Where should a car's brakes be?

"OPPOsaurus WRX" (opposaurus) 07/29/2019 at 10:20 • Filed to: None

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Specifically i’m asking about the calipers. A s I crawled to work on the highway this morning, I started noticing that the calipers on different vehicles were located at different locations. (infront of the axle , behind it, slightly above horizontal , even with axle or below . Is there any benefit from having the brakes either before or after the axle? Does it have something to do with the type of suspension? Is it just a case of ‘this is where it fits so that is where it goes’. I thought that maybe there is some physics involved that when the brakes are engaged it would effect braking ability / handling / grip or something (like if the brake is behind the axle, the disc is to some point pushing up on the caliper where as in front it would pull down on it and that would have effects on the car cuz science) so whats the deal?

Tripper > OPPOsaurus WRX 07/29/2019 at 10:26

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Yo! I also noticed this over the weekend! I think I have noticed it before, but this weekend when I saw an STi with the calipers at the back I thought “they’re mounted differently on lots of cars, I wonder what the science is behind that. Maybe it just comes down to convenience for the overall design?”

nerd_racing > OPPOsaurus WRX 07/29/2019 at 10:34

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I think it has more to do with packaging than anything else.  Where does it make the most sense to run the flex lines that connect the caliper to the hard lines on the body.  If you think about it, the friction and normal force from the brake pads and the hydraulic pistons are being applied tangent to the flat surface of the rotor, so clocking of the caliper shouldn’t matter all that much.  Unless you are ducting air to a specific place for cooling, or have packaging limitations for ducting or brake lines.  

Bylan - Hoarder of LS400's > OPPOsaurus WRX 07/29/2019 at 10:35

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Its mostly related to where the steering box attaches to the wheels hubs. Having the caliper toward the back of the wheel and the tie-rod from the steering ahead of  the front of the axle improves steering response and feel. Its kind of like comparing front wheel steering to rear wheel steering; obviously front wheel steering is a lot more intuitive and easy to control. Same idea here, excepts its forward of the center- line of the wheel compa red to behind the center line. I learned this crawling under some new lexus’ and noticing they all have the calipers behind the center- line.

Spanfeller is a twat > OPPOsaurus WRX 07/29/2019 at 10:38

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My understanding is that it’s a packaging thing and they bare little difference as far as performance goes. Visually though, they look better towards the center of the car in my mind.

Michael > OPPOsaurus WRX 07/29/2019 at 10:38

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Generally you want your calipers to be on the front of the rear axle and the rear of the front axle, to keep the weight towards the center of the car (low polar moment of inertia) . In most cases, though, this difference is negligible except in the highest performance sports cars, and falls by the wayside to things like others have said, like where the steering tie rods need to mount

RamblinRover Luxury-Yacht > OPPOsaurus WRX 07/29/2019 at 10:39

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Has more to do with the spindle design than any of the physics, I believe. The moment around the axis of the wheel isn’t best understood in terms of a force “up” or “down”, as each such force has an “up” or “down” couple force acting at the center of the axle. So there is a little bit of relative up/down twisting/pushing/pulling, but in terms of the location of those forces on the body, I don’t think it’s terrible consequential except for how the suspension manages the twisting.

Whereas, on the other hand, if the spindle connects to the steering on the rear, it’s more convenient for the caliper to be on the front for space concerns and being able to have compact arm geometry, which is an issue even more pronounced if the steering steers from the front - leading to a caliper on the back. Not convenient to put the caliper on top or bottom, either, as the top is either a strut or arm connection zone, and the bottom is the arm/lower ball joint.

Another concern could be cooling and the design of the cooling ducts, but the spindle design (front or rear steer) is the biggest one, because steering packaging is part of the packaging of the whole engine compartment. It’s also worth note that as all suspension rubber parts and to lesser extent steel parts flex and compress/extend in cornering, there are some natural advantages to front-steer spindles, though they’re harder to execute a lot of the time.

kanadanmajava1 > OPPOsaurus WRX 07/29/2019 at 10:43

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Why not both?

Usually the place is dic ta ted by the placement of steering gear. It’s possible to have the tie rod in the same side as the brake caliper but this is usually avoided. 

OPPOsaurus WRX > kanadanmajava1 07/29/2019 at 11:08

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why not both : weight

OPPOsaurus WRX > RamblinRover Luxury-Yacht 07/29/2019 at 11:10

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thanks.  i think this is what i was looking for.

LongbowMkII > OPPOsaurus WRX 07/29/2019 at 11:17

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Ive never seen calipers on the bottom. I guess the slightly lower CoG isnt worth the packaging headache

OPPOsaurus WRX > LongbowMkII 07/29/2019 at 11:28

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I didn;t mean below as in bottom 6 o’clock but more like below horizontal like at 8 o’ clock.  I dont think i’ve seen them at the bottom either.

RamblinRover Luxury-Yacht > OPPOsaurus WRX 07/29/2019 at 11:32

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In terms of the suspension design, many cars have a slightly leading arm design in the front so that braking counters its own dive somewhat. So that when the wheels resist, they try to cam the body of the car upward against the dynamic weight shift:

(laughably crude MS Paint)

The couple around *almost* the wheel axis is sort of able to contribute to this...

But if the couple is behind the point of the wheel (orange), while it has a more pronounced effect on the leading arm effective pivot, it’s more than dwarfed by the upward force at the wheel (yellow) - and if the couple is ahead of the axis (pink), it’s weaker around the leading arm pivot point.

This is even leaving out that plenty of cars have a neutral arm pivot response (and just expect to dive) or a trailing front arm response (for softer reaction to bumps). Such a design probably dives infinitesimally more with a leading caliper, but nobody cares.

Short answer: in theory, it could matter. In practice and for all normal purposes... not really.

OPPOsaurus WRX > RamblinRover Luxury-Yacht 07/29/2019 at 11:38

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beautiful pictures

My bird IS the word > OPPOsaurus WRX 07/29/2019 at 12:06

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I also would not underestimate brake line routing in the decision.

kanadanmajava1 > OPPOsaurus WRX 07/29/2019 at 12:13

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Two s ma ller calipers could offer similar performance than a single larger one. The heat dissipation from two units would be better than a larger one could offer. Racecars wouldn’t have used then if it would be just wasted weight. But the solution is quite rarely seen in any purpose.

Chan - Mid-engine with cabin fever > OPPOsaurus WRX 07/29/2019 at 12:48

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Inboard brakes FTW.

MarquetteLa > OPPOsaurus WRX 07/29/2019 at 12:59

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Miatas have their calipers placed toward the middle of the car for weight balance and center of gravity reasons.

gmporschenut also a fan of hondas > LongbowMkII 07/29/2019 at 13:06

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I would think it would be avoided so deep puddles don’t soak caliper/pads