Anyone done the 850/540 f/r brake swap?

[[1990]525]

Has anyone swapped the 850i brakes in front and 540i brakes in the rear and recorded some stopping distance numbers? Just curious...

[Jeff N.]

525']Has anyone swapped the 850i brakes in front and 540i brakes in the rear and recorded some stopping distance numbers? Just curious...

I wouldn't expect a shorter stopping distance. The factory brakes can already initate the ABS thus demonstrating more than adequate braking torque.

The reason to upgrade would be improved heat dissapation under repeated stops (ie: track use).

Jeff --> M5 front brakes and 540 cooling ducts, stock rear brakes

[Brandon J]

Most people here don't actually have the means to record before and after a modification.

The 850/540 brake set-up can shorten stopping distance. The larger 324mm rotor increases the scrub radius, meaning is it easier to stop the wheel as you get closer to the outside (larger diameter) than the inside. Think of stopping a bicycle wheel in the middle or at the tire.

The 850 front only works well with the 540 rear because the 540 rear has a larger piston cylinder. Thus, more pressure can be applied for the same size diameter.

These 2 together, larger front scrub radius and more pressure to the pad will allow the car to stop shorter with more control. A lot of cars can lock their brakes, but doing it in a progrssive controlled manner is key. One other thing to look at is the pad on the 850 caliper is larger than any single piston e34 caliper.

These items do make more of a difference when at the track, in stop and go, and when wet. Of course, tires should not be skimped since the whole braking, handling, and acceleration depends on good tires first.

[Sidney Rough-Diamond]

I know a mate who has done a swap to 840 Brembo 4 pots and the larger discs of the 8er. All just bolts up. I think he has a set of the calipers for sale right now. Pity I'm broke or I would go for this.

[Jeff N.]

If you can exceed the friction threshold of tire with either caliper, the limiting factor is the tire and not the brake torque.

Are you saying that you gain stopping distance by reducing the time it takes to exceed the friction threshold of the tire? I guess I could see that as a possibility with the 850 caliper and the larger amound of torque/scrub radius.

On a percentage change basis, once you can lockup the tire, using the correct tire for the conditions should make the bigger difference in stopping distance over larger brakes.

I guess I'm saying that while I could agree that you might make a small improvement by going to the 850/540 setup in distance reduction, the smarter return would be look at the tires and put your money there.

None of this applies of course when you go the track. :)



Most people here don't actually have the means to record before and after a modification.

The 850/540 brake set-up can shorten stopping distance. The larger 324mm rotor increases the scrub radius, meaning is it easier to stop the wheel as you get closer to the outside (larger diameter) than the inside. Think of stopping a bicycle wheel in the middle or at the tire.

The 850 front only works well with the 540 rear because the 540 rear has a larger piston cylinder. Thus, more pressure can be applied for the same size diameter.

These 2 together, larger front scrub radius and more pressure to the pad will allow the car to stop shorter with more control. A lot of cars can lock their brakes, but doing it in a progrssive controlled manner is key. One other thing to look at is the pad on the 850 caliper is larger than any single piston e34 caliper.

These items do make more of a difference when at the track, in stop and go, and when wet. Of course, tires should not be skimped since the whole braking, handling, and acceleration depends on good tires first.

[Brandon J]

Good, so we both agree that good tires are always the first thing to look at. However, even a small improvement on the street makes the difference between hitting someones bumper or having one foot of cushion space.

Now, good, bad, worn, new, tires always benefit from good modulation of brakes. I have 345mm front brakes and 328 rear brakes in my e39. The front M5 brakes in my e39 has the same size piston caliper as all e34s and single piston e31s. The e39 M5 brakes can stop so quickly without tire squeal lock-up. It modulates the brakes extremely well. We do both agree on brake temp and reoccurring stops such as at the track. As far as the friction threshold of tires, good brakes allow the tire to get close to the threshold faster and modulate so as not to exceed it.

Last week I went to Gingerman raceway after I met Gustav at the Tirerack. Our group had a track day compliments of Tirerack. I have M5 brakes all around with Axxis Ultimates. Tires are 25k+ miles on Pilot Sports. The e39 M5 I ran with could not gain nor pull away. We traded spots between 20 min. run sessions to see each other's lines.

Before going to Gingerman, our group had a 2hr tech session on tires, handling, and roads. Very informative and learned a lot of new things as well as learned that I was doing a lot of correct things.

I use the M5 all around with Axxis Ultimates and Pilot Sports, 245/45 17 front and 255/40 17 rear. The front is larger for the slightly wider tire patch and for more tire side wall. Tires are suppose to work and keep as much contact patch on the ground at all times. I chose the larger side wall to contour to the uneven surfaces of roads and to give a slightly better ride in Chicago pot hole streets. I had to balance so I use fixed camber plates. Both together allow for great contact patch on turns, better suspension angle, very compliant ride, and great handling. Ask, Kris (DV M5) or others who have ridden or driven it. I can "dance the car" because of the good balance. It was fun doing this at the track going 55mph on a turn. Sabine does an awesome job of dancing the e39 M5.

Ok, that's my contribution and hope [1990]525 or anyone else can use this information while they decide on upgrades.


If you can exceed the friction threshold of tire with either caliper, the limiting factor is the tire and not the brake torque.

Are you saying that you gain stopping distance by reducing the time it takes to exceed the friction threshold of the tire? I guess I could see that as a possibility with the 850 caliper and the larger amound of torque/scrub radius.

On a percentage change basis, once you can lockup the tire, using the correct tire for the conditions should make the bigger difference in stopping distance over larger brakes.

I guess I'm saying that while I could agree that you might make a small improvement by going to the 850/540 setup in distance reduction, the smarter return would be look at the tires and put your money there.

None of this applies of course when you go the track. :)

[Jeff N.]

However, even a small improvement on the street makes the difference between hitting someones bumper or having one foot of cushion space.


When I teach at CCA and PCA skills days, one of the drills is a threshold braking drill. I'm sure you've seen and done it - basically practicing a panic stop. We do straight line at first, then put in a turn after several runs. Point is to get familiar with the abilities and feel of the ABS.

It's pretty interesting to look at the common mistakes - there are two. The first is that novices tend to release the brake pressure as the car begins slow - fail to maintain full pedal pressure until a complete stop. This of course increases stopping distance. :) The second common mistake that when asked to turn the car under braking ("collision avoidance"), they tend to release some braking pressure during the turn then re-apply pressure as the car straightens out. Again increasing the stopping distance. :)

Speaking of modulation (I agree with your points there), it's interesting to observe the improvements in the newer ABS systems over our cars. The newer systems seem to have a much faster ABS modulation rate. For the driver this means less chatter in the pedal and I suspect faster stopping times.

PS - I want a Tirerack sponsored track day!!! Sounds like fun. Pretty nifty that your E34 can run with an E39 M5. Done quite a few laps in the right seat with those and they are no slouch. Impressive torque and straightline speed...

[Robin-535im]

Am I too late to chime in on this one? :)

Keeping in mind that my knowledge in this area is very slim... my limited experience is more on the feel of the brakes with the bigger diameter. I notice an extended range of more linear feel when applying the brakes, so you can eek closer to the ABS and play with it more, so a human in the loop can fine tune the braking better.

The engineer in me has always wondered about the stopping distance though, because lock-up is lock-up, no matter the brake radius.

Good info in this thread - items like this are why I love this board.