A Question for You

Daytona Vette

CCCUK regional rep
A question for the Physicists and Engineers amongst you

As both are out there - Which is better; having the Brake Calliper infront of the Hub or to the rear of the Hub on the front axle and then what about the rear axle (assuming the vehicle always travells in a forward motion) ?
 

Roscobbc

Moderator
Depends on respective geometry of other suspension components - caliper behind theoretically would cause suspension to lift under braking - Rolls Royce Shadows have two calipers - one in front - one behind. Probably the most powerful brakes of 'the day'.
 

Daytona Vette

CCCUK regional rep
Not thought of suspension lifting, but certainly under breaking you want the nose down, so I can see Calliper at the front for that - I can not help thinking that a disc rotating into a Calliper from the top is better than rotating into the Calliper from the bottom, but I can not put any science to it - Yes the Calliper should be mounted as low as possible (stability and C of G) and you need the bleed nipple positioned at a high point of the calliper, just looking for understanding and answers.
 

Roscobbc

Moderator
Just reading about some of the experimental braking systems that GM were playing with a few decades ago - on was a clutch/disc hybrid affair - where there were two 'clutch' plates operating on either side of a single disc. Imagine that - 360 degree contact with the swept braking surface. Heat would be a problem I'm guessing
 

Forrest Gump

CCCUK regional rep
Just reading about some of the experimental braking systems that GM were playing with a few decades ago - on was a clutch/disc hybrid affair - where there were two 'clutch' plates operating on either side of a single disc. Imagine that - 360 degree contact with the swept braking surface. Heat would be a problem I'm guessing

The original drum brake has shoes covering much of the brake surface area - just in a different plane. Trucks still use them to stop 44 tonnes.

Then there are “inboard” disc brakes, common on the rear of racing cars, reducing unsprung weight. Interstingly, the 1970’s Rover P6 had inboard rear discs.

Another factor is the effect of the energy recovery system on today’s hybrid cars. It takes some load away from the disc brakes.
 

Oneball

CCCUK Member
Some racing cars, Lotus 72 comes to mind, have onboard brakes on all 4 corners despite only being 2wd.
 

antijam

CCCUK Member
From the point of view of vehicle dynamics, calipers low down help to keep the car's CG low. Inboard brakes reduce unsprung weight and also minimise polar moment of inertia, as does positioning the fronts behind the wheel and the rears in front as oneball points out . Both these factors aid handling The position of the calipers has no effect on suspension lift or fall as all rotor/pad braking loads and moments are reacted within the hub assembly. On ordinary commuter cars factors like suspension design and cost limitations are more influential while performance cars may pay more attention to brake cooling constraints for example.

There's an interesting review of caliper positioning in this video....

Pondering this subject reminded me that nearly 50 years ago I had a Citroën GS with inboard front power brakes - as well as an air-cooled 1015 cc flat four engine and front drive transmission and the best ride I've ever experienced in a car - I miss it.
 

Roscobbc

Moderator
From the point of view of vehicle dynamics, calipers low down help to keep the car's CG low. Inboard brakes reduce unsprung weight and also minimise polar moment of inertia, as does positioning the fronts behind the wheel and the rears in front as oneball points out . Both these factors aid handling The position of the calipers has no effect on suspension lift or fall as all rotor/pad braking loads and moments are reacted within the hub assembly. On ordinary commuter cars factors like suspension design and cost limitations are more influential while performance cars may pay more attention to brake cooling constraints for example.

There's an interesting review of caliper positioning in this video....

Pondering this subject reminded me that nearly 50 years ago I had a Citroën GS with inboard front power brakes - as well as an air-cooled 1015 cc flat four engine and front drive transmission and the best ride I've ever experienced in a car - I miss it.
Had a new poverty spec one of those as a company vehicle. Bright yellow with rubber mats instead of carpets. Quite an advanced ohc flat 4 1015 cc aur cooled engine that would rev quite high (unlike a Beetle or perhaps later Scooby) it had zero torque and a 4 speed 'box to get any actually forward motion you actually HAD to rev the nuts off the thing and MPG was disastrous as I recall - sub 18mpg in town and 22-25 mpg on motorway/A road use.
 

antijam

CCCUK Member
Had a new poverty spec one of those as a company vehicle. Bright yellow with rubber mats instead of carpets. Quite an advanced ohc flat 4 1015 cc aur cooled engine that would rev quite high (unlike a Beetle or perhaps later Scooby) it had zero torque and a 4 speed 'box to get any actually forward motion you actually HAD to rev the nuts off the thing and MPG was disastrous as I recall - sub 18mpg in town and 22-25 mpg on motorway/A road use.

It certainly wasn't powerful but that flat 4 had a turbine smoothness. I bought my GS - an estate version no less - and it was a great workhorse. Skinny 145 section Michelins on front drive 15" wheels helped it go anywhere. In snowy winters I lost count of the number of times I'd pass water cooled, fat tyred cars stranded by the side of the road. After the GS I talked my employer into providing me with a GSA Club as a company car. The larger 1220 cc engine was (slightly) more powerful but was nowhere near as smooth and mods to the suspension had compromised the ride; it was something of a disappointment after the GS. Still, as a technological tour de force - there was even a rare birotor Wankel engined version - together with the SM and the CX it helped Citroën into bankruptcy.
 

Roscobbc

Moderator
Yes the flat 4 was (for an air cooled engine) remarkable quiet and smooth running - seemingly unburstable.....
 

Vetman

CCCUK Member
Callipers behind the front wheel and forward of the rear wheel promotes anti- dive. The forward force at the centre of gravity is resisted by the braking force between tyre and road. This force x the height of the CoG creates a couple that is balanced by the increased vertical force of the front tyres and reduced vertical force of the rear tyres x the wheelbase dimension. This is independent of where the callipers are are. Simples!
 

Vetman

CCCUK Member
Further consideration. I think the anti- dive arrangement of the callipers for sporting vehicles reduces suspension travel and maintains a better tyre contact patch. The upward force on the front wheel calliper reduces the coil spring load and its compression. Opposite at the back. Might also prevent a low skirt from grounding.

Better theory: Anti- dive calliper arrangement was introduced just after introduction of cup holders, to prevent coffee spillage no doubt.
 

Roscobbc

Moderator
Rear end caliper location effects are surely based on what type of axle is fitted and how it is suspended. Simple live axle on leaf springs will not have any effect on caliper location as axle will try to rotate around U bolt locations. Not that any of it really matters too much with rear end on doing 20% or so of total braking effort.
 

Vetman

CCCUK Member
I think a forward calliper at the back will tend to hold the back of the vehicle down regardless of suspension design.
 

Roscobbc

Moderator
Perhaps a 'forward' caliper on a simple live rear axle would cause the rear-end to pitch-up as the axle effectively 'winds-up' - tbh probably much the same unless one had additional trailing links.
 

Vetman

CCCUK Member
If you accept that the rear forward mounted calliper is dragged down by friction at the pads then the rear suspension has to compress to generate the necessary reaction force. The back end of cars go down when you compress the suspension.
 

antijam

CCCUK Member
The braking force generated at the pad/disc interface and the associated moment are reacted entirely within the caliper/hub assembly. The force has no effect on the movement of the suspension. Where it does have an effect is on the hub bearings. The weight of the car results in an upward reaction at the wheel bearings. If the caliper is in front the frictional force adds to the weight reaction on the bearings; if it's mounted at the rear the force is subtracted, reducing the bearing load.
 
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