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Old 10-22-2019, 07:31 AM   #1 (permalink)
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wheel discs only on the front?

Ok, can anyone answer why some cars seem to only have discs on the front?

e.g. mclaren speedtail



.. and random race car, mazda 787



I would have thought that if they were good for aero, it would be best to have these on all 4 wheels? If you were only having one axle with covered wheels, surely makes more sense to cover the rears (leaving fronts open for brake cooling?)

I'm not saying these guys are wrong, i'm sure there is a whole bunch of work to come up with this set up but not sure if i understand the logic?

Any one care to comment?

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Old 10-22-2019, 10:56 AM   #2 (permalink)
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The rear is in a turbulent area that doesn't improve handling with a disc?

It's not like they are fuel efficient.
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Old 10-22-2019, 11:30 AM   #3 (permalink)
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I think those style of wheel covers do two things.

1. They are aerodynamic.

2. They cool the brake via turbine fins

PLUS

3. When the front wheels cut for a turn they are still doing all of the above - the rear wheels stay in plane with the body however.

The rear wheels also get cooling via dedicated ductwork typically in high performance cars.

An example below of turbofan wheels from the early 1980's...

https://www.jdmeuro.com/tag/turbofans/


Jan 2019
Turbofan wheels do more than just look good
https://www.hagerty.com/articles-vid...just-look-good
Quote:
In terms of ‘70s/’80s racing ephemera, turbofan wheels are rare air. Turbofan wheels, named after the jet engine, were conceived as an aerodynamic boon.................

Managing tire and brake temperatures on a racecar is an exact science. Both need to be kept warm without getting too hot. As modern disc brakes grew larger, more complex (vented, slotted), and more powerful, overheating became an issue. Good brakes are the difference between victory and an inglorious crash into a tire barrier. And with brakes, the right temperature is key: Cold brakes can’t stop as effectively, hot brakes lead to fading, and overheated brakes are downright dangerous. But getting tires and brakes up to operating temperature quickly is also necessary, especially in qualifying, where time is a constraint.

Turbofan wheels work to passively pump air inside the wheel toward the brakes. The outside covers direct fresh cooler air inward. Perpendicular louvers underneath the cover guide the air directly at the brake rotors. Ambient air moves faster over the outside cover than the hot air inside, and the low pressure generated by the outside air draws the hot air out.
This is interesting....................

Quote:
Eventually, turbofan wheels were banned by most racing series, deemed “moveable aerodynamic devices.” Rumor has it that some wheels were designed to evacuate air from the entire bottom of the car, thereby increasing downforce.
I think from a marketing viewpoint, too many aero-treatments make the car look fussy or delicate, and not very macho. If only 1/2 of the car has aerodynamic wheel covers, the people that want none of it, and the people than want them on all four may be able to live with it, hence a wider audience acceptance.
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Old 10-22-2019, 11:42 AM   #4 (permalink)
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Quote:
Originally Posted by rob.e View Post
I'm not saying these guys are wrong, i'm sure there is a whole bunch of work to come up with this set up but not sure if i understand the logic?

Any one care to comment?
Look at as many CFD images of street cars that you can.

There is a typical pattern of the front wheels generating more drag than the rear wheels - on most cars. I'm sure that you will find an exception or two, but look for a majority pattern.

https://www.simuleon.com/event/intro...-to-xflow-cfd/


EDIT:
Photobucket has blurred out the CFD of my Pick Up truck, but you can still see more drag on the front wheels.

Pickup Truck Aerodynamics - CFD Study Chevy S10
https://ecomodder.com/forum/showthre...s10-35043.html


As to the cause of this phenomena, I'd wager to say there is more upset air at the front of the car as the pressure and air velocity first attempt to stabilize. Just more of a party going on up front.

Keep in mind that high pressure up front means low air velocity, and high air velocity means lower air pressure. This is not very intuitive, Mr Bernoulli got it right though.

Bernoulli's principle
https://en.wikipedia.org/wiki/Bernoulli%27s_principle
Quote:
In fluid dynamics, Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy.
So now we want to ask if it's the pressure causing the drag or the air velocity.

This is what NASA has to say.

What is Drag?
https://www.grc.nasa.gov/www/k-12/airplane/drag1.html
Quote:
Drag is generated by the difference in velocity between the solid object and the fluid.
So it's Velocity and not Pressure that causes drag.

Can we conclude that there is more velocity at one corner of the vehicle verses the opposite corner, or will they be equal?

Do we look at pressure CFD illustrations or velocity CFD illustrations or both?

Like I said, I think one needs to keep in mind that the front wheels pivot and the rear wheels do not, and maybe just leave it at that.
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Last edited by kach22i; 10-22-2019 at 11:58 AM..
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Old 10-23-2019, 03:48 AM   #5 (permalink)
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Good info, kach22i. I've learned something new today.
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Old 10-23-2019, 06:08 PM   #6 (permalink)
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all wheels

Quote:
Originally Posted by rob.e View Post
Ok, can anyone answer why some cars seem to only have discs on the front?

e.g. mclaren speedtail



.. and random race car, mazda 787



I would have thought that if they were good for aero, it would be best to have these on all 4 wheels? If you were only having one axle with covered wheels, surely makes more sense to cover the rears (leaving fronts open for brake cooling?)

I'm not saying these guys are wrong, i'm sure there is a whole bunch of work to come up with this set up but not sure if i understand the logic?

Any one care to comment?
GOOGLE for an image of the 1987 OLDS AEROTECH long tail,land speed record car.This car made more concessions to drag reduction.
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Old 10-23-2019, 09:32 PM   #7 (permalink)
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Old 10-25-2019, 12:19 PM   #8 (permalink)
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Damn.. those olds wheel covers are exactly what i want for my car


Some great info here - thanks chaps.

I'm thinking of making my own covers, just a couple of them first. I can then try these out on front axle and rear axle separately to see which works best.

I had been thinking about just adding to rear (to leave fronts open for brake cooling) but based on those cfd flow images i think that's flawed - fronts look to be most benefit.

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Old 10-27-2019, 03:55 AM   #9 (permalink)
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Quote:
those olds wheel covers are exactly what i want for my car
Which one? The upper example looks like Moon disks. The lower example could be recreated with a Moon disk pizza pans hole-sawn and dimple-died, or better yet chucked up in a lathe and spin-formed like the disk was originally.
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Old 10-27-2019, 10:09 AM   #10 (permalink)
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Pizza pans are 2 stage die stamped then have the edge rolled on a can opener like device. The only reason to spin one would be you have a plywood buck pattern at home on your drilling device.

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