17s with 13" brakes or 18s with 14" brakes, is there a benefit in stopping power?

[Henesian]

I've been pondering this for a while. There are better combinations of 17" rim tire sizes than there are of 18" rim tire sizes. The only reason that I can see to going to 18" rims is for the look and 14" brakes over 13" brakes infront.

Is there any performance benefit from the 14" brakes over the 13" brakes, if you take into account the weight increase from the upsize? Or is it just for looks?

The price difference for the brakes is very minimal.

[rjsjea]

I'm not sure there are more 17" tires available over 18"s..........example is the popular Michelin PS2's. Bunch more 18's available than 17's

A whole bunch more billet 2/3 pc wheels available in 18"s.....

[clancj]

many more options in 18" tires/size over 17".

[Derek69SS]

Tire options are better in 18" but they're also more expensive.

I like 17s, but fear they'll be obsolete in a few years.

As for the brake size thing, be honest with yourself... will you ever overheat 13" brakes with track-pads?

[ProdigyCustoms]

Derek started to touch on this with the heat question. A 14 will take longer to heat up then a 13. Realistically a 14" brake MIGHT just add to the MOI (moment of inertia) by making the rotor more heavy / larger diameter. So in reality the brake caliper has to work harder to stop more weight. So technically by going from 17s / 13s to 18s /14s raises the MOI significantly, so braking MIGHT be poorer with the bigger brake kit. Who'd a thunk it?

So why 14s? Because there is more surface area and they take longer to heat up. And they look badd ass.

So unless your running the 24 hours of Daytona, do you need 14s? No.

[ROBS6T8]

Tire options are better in 18" but they're also more expensive.

I like 17s, but fear they'll be obsolete in a few years.

As for the brake size thing, be honest with yourself... will you ever overheat 13" brakes with track-pads?

I fear the same thing! Hopefully I'll be tired of my Rushforth 17's in a few years. Not!!

[DarkoNova]

I wouldn't worry about 17's becoming obsolete.

Look at old BMW CCA racing that restricts tire size to the OEM 14" wheels. There's still plenty of rubber to choose from in that size.

[Derek69SS]

Derek started to touch on this with the heat question. A 14 will take longer to heat up then a 13. Realistically a 14" brake MIGHT just add to the MOI (moment of inertia) by making the rotor more heavy / larger diameter. So in reality the brake caliper has to work harder to stop more weight. So technically by going from 17s / 13s to 18s /14s raises the MOI significantly, so braking MIGHT be poorer with the bigger brake kit. Who'd a thunk it?

So why 14s? Because there is more surface area and they take longer to heat up. And they look badd ass.

So unless your running the 24 hours of Daytona, do you need 14s? No.

I think the mechanical advantage of the caliper being farther from the axle center-line would offset any MOI drawback of larger brakes... but that is still rotating/unsprung weight that will have negative effects on acceleration and handling.

I wouldn't worry about 17's becoming obsolete.

Look at old BMW CCA racing that restricts tire size to the OEM 14" wheels. There's still plenty of rubber to choose from in that size.

Classing restrictions will be what keeps GOOD 17" tires around... Unfortunately, most of them will be 265mm and narrower for SCCA STX class.

[aronhk_md]

[QUOTE=Derek69SS;762054]I think the mechanical advantage of the caliper being farther from the axle center-line would offset any MOI drawback of larger brakes... but that is still rotating/unsprung weight that will have negative effects on acceleration and handling.

QUOTE]

I agree that having the caliper farther from center line outweighs any increase in MOI. Essentially just look at brake upgrades on a C6 vette versus C5. Any time the factory manages to put a larger rotor with the same caliper the net result is faster stopping AND less heat. I imagine too that while a larger wheel weighs a bit more and the MOI is increased due to mass a bit farther out...the tire weighs a drop less because almost invariably a lower profile is used. Dont know what that adds up to though

Either way, I'll take the 18 inch wheels with 14 inch brakes over 17's and 13's.

[JRouche]

Is there any performance benefit from the 14" brakes over the 13" brakes,

Not that you will notice unless you are tracking the car regularly and stay on top of the pad changes.

Brake pads WILL show a difference. The rotor size is minor when you are talking about 13 and 14 rotors. Not to mention how the rotors are used. Solid rotor and hat, cast unit? Separate rotor and hat? Size doesent mean ANYTHING!!

The type of brakes mean more. Even more important are the calipers. Single sided calipers, where the pads are forced together with the force of one side (sliding caliper) are less efficient. A decent caliper that forces the pads on both sides of the rotor with hydraulic pressure is better IMO.

A 13" rotor with a superior caliper will out preform a 14" rotor braking system that uses a sliding caliper and stock type calipers.

And toss in some really nice pads and you will be able to control your brakes from full lock up to a feathering of the brakes.

Larger rotors are nice when brake fade becomes an issue. I dont think most of the folks here have a problem with brake fade with their 13" rotors. If they do then its a brake pad problem, not a rotor size issue.

IMO 14" rotors are over kill and a source of unsprung weight, specially if they are a cast hub and rotor. Talk about some weight, thats a heavy unit.

I like an aluminum hat with a bolted on rotor. No reason the hat or hub needs to be cast iron, its just heavy. But its expensive to make a separate a cast rotor that will mount to various hats or hubs. Not so much the cast rotor, but the aluminum hat.

And Id like to see how many of the major castings of rotors integrated with the hubs are out of china. Bet its a large portion. Im talking about the after market products. Cast in china and maybe shipped out here for final machine work. Maybe not even that. Cast and machined in china. Wouldnt surprise me.

A large cast iron rotor is a lil excessive. And if its a cast iron rotor and hat!! Big time excessive. Ive put them on the scale. Some major weight there. JR

[Bryce]

Ok increase in MOI form 13" to 14" brake. everything else is equal, same thickness and material and internal venting, is 25%

Increase in torque form 13" to 14", assuming same clamping force from the caliper is 7.2%

Draw your own conclusions.

[Derek69SS]

Ok increase in MOI form 13" to 14" brake. everything else is equal, same thickness and material and internal venting, is 25%

Increase in torque form 13" to 14", assuming same clamping force from the caliper is 7.2%



Draw your own conclusions.

25% increase in the weight of the rotors alone, vs 7.2% mechanical advantage in braking the weight of the WHOLE CAR. I know which one I'd choose.

[aronhk_md]

25% increase in the weight of the rotors alone, vs 7.2% mechanical advantage in braking the weight of the WHOLE CAR. I know which one I'd choose.

Hmmmm.....I agree with Derek. If your rotors weighed 5lb in a 13, and then 6.25lb in a 14, but you net a 7.2% mechanical advantage in braking a 3500-4000 lb car....I also know I'd choose the larger brakes. Obviously not talking cast rotors here.....and the weight was just an imagined one, but still.

[SIK02SS]

Derek started to touch on this with the heat question. A 14 will take longer to heat up then a 13. Realistically a 14" brake MIGHT just add to the MOI (moment of inertia) by making the rotor more heavy / larger diameter. So in reality the brake caliper has to work harder to stop more weight. So technically by going from 17s / 13s to 18s /14s raises the MOI significantly, so braking MIGHT be poorer with the bigger brake kit. Who'd a thunk it?

So why 14s? Because there is more surface area and they take longer to heat up. And they look badd ass.

So unless your running the 24 hours of Daytona, do you need 14s? No.

Listen to this man. Only way a 14" rotor setup is better than a 13" rotor setup, is if the caliper on the 14" is itself different and better than what's on the 13". Besides, no one wants their over sized brakes slowing them down as a result due to more unsprung weight with rotational mass

[SIK02SS]

[QUOTE=aronhk_md;762150]

I think the mechanical advantage of the caliper being farther from the axle center-line would offset any MOI drawback of larger brakes... but that is still rotating/unsprung weight that will have negative effects on acceleration and handling.

QUOTE]

I agree that having the caliper farther from center line outweighs any increase in MOI. Essentially just look at brake upgrades on a C6 vette versus C5. Any time the factory manages to put a larger rotor with the same caliper the net result is faster stopping AND less heat. I imagine too that while a larger wheel weighs a bit more and the MOI is increased due to mass a bit farther out...the tire weighs a drop less because almost invariably a lower profile is used. Dont know what that adds up to though

Either way, I'll take the 18 inch wheels with 14 inch brakes over 17's and 13's.

The C5 and C6 also have different calipers, the C6 being better..

[aronhk_md]

I disagree...lol. Frank is knowledgeable but not the wizard of performance. Type of caliper is important too. But taking the same piston/type caliper........as long as a cast hub/rotor is NOT being used, there isnt some huge increase in weight or MOI from going to a 14" rotor from 13". Nor is there much increase in unsprung weight. The major increase in unsprung weight is from having to use an 18" wheel over 17". And the increased mechanical advantage the caliper has on the rotor is considerable. Even a 5% increase would be considerable when you are talking about stopping a 4000lb car.

Yes, I know the calipers are different C6 to Z06......thats what I meant. Sorry for the confusion. C5-C6 is the same. But I was referring to the Kore brakes for example.......where one can choose the C6 brake AND caliper, or choose the C6 caliper with a Z06 rotor.

P.S. - I've driven the newer Z06. I dont really think the big wheels and big brakes slow it down or make it less responsive in any measureable way over the base C6. Not going to compare braking because I know its a different caliper.

[Derek69SS]

Way too many variables to say one is going to make you faster than the other.

Type of course:
Average autocross, even 13" brakes are overkill... I've made 30+ autocross runs in one day in 90+deg weather at a test-n-tune, and never experienced ANY fade in my C5 brakes. Tires got greasy, but brakes never faded. Advantage - Smaller brakes
Put the same car on a big roadcourse with some speed and some really heavy braking zones to get the rotor temps up... Advantage - Bigger brakes

Horsepower:
300hp car - MOI is really going to effect your acceleration. Advantage - smaller brakes.
600hp car - MOI isn't going to matter so much, you can only accelerate as much as the tires will let you anyway. Draw.

Weight of car:
1800lb Lotus - doesn't need big brakes and an increase in unsprung weight would be easily felt.
3700lb Chevelle - more weight to stop, and the weight increase would be a smaller percentage of its total weight, so it would be less noticeable.

As with any part of your suspension combination, it's all about sacrificing one aspect to gain in another, and balancing those sacrifices to get the best possible combination for your usage.

If all I have to do is stop, I want the biggest ones possible.

[aronhk_md]

Very well put Derek.......and well thought out. I do tend to think of things from my perspective. I will be seeing higher speeds than some, and I do have a heavier type car. Thanks for putting that out there....I think it does the most to really qualify why one might want the larger brakes.....or the smaller.

[SLO_Z28]

Autocrossing ive never even come close to fading out my 10 inch brakes, and my braking is tire limited. I am switching to a 12 inch setup with a SL4 caliper on a aluminum hat but thats mostly to lose weight while maintaining pad size and increasing PSA. Derek is dead on a few posts up, the reason my brakes work for me are correct pad choice and usage.

[ProdigyCustoms]

So in summary, 14" over 13", brake package MOI and unsprung weight rise slightly, slight mechanical advantage. 18" wheel and tire package, MOI of the wheel / tire package increases slightly, unsprung weight increases slightly, no mechanical advantage. In summary, probably a unmeasurable difference.

I call for a test! A magazine needs to do this. It would be really simple with Wilwoods. The 13" kit is converted to a 14" kit with a rotor and a couple caliper spacers, could be done in less then a hour. Get 2 sets of tires and wheels, same brand / style wheel / tire. Swap the rotors, tires. My bet is there are within feet of one another, if not inches.

Steve?