 horsepower to slow a 3360 pound car from 60 to zero in 2.05 seconds.

It is assuming a 1 foot diameter wheel and 100% efficiency brakes (so real number is likely a good bit higher).  Will wheel diameter affect that number?  I would not think so.

Anyway, interesting problem, and a great use for AI.

MiniDave
MiniDave HalfDork
2/14/24 5:28 p.m.

Well, pretty much all the arguments are already posted, but there's a reason I run 13's on my Mini and not 10's or even factory 12's (on later cars when they upped the size of the brake  discs) and that Is I like the looks!

They say it cuts down on the top speed (when would I ever find out?) and that the car doesn't handle or accelerate as quickly as on 10's (probably true) but 10's won't clear the 8.4" discs (as compared to the 7.5's) plus - I like the looks! (didn't I say that before?)

Tires are an interesting thing as more 13's are available these days than before......but most are not high performance. I get Yokohama 539's from England.....

Snrub
Snrub Dork
2/15/24 1:56 p.m.

For powerful cars, adding overall diameter helps with traction. I'm sure the ideal size varies with the vehicle.

It's amazing how much heavier the wheel tire packages are on modern quick cars vs. the old 23.5" standard. When changing wheels/tires, it's the difference between"that's easy to carry " and that hurts my back." They're not just doing that for style.

gencollon
gencollon Reader
2/16/24 12:11 a.m.

In reply to aircooled :

Ah, but the tires are scrubbing and slipping and absorbing a good amount of energy as heat and the breaking down of the rubber molecules. 

If you apply enough torque to stop the tires rotating entirely, then you still stop pretty fast and it costs the brakes 0 hp to do it. ;)

If I had to take a WAG at it, I'd say the tires are doing ~20% of the work maybe in a really quick stop?

Oapfu
Oapfu GRM+ Memberand Reader
2/18/24 8:31 p.m.
aircooled said:

That's an interesting concept.  How much "horsepower" do the brakes generate stopping a car?  Pretty easy to calculate (I think) with just the distance and the weight of the car.

Here is a link to some MT best of's:  https://www.motortrend.com/features/20-best-60-to-0-distances-recorded/

2014 Corvette: 60 to 0 = 90ft
Curb Weight: 3360 lbs.

Bing AI says that represents 1.33 g's and 2.5 seconds. [[maybe a typo on 2.05 ??]]

It also says that will take 962.67 horsepower to slow a 3360 pound car from 60 to zero in 2.05 seconds.

  1. It is bloody amazing that AI can even try to do this at all.
  2. My suspicion is that Bing took the 2.05s 60-0 time, plugged it in to one of the 0-60 calculators, and used that to solve for power.  I have NFC where the number came from otherwise.
  3. There is huge amount of uncertainty and reality vs. oversimplified physics going on here.

My first thought was to scream at the idea of brakes "generating" power.  My second thought was that the 962 HP number could be reasonable, based on some of the stupid HP and 0-60 times for a Tesla or whatever.

Then I tried to do some math on my own, eventually pulled up Excel, and I came up with 358 HP based on initial kinetic energy at 60mph.  I fully expect this is a little low, but I also think Bing's number is really high.  I messed with a few of the online 0-60 calculators as a reality check, and it would be pretty sketchy trying to use one to back-figure HP for braking.

I can match Bing's numbers for time and acceleration, if "2.5" was a typo for "2.05".  Green are the 'givens', orange is Bing's calcs, everything else is my calcs based on simple motion formulas or SI unit definitions.

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