Reverse cooling on A8 discs!

[Henka]

All of you guys running A8 314mm discs. Have you noticed that they run conciderably hotter than the 323mm discs? Apperently they are supposed to have a reverse cooling pattern. Supposedly they take air in by the wheel and spits it out in the center whereas the 323's take the air in by the center and spits it out at the wheel. Anone care to comment?

Zlobo claims that his 314's run much hotter than the 323's and his conclusion was that it had to do with the cooling direction.

Anyone got pics of this mess??

I'm still a bit confused though. I mean, a disc taking in cool air by the wheel will force the air agains't it's natural directions - cool air want's to go where it's hotter, the hub is likely to be cooler than the friction area, so the natural flow should be fron center and out... Confusing.

[dan_b]

314s are likely to be hotter than 323s if you're braking the same car at the same braking force from the same speed: the 323mm disc is a bigger heat sink: it can soak up, and disperse, a greater amount of heat in the first place due to its bigger size and mass. A larger disc (both diameter and thickness) will always be superior in this way.

I don't know anything about direction of airflow within forced induction in a brake disc though, there's probably lots of fluid dynamics science going on!

[Keeper of The Kitten]

It's really about the vanes in between the disc faces - just think of them as turbines a bit like the thing hanging off the RH side of your engine! If the vanes are "leaning back" (imagine the way they would look at the top part of the disc when viewed from the side of your car) they are drawing air from the centre and blowing it out of the periphery of the disc.

__MDC__

[Henka]

Hmmm.... So this is what happens when you turn them the wrong way around then? Or?

[Keeper of The Kitten]

Yep, if they were put on the wrong side they would be rotating the wrong way!

__MDC__

[Henka]

But would they reverse the cooling air direction??

[Keeper of The Kitten]

Yes, I think so, sort of - but it would be less effective anyway, because the "normal" direction takes advantage of inertia to throw the air outwards in exactly the way a compressor does.

With the vanes angled in the reverse direction they are trying to scoop air in at the outer circumference of the disc and move it towards the centre - but that is not a natural way for the air to flow. I think it is more likely that they would just disturb the air at the outer edge of the disc and not really succeed in moving it to the centre. The angle of the vanes would have to be a lot more aggressive to achieve the reversed airflow.

I think the tendency to move air outwards would be much easier, even with practically no angle in the vanes.

The more I think of it, the idea of using reversed airflow sounds like a very bad idea...

__MDC__

[Henka]

Yeah, and I can't for the life of me understand why ANYONE would do that. It goes against all sense. Has to be a myth!

[Stick Insect]

Put them on the other side of the car then?

David

[Stick Insect]

Put them on the other side of the car then?

David

sorry
Didn't read previous quotes....

David