Which set of wheels

[sawood12]

ORIGINAL: 944 man

ORIGINAL: Hilux
Not entirely as this can be offset by the increase in (or regaining back) the mechanical advantage by using larger discs BUT at the disadvantage of increased unsprung weight.

Of course. My point is that increasing the rolling radius without increasing the size of the drake disc, reduces the braking effort felt.

It works opposite way - i.e. you have to increase braking force if you increase the wheel dia. Think about it this way. Braking is all about energy. The energy the brakes have to generate is the same irrespective of the size of the wheel relative to the size of the brake disk - it's all about the weight and speed of the car. All brakes do is to convert kinetic energy to friction (i.e. heat). The energy generated by the brakes is dependant on the area of the frictional surfaces (i.e. brake pad area), the force applied by the caliper (i.e. pedal pressure) and the speed of the brake disc surface travelling past the brake pad (i.e. wheel rpm).

Therefore if you increase your rolling radius of the road wheel whilst keeping the brake disc dia and brake pad area constant you slow the wheel RPM down for a given speed, therefore the brake disk surface is travelling past the pad more slowly therefore less frictional energy is being generated and your decellaration rate will be slower. So to compensate you need to press harder on the brake pedal to increase force applied to the brake pads.

So using the same principle as long as you keep the pad area the same and the road wheel rolling diameter the same then varying the disk size wont matter to the braking force applied.

 

[944 man]

Im not sure what your point is Scott. If you increase your overall wheel and tyre diameter then your brakes will be able to provide less effort. This is what I have stated and it is a fact.

 

[John Sims]

ORIGINAL: Hilux

Comments on this forum are entirely the personal opinion of the poster and Porsche Club GB does not recommend sticking your fingers (or any other part of your, or anybody else's, anatomy) in the spokes of spinning wheels. [8|] [] []

[FONT=arial"]'Nanny' and 'State' come to mind [&o][&o]

Or perhaps it was a joke? Obviously we wouldn't recommend sticking things in the spokes of spinning wheels but that would go without saying. Except perhaps it didn't as by saying it apparently some didn't appreciate saying it was a joke so it might have been better that it was said any way. [8|] [] (PS A winking smiley following a comment is often employed to demonstrate the preceding comment should be read as a joke or sarcasm) []

[FONT=arial"]How long have I been a "guest" of the club [] ....................only just noticed [8|]

I don't know. How long have you been using the forum as a non member? []

 

[barks944]

ORIGINAL: sawood12

ORIGINAL: 944 man

ORIGINAL: Hilux
Not entirely as this can be offset by the increase in (or regaining back) the mechanical advantage by using larger discs BUT at the disadvantage of increased unsprung weight.

Of course. My point is that increasing the rolling radius without increasing the size of the drake disc, reduces the braking effort felt.

It works opposite way - i.e. you have to increase braking force if you increase the wheel dia. Think about it this way. Braking is all about energy. The energy the brakes have to generate is the same irrespective of the size of the wheel relative to the size of the brake disk - it's all about the weight and speed of the car. All brakes do is to convert kinetic energy to friction (i.e. heat). The energy generated by the brakes is dependant on the area of the frictional surfaces (i.e. brake pad area), the force applied by the caliper (i.e. pedal pressure) and the speed of the brake disc surface travelling past the brake pad (i.e. wheel rpm).

Therefore if you increase your rolling radius of the road wheel whilst keeping the brake disc dia and brake pad area constant you slow the wheel RPM down for a given speed, therefore the brake disk surface is travelling past the pad more slowly therefore less frictional energy is being generated and your decellaration rate will be slower. So to compensate you need to press harder on the brake pedal to increase force applied to the brake pads.

So using the same principle as long as you keep the pad area the same and the road wheel rolling diameter the same then varying the disk size wont matter to the braking force applied.

Isnt it a bit to do with moments? If your rolling radius is increased the wheel has more leverage and produces a larger moment for a given force at the road.

moment = force * distance from pivot

Assuming the brakes are unchanged they will produce the same moment for a given pedal pressure. This moment is then applied to the wheel and due to the increased distance from the pivot (centre of wheel hub) a larger wheel exerts a smaller force upon the road surface and hence produces a decreased acceleration to a smaller wheel.

The same thing explains why your acceleration is decreased with a larger wheel as the same torque applied at the wheel hub by the engine results in a smaller force applied at the road surface the larger the rolling radius of the wheel.

The factor of increasing or decreasing the wheel RPM must as you say have an effect on braking force as frictional force is a function of the speed two surfaces are passing each other isnt it? So if i keep my foot on the brake in the same position as the car slows down the braking force applied to the road gets less and less the slower the car gets. Any ideas how this effect compares in magnitiude to changing the leverage?

I also summise that a bigger brake disc has a 3 fold benefit then. It increases the avg moment produced as the average diameter the pad is acting on the hub is increased, allows a larger pad area and increases the average velocity of the disc in relation to the brake pad.

 

[944 man]

Spot on Tom. The brake disc/caliper's effort is constant and by increasing the rolling radius of the wheel and tyre you are increasing the size of the 'lever' that it has over them. The nett result is that increasing the rolling radius of the wheel and tyre has the same effect as fitting a smaller disc to your existing wheel, less braking; in exactly the same way that youd expect increased braking effort if you fitted larger diameter discs.

 

[John Sims]

Bigger brakes give considerable increase in surface area relative to diameter and also aid cooling by allowing more of the disc to be exposed. For example a 4" long pad may cover a quarter of the inner circumference of a disc but only and eighth or less as you get to the outer edge.

While I also accept that the outer edge of the pad will have greater leverage isn't there a trade off with the increased velocity of the outer edge. It must take more energy to stop something going faster than something going slower?

All in all it does seem that larger diameter brakes are better for (prolonged aggressive) stopping than smaller diameter brakes. There is obviously a dynamic trade off with weight and gyroscopic damping. What you consider is the best compromise is up to you - if you don't kill your current brakes you don't need bigger ones.

 

[barks944]

ORIGINAL: John Sims

Bigger brakes give considerable increase in surface area relative to diameter and also aid cooling by allowing more of the disc to be exposed. For example a 4" long pad may cover a quarter of the inner circumference of a disc but only and eighth or less as you get to the outer edge.

While I also accept that the outer edge of the pad will have greater leverage isn't there a trade off with the increased velocity of the outer edge. It must take more energy to stop something going faster than something going slower?

All in all it does seem that larger diameter brakes are better for (prolonged aggressive) stopping than smaller diameter brakes. There is obviously a dynamic trade off with weight and gyroscopic damping. What you consider is the best compromise is up to you - if you don't kill your current brakes you don't need bigger ones.

The disc will have its own inertia too and indeed it will have a detrimental effect on braking. But I think the increased velocity also gives an increase in the frictional force applied for the same pedal pressure. A bit like when your car speeds up the air resistance increased (im not sure the same principal applies to a brake). So as you say its a compromise of what gives the best benefit for the trade off, there are loads of dynamics at play its just a question of which ones have the greatest effect overall.

 

[edh]

ORIGINAL: 944 man

ORIGINAL: ikillcopiers

I got my '44 Turbo over a year and a half ago, and it came with Boxster wheels on it.
They're even the right size (205/55R16 Front & 225/50R16 Rear)

Your wheels should be 7" & 8" or 7.5" & 9", depending on model year. Those 16" Boxster wheels are/have 6" fronts and 7" rears, so whilst you may have the correct sized tyres; youre seriously under-wheeled. Ive fitted a set to my 924S, although Id prefer a set of ABS offset Turbo 7"x16" ATS telephone dials; or equally as rare, four 7"x16" D90s...

Simon

- or even 4 x 7.5" CS rims? - If you come across a set I'll swap you two fronts [] - I'm looking for 2 more 9's for a set of track wheels.

 

[944 man]

A generous offer Ed, but if I come across a set of forged wheels I'll be keeping them in my garage... [] I think that I will settle on a set of 7" 53mm offset 15" ATS telephone dials; a size that Porsche fitted on the Le Mans limited edition cars.

 

[Hilux]

equally as rare, four 7"x16" D90s...

I`ve got two []

 

[944 man]

Every 944 S2 came with two, but theyre still far harder to come by than 6"s or 8"s. The ideal wheels would be four front ABS offset Turbo 16" ATS telephone dials. Id happily swap the Compomotive 17"s on my track car for for a set of 7" & 8" 23mm offset 16" Turbo dials too...

 

[barks944]

Anyone know what the offset might be on a set of cookie cutters found on an 84 lux? The rears have spacers on them I'd say around 20mm at a guess.

 

[edh]

ORIGINAL: 944 man

A generous offer Ed, but if I come across a set of forged wheels I'll be keeping them in my garage... [] I think that I will settle on a set of 7" 53mm offset 15" ATS telephone dials; a size that Porsche fitted on the Le Mans limited edition cars.

Well it was worth a try [] - I'll just have to find a third set somewhere. Although don't forget that the forged wheels are cracking, so really ought to be disposed of []

I put some 16" x 7J 993 cups on my 924S, but the steering was certainly better on the 15" teledials. It was even sharper & lighter when I had some 195/45/15's on the fronts of my track wheels (some tyres left over from a previous track car that needed using up)

 

[944 man]

ORIGINAL: barks944

Anyone know what the offset might be on a set of cookie cutters found on an 84 lux? The rears have spacers on them I'd say around 20mm at a guess.

23mm.

 

[John Sims]

As everyone is getting excited about 16" wheels, I have a set of outrageously light Turbo option 928 style (Drain lid [8|]) wheels sitting in my shed. Better still they are shod the barely used Pirelli P Zero C track tyres.

I am sick of tripping over them and would look to them departing my premisses for fairly insubstantial remuneration - ie pretty cheap. The tyres are superb on the track and road legal so you don't have to chuck a second set of wheels in the car.

 

[edh]

What size tyres John? - I have loads of P Zero C's in 225/50/16 but none any wider

They are a good track tyre - grippy but still quite forgiving when they do start to slide.

 

[944 man]

Not a greatly desirable wheel, but theyre made by Fuchs I believe and are brilliant...

 

[Neil Haughey]

ORIGINAL: John Sims

While I also accept that the outer edge of the pad will have greater leverage isn't there a trade off with the increased velocity of the outer edge. It must take more energy to stop something going faster than something going slower?

A guy I knew years ago explained to me that this is a really good thing, increased pad area and larger diameter disks make it much less likely for the pad to bind onto the disk and hence cause the brakes to lock up. He reckoned this was the limiting factor in a lot of brake systems, you can get easily get enough brake purchase to lock the brakes but this is exactly what you don't want, you want maximum smooth deceleration without locking. He said this was one of the main drivers towards bigger wheels on modern cars, that of course and aesthetics.

On your second point the total energy required to stop the vehicle has nothing to do with the size of the wheels, brakes or anything else its simply converting the vehicles kinetic energy i.e. its mass times its velocity into heat, hence why using more disk surface to do this is better and less likely to cause the brakes to bind on.

The example given to me years ago was a bit extreme but explains the point perfectly. If one images pushing a nail against the disk it will do practically nothing to slow the car down until you bash it in hard enough to either stop the disk solid or punch through it. Bigger pads and disks equals smoother braking and hence shorter stopping times.

 

[Suffolk944]

As everyone is getting excited about 16" wheels, I have a set of outrageously light Turbo option 928 style (Drain lid [8|]) wheels sitting in my shed. Better still they are shod the barely used Pirelli P Zero C track tyres.

I am sick of tripping over them and would look to them departing my premisses for fairly insubstantial remuneration - ie pretty cheap. The tyres are superb on the track and road legal so you don't have to chuck a second set of wheels in the car.

Oooh interesting - would the offsets be suitable for my car ?

 

[sawood12]

ORIGINAL: John Sims

All in all it does seem that larger diameter brakes are better for (prolonged aggressive) stopping than smaller diameter brakes. There is obviously a dynamic trade off with weight and gyroscopic damping. What you consider is the best compromise is up to you - if you don't kill your current brakes you don't need bigger ones.

That is the main reason for installing larger brakes - it isn't for the larger and more powerful calipers, it is to support larger pads as the larger surface area will generate less heat for the same braking. Braking is limited by the tyre grip. Assuming you are braking to the limits of tyre adhesion then that sets the amount of energy (i.e. heat) the brakes have to generate. With a larger pad that energy is spread over a larger area therefore the effect is to reduce the temperature as a larger area of a cooler temperature will have the same energy as a correspondingly smaller area at a higher temperature.

For example, If you look at the 6 - pot calipers on the GT3 mk2 the sum of the piston area is less than on the 4 pot GT3 mk1 calipers (therefore less powerful), but yet the pad area is significantly increased which is what makes these brakes better at resisting brake fade.