Rear Discs

[LondonTom]

I've got a 280mm Wilwood four pot set up on the front, but standard rear 325i brakes on the rear.

I need to change the discs, but should I just stick with the standard discs or are there worthwhile upgrades?

Thanks.

Tom

[MillRat]

Nope, standard is fine (they don't do any work compared to the fronts).

Some disc manufacturers make slotted rear rotors (DBA for example), however these are still solids (I.e. not ventilated).

http://www.dba.com.au/product-search

For interest, in the 80's when they raced the E30's in the Australian Touring Car Championship, they would beef up the fronts, but still run the factory rear brakes.

A good set of pads, and refreshing the calipers should be fine.

[Black_Potato]

what he said

[LondonTom]

Cheers guys, really appreciate it.

[GeoffBob]

Tom,

I posted the details of my rear brake upgrade here, to which not one miserable bastard posted a reply

'Fraid I am not in full agreement with the other two posts above though. There is in fact one very good reason to upgrade the rear brakes, and it's NOT necessarily to improve the rear braking power, but rather to return some semblance of normality to the cars brake bias!

The simple fact is that callipers with larger diameter and/or more pistons require less hydraulic fluid pressure to achieve the same braking force upon the disc. This is why large/multiple piston calipers are such a blessing to people like me who run non-servo assisted brakes, such as on my track car. The larger pistons effectively return some of the "mechanical advantage" that was lost when the servo was removed (and there is only so much room to make the pedal longer in the drivers footwell). This is the primary reason I have both larger callipers and discs on both the front and rear of my car.

In your case, however (and I realise that my circumstances don't apply to your car), unless you have gone to the trouble of resizing your dual master cylinder, your front brakes now require less pedal force (with your larger discs and callipers) to achieve the same braking force as you had previously (with the OEM front brakes). Hence, without realising it, you are now pressing the brake pedal a smidge softer than you used to (to get the same required stopping distance), and so your back brakes are now working LESS than they used to, which is why some conclude that there is no need to upgrade the back brakes because they do so little work anyway (no signs of any brake dust). As a general rule BTW, your back brakes will always do less work than your fronts, due to the distribution of weight to the front while braking. However, this situation is exacerbated by upping the size of the front brakes, but not the rears!

Having said all this, however, I am still in agreement with the others that you don't need to upgrade your rear brakes. If you feel that your rear brakes aren't doing enough it's because you need to sort out the size of your M/C (specifically the front to rear bias) that arose when you upgraded your front brakes. If you don't, the only way you will ever get your back brakes to do any real braking, is well after you have pushed the brake pedal so hard that your front wheels have already locked up. And that is a fact.

Hope this all makes sense and is of some help to you.

Regards
Geoff


EDIT: I once took up this issue with KeriWMS who sells FRONT brake upgrade kits here on the Zone. Keri then went to great lengths to demonstrate to me that, despite the bigger disc and increased number of pistons on his WMS front upgrade kit, the total piston size was no different to the OEM brakes!! This is so important if you expect to maintain your cars correct brake bias.

Having fitted 4-pot Willwoods to your own car you have, probably unknowingly, upset your cars brake bias.

[keri-WMS]

I've been working on bias a bit recently, ONLY as a simplified "brake setup vs brake setup" comparison, not an overall car study - so this ignores everything other than the caliper/pad/disc.

Here are four examples, interesting results I thought!

Standard E30 saloon (not Touring!)


...fitted with 280mm disc / 1.25" piston WMS kit:


...fitted with 298.5mm disc / 1.25" piston WMS kit:


...fitted with 280mm disc / 1.38" piston WMS kit, this is the earlier spec and is still available for people wanting more front bias inside the 15" BBS (due to running semi-slicks for instance):

[Mikey_Boy]

Interesting stuff indeed! Agree with Geoff about Master cylinders and the others about standard rear brakes - when I upgraded my front brakes, I also installed an E32 750il master cylinder to improve pedal feel. Coupled to standard rear brakes this isn't perfect, but a helluva lot better than the standard master cylinder!

Cheers,
Mike