Rear caliper position varies on different vehicles

[InflatableBuddha]

On vehicles equipped with four-wheel disc brakes, I have noticed that the rear calipers are in different positions on different models.

The calipers are positioned on the rotor at one of a number of angles along the top half of the rotor. If you imagine a clock face, I have seen the calipers at 9:00, 11:00, 12:00, 1:00 and 3:00 (roughly 0, 45, 90, 135 and 180 degrees).

In contrast, regardless of model, the front calipers are usually at 0 degrees (front of the rotor). I know that the front brakes perform most of the braking, so presumably having the caliper at the front is the most effective position (although I have seen some Corvettes and AMG Mercedes with front calipers at 180 degrees ).

If this is true, why vary the caliper position on the rear rotors? Why not always position them at the front? All other factors being constant, is the performance of the rear brakes worse if the calipers are at any position other than 0 degrees?

 

[BassBomb]

unsprung weight balance?

 

[Zenmervolt]

It makes no difference at all where the caliper is located. Caliper location is dictated by space concerns. For the front brakes, this usually means at what you are calling 0 degrees or 180 degrees because of the necessity of mounting other hardware (tie rods, etc) to the front suspension.

ZV

 

[InflatableBuddha]

OK, so it's a design consideration, not a performance issue.

Thanks.

 

[Apex]

The ideal position is at the bottom, to lower the center of gravity.

Furthermore, it should be towards the center of the car, to lower the car's polar moment.

This being said, very few manufacturers care about these things more than basic space/design concerns.

 

[bruceb]

Can't say about which spot is ideal, but on my GM Grand Prix all 4 calipers are at the 3 o'clock position. I myself have never seen any at 12 or 6 o'clock.

 

[Zenmervolt]

Originally posted by: Apex
The ideal position is at the bottom, to lower the center of gravity.

Furthermore, it should be towards the center of the car, to lower the car's polar moment.

This being said, very few manufacturers care about these things more than basic space/design concerns.

Given the weight of a brake caliper compared to the overall weight of the entire car and the fact that you're moving it only about 12 inches at the absolute most the effects of a brake caliper on center of gravity and polar moment are so incredibly small that they aren't even worth considering.

In fact, it's not even important enough for Formula 1 cars, which carry both front and rear calipers at the rear of the disc (the 3 o'clock position). Heat dissipation concerns take precedence in that situation (having the caliper at the rear of the disc makes it easier to route cool air over the disc).

Furthermore, there are differing schools of thought on polar moment. Taking my own two fun cars as examples, the mid-engine 914 has an extremely low polar moment of inertia, while the front engine/rear transaxle 951 has a relatively high polar moment of inertia. Both are considered to be among the best handling cars ever made. While the 914 is a little more nimble, it is also more prone to snap-oversteer because of its low polar moment of inertia. The 951, on the other hand, can make hamfisted drivers look like heros because its higher polar moment of inertia makes it less prone to bad habits.

ZV

 

[InflatableBuddha]

Originally posted by: bruceb
Can't say about which spot is ideal, but on my GM Grand Prix all 4 calipers are at the 3 o'clock position. I myself have never seen any at 12 or 6 o'clock.

Fourth generation Dodge Caravans (SXT; possibly other trims) have the rear calipers at 12:00. I have not seen any vehicles with the calipers located at 6:00.

 

[Apex]

Originally posted by: Zenmervolt

Originally posted by: Apex
The ideal position is at the bottom, to lower the center of gravity.

Furthermore, it should be towards the center of the car, to lower the car's polar moment.

This being said, very few manufacturers care about these things more than basic space/design concerns.

Given the weight of a brake caliper compared to the overall weight of the entire car and the fact that you're moving it only about 12 inches at the absolute most the effects of a brake caliper on center of gravity and polar moment are so incredibly small that they aren't even worth considering.

In fact, it's not even important enough for Formula 1 cars, which carry both front and rear calipers at the rear of the disc (the 3 o'clock position). Heat dissipation concerns take precedence in that situation (having the caliper at the rear of the disc makes it easier to route cool air over the disc).

Furthermore, there are differing schools of thought on polar moment. Taking my own two fun cars as examples, the mid-engine 914 has an extremely low polar moment of inertia, while the front engine/rear transaxle 951 has a relatively high polar moment of inertia. Both are considered to be among the best handling cars ever made. While the 914 is a little more nimble, it is also more prone to snap-oversteer because of its low polar moment of inertia. The 951, on the other hand, can make hamfisted drivers look like heros because its higher polar moment of inertia makes it less prone to bad habits.

ZV

F1 vehicles have varying placements for calipers, but the vast majority of the modern ones have them at the bottom, many slightly towards the inside:

You can glance over all of the models from year to hear here (not all of the pictures show the calipers, but many do):

http://www.f1-fansite.com/wall...2009/wallpaperLA09.asp

The 914 has a relatively small polar moment, great weight distribution (46f/54r). IMHO (and I have more experience with modern Porsches than vintage ones), while small polar moments do increase responsiveness at the expense of stability, the 914's handling quirks were mainly due to a few things:

1. Improper alignment (higher rear negative camber and caster helped things, as does 1/16" toe in per side. Front should be a bit less negative camber than rear, maxed caster, and 1/16" toe out per side).
2. Too much deflection in rear rubber bushings
3. Insufficient bracing for trailing arms
4. (most important one): Improper driving style for mid-engined vehicles

The 914's main handling downfall is not really the lower polar moment or weight distribution, but rather the suspension/chassis. While good for that time period, time has marched on. Hop into an Exige, Cayman, or any number of modern choices, and it's amazing how well behaved these vehicles are. The Noble has a mid-engined setup with more weight towards the rear is extremely well behaved and easy to drive.

I've never had the pleasure to drive a 951, but I did spend some time in a 944 S2. Great cars, definitely fewer handling quirks than the 911's of that era.

 

[Apex]

Originally posted by: InflatableBuddha

Originally posted by: bruceb
Can't say about which spot is ideal, but on my GM Grand Prix all 4 calipers are at the 3 o'clock position. I myself have never seen any at 12 or 6 o'clock.

Fourth generation Dodge Caravans (SXT; possibly other trims) have the rear calipers at 12:00. I have not seen any vehicles with the calipers located at 6:00.

http://www.f1-fansite.com/wall...RedBull-RB4-1-1680.jpg

 

[Black88GTA]

I would think that brake calipers being placed in the 6:00 (bottom) position on the rotor would be more prone to damage from road debris. The vital soft hydraulic lines required for brake operation would be closer to the ground, and would not have the mass of the rotor / spindle or other suspension components below it or to the side to deflect any mobile debris away from the fittings / soft parts. This may work well for an F1 car, which is operated only on a well groomed, perfectly paved, closed track - but not as well for a commuter that operates daily on public roads with trash, tire shreds, roadkill, etc out there waiting to be hit.

As ZV said, any polar moment / center of gravity benefit that would be gained by different mounting points of the calipers on a standard car would be insignificant, especially when compared with the benefits of mounting them in other positions.

 

[Apex]

Virtually all sports cars have it between the bottom and inboard, normally closer to the inboard. In other words, if you're looking at the side of the car and the car is facing to the right, the front caliper will be at the 7-8 o'clock position, and the rear will be at the 4-5 o'clock position. Ferrari, Lamborghini, Pagani, Koenigsegg, Porsche, Corvette, etc. All of em.

http://www.seriouswheels.com/2...low-Side-1600x1200.htm

 

[InflatableBuddha]

Interesting point about the airflow across the rotor in high-performance cars, ZV.

Thanks for the pics Apex. I admit I had regular passenger vehicles in mind when I posted this topic, not F1 cars or high-performance sports cars.

 

[Zenmervolt]

Originally posted by: Apex
F1 vehicles have varying placements for calipers, but the vast majority of the modern ones have them at the bottom, many slightly towards the inside:

You can glance over all of the models from year to hear here (not all of the pictures show the calipers, but many do):

http://www.f1-fansite.com/wall...2009/wallpaperLA09.asp

Most of the photos I looked at showed the rear calipers slightly to the rear. The positioning seems mostly to do with keeping the calipers away from areas where they might interfere with airflow. I was looking at older cars and had not seen the placement on the newest vehicles. This photo, specifically, was one that stood out when I did a cursory Google search for "Formula 1 brake". I'll concede your point on a theoretical level, but the simple fact is that there's just not going to be any sort of measurable difference in handling due to caliper placement on street-legal vehicles.

Originally posted by: Apex
The 914 has a relatively small polar moment, great weight distribution (46f/54r). IMHO (and I have more experience with modern Porsches than vintage ones), while small polar moments do increase responsiveness at the expense of stability, the 914's handling quirks were mainly due to a few things:

1. Improper alignment (higher rear negative camber and caster helped things, as does 1/16" toe in per side. Front should be a bit less negative camber than rear, maxed caster, and 1/16" toe out per side).
2. Too much deflection in rear rubber bushings
3. Insufficient bracing for trailing arms
4. (most important one): Improper driving style for mid-engined vehicles

The 914's main handling downfall is not really the lower polar moment or weight distribution, but rather the suspension/chassis. While good for that time period, time has marched on. Hop into an Exige, Cayman, or any number of modern choices, and it's amazing how well behaved these vehicles are. The Noble has a mid-engined setup with more weight towards the rear is extremely well behaved and easy to drive.

I've never had the pleasure to drive a 951, but I did spend some time in a 944 S2. Great cars, definitely fewer handling quirks than the 911's of that era.

I've never known a mid-engine car that was not at least slightly prone to snap-oversteer. Even a 951 can be provoked into snap-oversteer if a driver is really overcooking things. While a good suspension setup for a mid-engine vehicle can mitigate that tendency somewhat, it remains an inevitable consequence of a low polar moment of inertia. It simply takes less lateral force to get the ends of the car to change direction. This is a positive thing in terms of how "nimble" a car feels, but, as you correctly pointed out, does require that a driver be more aware of what he or she is doing. In a 951, one can abruptly lift the throttle mid-corner to compensate for the mistake of coming in too hot, while the 914 tends to get a little upset in that scenario. It's not something that is endemic to the 914, but rather a common trait for all mid-engine cars.

A mid-engined car will always be somewhat "twitchier" than an equivalent car with a front engine, rear transaxle layout, but that is not, IMO, a drawback. I certainly don't consider my 914 to be anything other than "well behaved", it just offers a convenient counterpart to my 951 when describing the benefits and tradeoffs associated with high or low polar moments of inertia. Both high and low polar moments have their advantages and disadvantages.

I think we can definitely both agree though that a car that has a high polar moment on one and a low polar moment on the other end can be a handful.

ZV

 

[Zenmervolt]

Originally posted by: Apex
Virtually all sports cars have it between the bottom and inboard, normally closer to the inboard. In other words, if you're looking at the side of the car and the car is facing to the right, the front caliper will be at the 7-8 o'clock position, and the rear will be at the 4-5 o'clock position. Ferrari, Lamborghini, Pagani, Koenigsegg, Porsche, Corvette, etc. All of em.

http://www.seriouswheels.com/2...low-Side-1600x1200.htm

Porsche, at least, tends to keep their calipers at either the 9 o'clock or the 3 o'clock position. Sometimes they even go with the 2 o'clock or 10 o'clock position.

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It's worth nothing that there are packaging benefits to positioning the calipers closer to center as this can reduce the complexity of the brake line routing.

ZV

 

[Apex]

Originally posted by: Zenmervolt

I've never known a mid-engine car that was not at least slightly prone to snap-oversteer. Even a 951 can be provoked into snap-oversteer if a driver is really overcooking things. While a good suspension setup for a mid-engine vehicle can mitigate that tendency somewhat, it remains an inevitable consequence of a low polar moment of inertia. It simply takes less lateral force to get the ends of the car to change direction. This is a positive thing in terms of how "nimble" a car feels, but, as you correctly pointed out, does require that a driver be more aware of what he or she is doing. In a 951, one can abruptly lift the throttle mid-corner to compensate for the mistake of coming in too hot, while the 914 tends to get a little upset in that scenario. It's not something that is endemic to the 914, but rather a common trait for all mid-engine cars.

A mid-engined car will always be somewhat "twitchier" than an equivalent car with a front engine, rear transaxle layout, but that is not, IMO, a drawback. I certainly don't consider my 914 to be anything other than "well behaved", it just offers a convenient counterpart to my 951 when describing the benefits and tradeoffs associated with high or low polar moments of inertia. Both high and low polar moments have their advantages and disadvantages.

I think we can definitely both agree though that a car that has a high polar moment on one and a low polar moment on the other end can be a handful.

ZV

Hmm, I think we agree in practice, but merely have a slightly different definition of snap oversteer. For me, I think the 951 doesn't really snap, though it can be provoked into a relatively benign slide. There are definitely mid-engined vehicles like this; the tail comes out in a relatively controlled fashion for all but the most hamfisted drivers (and I'm often in this category).

For snap oversteer, I think the old 911's are like this, stick, stick stick stick, then swing around fast enough to require a change of underwear.

Originally posted by: Zenmervolt

Originally posted by: Apex
Virtually all sports cars have it between the bottom and inboard, normally closer to the inboard. In other words, if you're looking at the side of the car and the car is facing to the right, the front caliper will be at the 7-8 o'clock position, and the rear will be at the 4-5 o'clock position. Ferrari, Lamborghini, Pagani, Koenigsegg, Porsche, Corvette, etc. All of em.

http://www.seriouswheels.com/2...low-Side-1600x1200.htm

Porsche, at least, tends to keep their calipers at either the 9 o'clock or the 3 o'clock position. Sometimes they even go with the 2 o'clock or 10 o'clock position.

Text
Text
Text
Text

It's worth nothing that there are packaging benefits to positioning the calipers closer to center as this can reduce the complexity of the brake line routing.

ZV

Hmm, you're right about Porsche. Sad, I just glanced at my wife's 996TT and the front calipers are at 2 o'clock. :disgust: I'm getting old and forgetful.

 

[Zenmervolt]

Originally posted by: Apex
Hmm, I think we agree in practice, but merely have a slightly different definition of snap oversteer. For me, I think the 951 doesn't really snap, though it can be provoked into a relatively benign slide. There are definitely mid-engined vehicles like this; the tail comes out in a relatively controlled fashion for all but the most hamfisted drivers (and I'm often in this category).

For snap oversteer, I think the old 911's are like this, stick, stick stick stick, then swing around fast enough to require a change of underwear.

I think I'm thinking more of lift-throttle oversteer (that is, oversteer that is provoked by the driver) than of the old 911s' oversteer that introduced all too many drivers to the laws of physics (and to ARMCO barriers).

I've gotten the 951/944 to snap, but it was the result of monumental stupidity on my part. Hairpin corner plus way too fast an entry speed plus suddenly stabbing the brakes equals an OH SH*T moment.

My 'teener will come around quite predictably and tends to want to settle into a benign drift if I ask too much of it, but it's definitely twitchier.

Then again, I tend to keep my cars set so that they understeer slightly with acceleration, oversteer slightly with lifted throttle, and are close to neutral in steady-state, so they tend to be a little more twitchy than most stock setups.

If you're ever in Seattle, drop me a line. We can grab a couple beers and BS about cars for a while. :beer:

ZV

 

[Apex]

Originally posted by: Zenmervolt


If you're ever in Seattle, drop me a line. We can grab a couple beers and BS about cars for a while. :beer:

ZV

Definitely sounds like a plan!