Wheel size vs Brake size

[EdLeake]

I'm in a quandry about wheel versus brake sizes.

The options are to run a 16" wheel with a 300mm disc, the AP 4 pot is lighter, the wheels are lighter, the tyres and naturally the discs too.

Tyres ~2kg, wheels 1kg and brakes 2kg lighter.

With 17" wheels I can run 330mm discs with slightly larger AP 4 pots, but is this extra brake capacity (performance and cooling) worth the weight disadvantage of the smaller setup?

The car in question is a 1140kg (with driver) 4 door saloon with a 68/32 split.

Opinions welcome.

[Chris Y]

Without overthinking this... In my experience, a 300mm disc and AP calipers should be more than good enough for the weight of car in question, assuming no more than say 170bhp.

I'm not sure 30mm extra on the disc would be worth the extra weight and expense of a 17 inch wheel size.

[EdLeake]

Ah well there comes the problem, your power suggestion.

The current production block 'fettled' but stock runs 220hp/165tq (209/158 stock factory), with a WTCC (S2000 spec) engine being built to 270hp/200tq for later in the year/early next.

Hence I was looking at running the same setup as WTCC/S2000 spec cars, 330 front and 260 rear, AP 4 pot and 2 pot respectively.

When the car was heavier (1330kg) I was having serious problems with brake heat, I'm a late braker and do work my brakes VERY hard albeit for shorter periods, but I eat discs through warping and heat.

That was running 2 pot fronts and to be honest carrying considerable weight.

I'm hopeful 300mm with AP's and cooling ducts will be suffice, but if they were why don't BTCC or WTCC run them... and why back in the late 90s did the Super Tourers run MASSIVE front brakes - because the regs allowed!

300 to 330 is 10% more surface area for braking and cooling, also the larger wheel would mean slightl more clearence and therefore more airflow.

I do however like the idea of 16s because the set I currently have are just 6kg each.

[andypipe]

Also consdider tyre size avaliabilty in each wheel size

[EdLeake]

Originally Posted by andypipe

Also consdider tyre size avaliabilty in each wheel size

I can run the required width in both 16 and 17", naturally 17's will have slightly smaller walls and therefore you'd expect slightly reduced flex.

[AU N EGL]

Removing as much unsprong wt is always better.

Quick caculations, for each 1 kg removed of unsprono wt is almost like removing 2.5 Kg of sprong wt.

or 1 US pound to ! 6.5 US pounds ( wt )

So wt savings in the wheels and brakes is always a good idea.

down side: tires, rims, easy to get or not?

wider front tires provide much better braking.

Overall wheel diameter effects gear ratio. Even as much as one inch has an effect on Gear ratio

Good Luck

[VT-R]

Had a similar quandary on my car and in the end went for the smaller wheels, few reasons off the top of my head...

1) Less weight
2) Lower cog. (based on available tyres)
3) Lower cog. as able to lower car further and still retain bump travel & driveshaft angle
4) Can use wider rim/tyre with fewer clearance issues.
5) Large cost saving on multiple sets of wheel/tyres

Probably could think of a few more if I tried... A lot of the problems above wouldn't be problems if you were designing from scratch, but things are different when you're trying to work with what you've got.

If rotors aren't big enough to dissipate heat you can always increase cooling by using water cooled calipers and increasing air flow to caliper/rotor. On the flip side of that if the rotors are too big, there's not much you can do about the weight.

[tristancliffe]

And that will then have an effect on your springing and roll resistance rates, as you would be altering the ability of the tyre to compliment (or otherwise) your chassis springs. Stiffer tyres require softer springs to maintain the same TOTAL wheel rate (at the contact patch) and frequency.

[EdLeake]

Sorry I should have said tyre, suspension and gearing elements aside, I'm considering just the wheel and brake elements here.

[tristancliffe]

I would say, in that case, get the biggest brakes that fit inside your wheels without being silly or stupidly heavy.

Then look at the other things (suspension, gearing etc), and cancel the order, and make a compromise or two. Then place an order. Then regret it when it arrives as you change your mind. Then find it works really well on the car, so your happy. Then find everyone else (who bought another option) is faster than you, and hate your decision making process...

There are no easy answers. Every change effects many different things. What are you prepared to compromise?

[Notso Swift]

Rotational mass or not you are only talking about less than 1%
Small things tend to be cheaper. I can run 17" wheels on my car (or 16") but choose to run 15" because, among other reasons (like weight) I can buy 6 tyres for the price of 4, so I go faster on the same (meager) budget


Yes the pro's use the biggest and best they can get, but they have different drivers to you and I... and do not assume they actually need the parts they have they sometimes have other reasons. Such as feel of the brakes. If they are under weight anyway something like this may be a consideration. If they were in trouble with weight I can say that this is one of the areas they will look at to loose some.
The stopping power is limited by your grip, Your car is about the same weight as mine, your power is less, but my top speed is limited (turbo restrictor) 300 mm disks should be more than capable, mine are 286 mm though I am on R tyres, not slicks. Since you have 16 already I would spend a little time on pad compounds and, importantly, ducting.

[AU N EGL]

Originally Posted by Notso Swift

Rotational mass or not you are only talking about less than 1%

of the TOTAL wt, not the rotational wt.

Removing rotational wt or unsprong wt is far more important then removing sprong wt.

If drive the same car with two different sets of wheels and one set of wheels are 1kg lighter per wheel then the other set, you feel the difference, you notice the acceleatation differene and the braking differences.

Go with the lighter set

[Goran Malmberg]

Just from memory and without beeing very seintific about equal tire grip etc, a 10 sec strip car changing from 15 to 18 wheels would be up to 0,5 sec slower. Much dependent on that the weight is farther out from the centre of the wheel. So, even if the weight of the wheels are about the same, a larger
wheel slows the car down.

We may notice that larger wheels also load the braking system.

I am not saying that lap times will slow down as there are other facors involved under these circumstances.

Goran

[ART Racing]

I run 16" wheels on my old touring car, and we run 309mm AP discs but there is room for more - I suspect the 315mm APs would fit under there, so don't limit yourself to thinking about 300mm discs as the largest you can fit.

John

[Notso Swift]

Originally Posted by AU N EGL

of the TOTAL wt, not the rotational wt.

Removing rotational wt or unsprong wt is far more important then removing sprong wt.

If drive the same car with two different sets of wheels and one set of wheels are 1kg lighter per wheel then the other set, you feel the difference, you notice the acceleatation differene and the braking differences.

Go with the lighter set

I understand rotational mass.
But we are talking 1" total on the diameter, that means if the same weight was on the rim for both wheels it is a 13% uplift, if it was a 18 inch wheel, then it is 45%, for the same weight COMPARED to the other wheel. (And in fact it will be less because not all of the weight is on the rim, it is distributed across the radius)
Assume the wheel combos are 20 kg and guess what, for an 1140kg vehicle we are still talking about less than 1% changes to the total inertia of the vehicle.

Regardless, the point was that other changes may be be more significant to the total performance, such as fresh tyres

[AU N EGL]

Originally Posted by Notso Swift

I understand rotational mass.
But we are talking 1" total on the diameter, that means if the same weight was on the rim for both wheels it is a 13% uplift, if it was a 18 inch wheel, then it is 45%, for the same weight COMPARED to the other wheel. (And in fact it will be less because not all of the weight is on the rim, it is distributed across the radius)
Assume the wheel combos are 20 kg and guess what, for an 1140kg vehicle we are still talking about less than 1% changes to the total inertia of the vehicle.

Regardless, the point was that other changes may be be more significant to the total performance, such as fresh tyres

Whole other discussion. Measure the inertia of the wheels seperate from the total inertia of the vehical.

Measure the wt of each rim 17" and 18" and if you can meanuser where that mass is located? Mass to the center of rotation vs mass on the outer edge of rotation is a very big difference even if the total wt is the same. Plus tires wts.

Yes fresh tires makes a huge difference.

One then can look at rotor sizes and inertia of a 13" disk vs 14" or even a 15" disk. Yes larger disks give a greater sweep area for braking, but is it worth that extra unsprong wt ?

Comes down to what the rules allow and what is avaible?

[EdLeake]

Interesting advice and informative, thanks so far guys, plenty for me to think about.

[JohnD]

Ed,
If the problem is not stopping but keeping the brakes from overheating, then air, air and more air! Improve your brake ducting.

Also, recirculating brake fluid, with a bigger reservoir?
Water cooling? A fine spray into the vents of the discs. Water has an enormous heat of evaporation (2260 joules/gram) so only a little can help, and you don't want a lot or risk shock cooling. Set up the spray to come on wit the brake lights!
John

[Al Weyman]

I only have 12" (305mm) rotors on both my cars, for one Chevrolet do a nice 12" rotor in there comps parts that bolts straight up and is reasonable and 2 they fit inside 15" rims although I do have 18" rims on the black car we have to be able to fit the standard wheel over the brake if required (my suggestion incidently after seeing people spending needlessly on huge brakes on little Mk 2 Escorts and stuff in ModProds when we allowed any diameter rim to be fitted. I have a nice big set of 6 pots on the heavier older car and that pulls up well from arouns 140mph at the end of the Snett staight and weighs in at about 1500kgs and can easly lock a wheel if pushed hard but I only Wilwood 4 pots on the black car I do feel although this car is a lot lighter it could do with bigger calipers.

The only time I ever had a problem was using a standard 'racing' brake fluid, then someone told me about Castrol SRF and I now use it in both cars and they have never failed since. A mate of mine has or had the big AP 13" set up on his SDI and I believe he has or was going to change them for smaller ones as he reconned they were too heavy and caused too much rotational inertia. However I will say none of the racing he or I do was with slick tyres and as has been pointed out that may be the deciding factor, the limit of adhesion of the tyres during braking.

[Goran Malmberg]

Originally Posted by EdLeake

1
The options are to run a 16" wheel with a 300mm disc, the AP 4 pot is lighter, the wheels are lighter, the tyres and naturally the discs too.
2
The car in question is a 1140kg (with driver) 4 door saloon with a 68/32 split.
3
With 17" wheels I can run 330mm discs with slightly larger AP 4 pots, but is this extra brake capacity (performance and cooling) worth the weight disadvantage of the smaller setup?

Opinions welcome.

1
You should be able to run larger than 300mm disc, I run 300mm myself on my
Pantera with 15" wheels and AP calipers. Four pot is ok.
2
With 68% front weight you will have som 85% during braking, which means you are hardley using the rear discs.
3
It is not the diameter only that determines heat capacity, it is disc weight (thickness as well as diameter), and disc cooling system. Disc diameter alter friction speed vs Tq and needed clamping foce for the same heat production.
So in your case you must depend and calculate very much on front brakes only.
Regards
Goran

[Al Weyman]

Then you must factor in tyre choice/championship requirements when selecting wheel sizes, the only reason I went for 18 inch rims over standard 16 inch diameter is because Toyo ionly make a 225 width tyre in the 888 range we are obliged to run. The car felt quicker on a 16 inch rim with a 265 low profile Kumho. On reflection a 17 inch may have been better but the 888 is also limited on that size with a maximum 255 width.

[AU N EGL]

Al is correct. Look at the tech rules for your class and what tires you can get.

[Notso Swift]

And compound, in our class we can only use the A048's imported locally to Au, 17" and above are hard, while 16" are Medium.

[Goran Malmberg]

I suggest using a 15p disc at the front with your cars weight distribution in mind. The weight could be had using a smaller but thicker disc or a thinner but larger diameter one. Then you can calculate for the disc dimesion needed using this weight in a wheel from 15-20".
You should make sure that you have a well working efficent COOLING system,
which seem to be forgotten in many brake sizing discussion.
A good quality 4 pot caliper is ok in any case. Even if the pad area is not the greatest ther are number of pad material that should bring its temperature to usable level. Consult your brake pad representative for tips.

However, there are some difference in cooling capacity between different diameter discs of the same weight, where the larger disc has some advantage. But even larger pads do cower disc surface. In any case, a 15p 320 mm disc is pretty balanced. the trick is to get both pads and discs to work within the right temparature range, which means that even oversize of any component is bad.
Regards
Goran Malmberg

[Notso Swift]

Yes, I was talking about the wheels and I was working on the worst, ie all the weight on the outer rim. Same weight and only 15" to 16" , so increase of 12.5 mm radius is 13% increase of the "apparent" wheel weight, but compared to total weight, which still needs to be considered because we race a total car we are still looking at less than 1% extra "apparent" weight that you need at accelerate (+/-)
If you are making big changes to the wheels, like 15 to 18, much more of an effect, but the case we are presented with is a plus 1" scenario, in this case I am saying that other factors will have more of an effect than just the wheel size.
The example I gave was th cost advantage I see in my own case, 6 tyres for the price of 4 more than makes up for any other disadvantage, like fractionally smaller rotors or 1” on the wheel (although it is actually an advantage because the wheel is both smaller and cheaper)
The rotational weight is not the be all and end all, yes keep it as low as possible, but other things can be, and are, bigger factors.