Why 15" wheels?

[Spee]

Just a quick question!

I've noticed lots of Cliosporters go for 15" wheels for track days, what is the reasoning behind this?

Is it purely due to the cost of tyres, I know the 15" track tyres are cheaper etc, or is there a handling/performance gain I don't understand?

Ta

G

 

[Reyes]

Lighter.

 

[Daniel]

Theres a few reasons.

Lighter (depending on wheel) cheaper tyres, better handling (depends on the person judging imo!).

 

[ForceIndia]

Cheaper is the biggest reason. 15" tyres are dirt cheap, 16"s aren't much cheaper than 17".

 

[jenic]

Cheaper tyres, better handling and my overly soft suspension doesn't cause the tyres to rub on the bumper bracket when pushing it.

 

[no-name-person]

I was looking at 15's however yesterday when I had a quick look at the clearance on my 16's.. there's hardly any! It'll be a VERY tight squeeze with my (standard) brakes. Is this normal?

 

[jack_kemp]

The main handling difference is due to the reduced gyroscopic effect due to less wheel weight, the car will turn in sharper and should reduce understeer.

Also less weight to accelerate and decelerate... Unsprung weight makes 4x as much difference as weight in the car. So 5kg saved per wheel and youve got an 80kg weight saving.

Plus tyres are less than half the price in 195/50/15

 

[MarkCup]

Also less weight to accelerate and decelerate... Unsprung weight makes 4x as much difference as weight in the car. So 5kg saved per wheel and youve got an 80kg weight saving.

No.

There is not less weight to accelerate and deccelerate, there's less mass for the springs/dampers to have to deal with, which means the suspension can cope with what the road throws at it more efficiently.

I myself binned the 16s years ago. I understand that there's minor differences in how it feels, but I'm not Tony Hunter so can't say I've really noticed any difference, other than on my wallet.

 

[jack_kemp]

No.

There is not less weight to accelerate and deccelerate, there's less mass for the springs/dampers to have to deal with, which means the suspension can cope with what the road throws at it more efficiently.

I myself binned the 16s years ago. I understand that there's minor differences in how it feels, but I'm not Tony Hunter so can't say I've really noticed any difference, other than on my wallet.

It's both actually. For anyone that cares....

The laws of physics rule here. Energy is needed to bring the wheel/tire mass up to speed (mph) and to spin it to the related rpm. Consider a lighter tire/wheel combo:

m = mass reduction
I = inertia reduction
w = rotational speed
v = car speed
r = tire radius
E = total energy saved by mass reduction "m"
^2 = squared

E = 1/2 m v^2 + 1/2 I w^2

E = E(speed) + E(rotation)

Rotary Inertia is the sum of each bit of mass times it's radius squared. If all the mass reduction is at the tire OD (the theoretical but not practical limit), the inertia reduction is the maximum possible value:

I = r^2 m

The relation between car speed and tire rpm:

v = r w

Combine 3 equations above:

E = 1/2 m v^2 + 1/2 (r^2 m) (v/r)^2

E = 1/2 m v^2 + 1/2 (m) (v)^2

E = 2 x E(speed) ... the 2X limit

So if you drop 10 lbs per corner, the net total weight savings is 4 x 17 = 68 lbs. For a 3700 lb car and driver, that's only a 1.8% change.

The main benefit of reducing wheel/tire or any unsprung weight is handling and comfort, by increasing the tires ability to keep good contact with the road. Total unsprung weight might be 80 lbs per corner, and 10 lbs would be a 13% change ... very significant.

 

[jack_kemp]

^^^^
This is for linear acceleration, And assuming equal weight distribution across the wheel. The true equation is a horribly complicated logarithmic job. But the point is the same.

The difference is greater the more power you have too.

 

[ilovewilly]

Cos they is cool init.

 

[Kam.]

13" Wheels are going on my racecar small and light!

 

[MarkCup]

It's both actually. For anyone that cares....

The laws of physics rule here. Energy is needed to bring the wheel/tire mass up to speed (mph) and to spin it to the related rpm. Consider a lighter tire/wheel combo:

m = mass reduction
I = inertia reduction
w = rotational speed
v = car speed
r = tire radius
E = total energy saved by mass reduction "m"
^2 = squared

E = 1/2 m v^2 + 1/2 I w^2

E = E(speed) + E(rotation)

Rotary Inertia is the sum of each bit of mass times it's radius squared. If all the mass reduction is at the tire OD (the theoretical but not practical limit), the inertia reduction is the maximum possible value:

I = r^2 m

The relation between car speed and tire rpm:

v = r w

Combine 3 equations above:

E = 1/2 m v^2 + 1/2 (r^2 m) (v/r)^2

E = 1/2 m v^2 + 1/2 (m) (v)^2

E = 2 x E(speed) ... the 2X limit

So if you drop 10 lbs per corner, the net total weight savings is 4 x 17 = 68 lbs. For a 3700 lb car and driver, that's only a 1.8% change.

The main benefit of reducing wheel/tire or any unsprung weight is handling and comfort, by increasing the tires ability to keep good contact with the road. Total unsprung weight might be 80 lbs per corner, and 10 lbs would be a 13% change ... very significant.

Nice work.

I think my point has been missed though...I accept that there'll be a reduction in rotational inertia, and that the car could therefore build a wheel's rotational speed faster if it weighed less, but the mass of the car that the engine has to accelerate will only ever be reduced by the absolute weight saving of the wheel/tyre combo.

In other words, car weighs 1,000 kgs. That's the mass the engine has to drag up to speed.

Car weighs 980 kgs due to 5kgs weight saving per corner. The engine has to drag 980 kgs up to speed. It doesn't magically reduce the mass of the car down to 920 kgs or any other amount regardless of your clever maths which I've long since forgotten .

980 kgs is 980 kgs.

Isn't it?

 

[shiyan]

There's got to be some handling improvements of using 16" from lower sidewall height surely?

It's something I've always wondered, although I've not bought any 16" rims yet.

Or are 1*2s simply not powerful enough?

 

[Kam.]

Nice work.

I think my point has been missed though...I accept that there'll be a reduction in rotational inertia, and that the car could therefore build a wheel's rotational speed faster if it weighed less, but the mass of the car that the engine has to accelerate will only ever be reduced by the absolute weight saving of the wheel/tyre combo.

In other words, car weighs 1,000 kgs. That's the mass the engine has to drag up to speed.

Car weighs 980 kgs due to 5kgs weight saving per corner. The engine has to drag 980 kgs up to speed. It doesn't magically reduce the mass of the car down to 920 kgs or any other amount regardless of your clever maths which I've long since forgotten .

980 kgs is 980 kgs.

Isn't it?

overall mass is not the same as rotational mass. If this did not work no-one would fit lighter flywheels.

 

[jack_kemp]

No, because the wheels don't weigh 20kg under acceleration. They weigh more when they're under acceleration than when they're static.

 

[jack_kemp]

There's got to be some handling improvements of using 16" from lower sidewall height surely?

It's something I've always wondered, although I've not bought any 16" rims yet.

Or are 1*2s simply not powerful enough?

No, the sidewalls are normally stiffer on a larger profile tyre.

 

[shiyan]

hmm... in this test they got the best grip out of 18" rims

Now obviously the Golf uses an overall larger wheel/tire but the best result was obtained with tires of height ratio /40 and /45

http://www.caranddriver.com/features/10q1/effects_of_upsized_wheels_and_tires_tested-tech_dept

 

[jack_kemp]

It's tyre dependant, I'm assuming they used the same tyres across every test.
But you won't find a race car that uses low profile tyres, so that says it all for me.

 

[.Kev.]

Why not.

 

[shiyan]

It's tyre dependant, I'm assuming they used the same tyres across every test.
But you won't find a race car that uses low profile tyres, so that says it all for me.

I hope you're kidding :rofl:

Before you post a picture of an F1 car - they've been using high profile tires due to historic reasons, not technical.

There has been a push to move to lower profile tires in F1 as tire deflection is the biggest undamped "spring" in suspension tuning

 

[Kam.]

those are not as physically as low as many road tyres though...
though technically as the profile is worked out from the ratio of width to height they are 'low profile'
Race tyres run a stiffer side wall construction than a road tyre so the effect of reducing the side profile is less of a benefit.

 

[jack_kemp]

As kam says, those aren't really low profile tyres though are they? Also, driven wheels on race cars tend to have a deeper profile tyre due to their ability to store torque, aiding launch.

Having low profile tyres on a road also massively increases bump steer, tramlining and understeer/oversteer.

 

[shiyan]

Christ, if that's not low profile then I don't know :rofl:

Look carefully at the height of that tire in the top picture and divide it with its width - it's around 410/20 or 410/25 if you were to put it in road tire terms

A quick look at the tires fitted to various types of cars on the road is also a pretty good indication. Have yet to see a high performance car with /55 or above tires

 

[bill]

just go 15's Gareth, its gone far to technical for me now this thread lol, please close thread, my head is hurting now!

 

[jack_kemp]

You wouldn't run 55+ tyres, that's a ridiculous thing to say. 195/50/15 is what I've got. The difference between a 45profile and a 50 is tiny anyway.

You're just being a tool. That tyre is not a 20 or 25 profile tyre, 35 perhaps.

 

[Spee]

just go 15's Gareth, its gone far to technical for me now this thread lol, please close thread, my head is hurting now!

I know, I've poked a wasp nest!

I'm struggling to find 15"s, plenty of 16"s about though! I doubt it will make that much difference, despite the avatar, Senna I aint!

 

[Spee]

Just found this! http://www.evo.co.uk/news/evonews/236977/trackday_tyres_to_be_outlawed.html

Thats a worry! Track wheels and tyres in the back of the car from 2012 it seems. Suppose it will save wear.

 

[jack_kemp]

I know, I've poked a wasp nest!

I'm struggling to find 15"s, plenty of 16"s about though! I doubt it will make that much difference, despite the avatar, Senna I aint!

People are finally coming round to the idea that 15's are better so they're getting snapped up quickly now!

 

[lindleyp]

That complex equation posted earlier is spot on and equations speak truth, although ive seen that very same post on equations relating to this subject posted in a US forum, dont remember which, quite alot of smart people on there with bucket loads of knowledge, will try and find the thread for you.

 

[jack_kemp]

That complex equation posted earlier is spot on and equations speak truth, although ive seen that very same post on equations relating to this subject posted in a US forum, dont remember which, quite alot of smart people on there with bucket loads of knowledge, will try and find the thread for you.

That's where I took it from

http://www.mbworld.org/forums/wheel...ck/343981-effect-wheel-mass-acceleration.html

 

[d4nmc]

I love the feel of my car on 15s Vs 16s, the road noise decrease alone is worth the change. The price of tires is also a massive determining factor with them being sooooo much cheaper.

 

[lindleyp]

That's where I took it from

http://www.mbworld.org/forums/wheel...ck/343981-effect-wheel-mass-acceleration.html

knew it, most of the questions asked with regards to in-depth technical explanations are best found on other forums.:rasp:

 

[jack_kemp]

knew it, most of the questions asked with regards to in-depth technical explanations are best found on other forums.:rasp:

That equation is far too long for me to remember! Lol. I was going to just post a link to the website but figured peole wouldn't read all of that.... There's pages and pages and pages of it.

 

[st3ve]

Christ, if that's not low profile then I don't know :rofl:

Look carefully at the height of that tire in the top picture and divide it with its width - it's around 410/20 or 410/25 if you were to put it in road tire terms

A quick look at the tires fitted to various types of cars on the road is also a pretty good indication. Have yet to see a high performance car with /55 or above tires

That's because a sporty car with not so sporty wheels wouldnt help sales due to joe average thinking along the same lines as you.

Besides, apart from cheaper tyres, the added benefits of smaller wheels is mainly for track use and road cars are sold to be driven on the road.