Mechanical Engineers This Question Is For You!

[Tony-Bones]

Question: is there a direct correlation between the Caliper position and the steering rack relative to the vehicles type of drive.

I am involved in the development of a manufactured car that is rear wheel drive, it has the caliper in-front of the wheel and the steering arm behind, also this car owns upper and lower A frame wishbones.

To my eye this construction looks wrong but i cannot explain why!

By construction FWD/Transverse positions the steering rack is behind the drive shaft, caliper in front of the disk so under breaking the inertia will 'pull down on the tyre' and compress the steering rack....via the accelerated rolling resistance/compression.

RWD caliper behind the disk steering arm in front would offer the same assuming the toe countered the added rolling resistance under braking.

So i image that a car that is RWD and has the caliper in front of the wheel and the steering arm/rack behind will encounter 'brake steer' by compression....

My question is this... is there a true reason for the position of the caliper and the steering arm relative to the vehicles type of drive.. and is this force one that can be measured. :D ...... 'cats in'

[MacRS200]

IMHO it will make no difference where the caliper is physically placed. The forces induced by braking are torque related. The caliper is fixed to the wishbone and the the disk to the hub, under braking the mass of the caliper is insigficant.

[Monster-Mat]

and also some rear wheel drive cars, and fwd have the calipers in the trailing possition

[Tony-Bones]

IMHO it will make no difference where the caliper is physically placed. The forces induced by braking are torque related. The caliper is fixed to the wishbone and the the disk to the hub, under braking the mass of the caliper is insigficant.

What 'Torque'

The 'Mass' is insignificant i agree but the direction is subject to dispute.....

Example this 'why do ' some marques have split caliper braking....... this must be to counter something!

My question.... not argument is i wonder why there is this obvious disparity between drive type.... is it purely construction or something else....

I know forces are involved but my (non) Cray computer cannot conclude all the variables....

In my mind you cannot have a RWD car with a (in front caliper) and a behind steering rack..... to me it would read as.... if you brake and 'bump' the 'pull down' will add compression on the loaded tyre and steer.

Saying all that i don't actually know... manufactured cars are somewhat new to me.... (cats in)

and also some rear wheel drive cars, and fwd have the calipers in the trailing possition

Conclusively i ask 'why'!

[MacRS200]

The torque I am talking about is the effect of the caliper gripping the disk, I only refer to the front brakes as the majority of braking is on the front.

The disk is effectively connected to the road and the caliper to the car so when you brake the caliper tries to rotate the car around the disk, hub and hence wheel contact point on the road.

This applied torque lifts the back of the car meaning that more of the cars weight is being supported by the front suspension hence the front suspension compresses. An extreme example of this is a motorbike doing a "stoppie" or someone going over the handlebars on a pedal bike.

Any steering effects caused by all that will be more related to the change in geometry of the front suspension rather that the brake assembly. So IMHO the position of the caliper relative to the disk and/or rack would not be significant.

[Monster-Mat]

as far as i am aware, and ive worked on many cars, 99.9% of steering racks are behind the wheels.

an exception i can think of is the TVR

the relation of the rack is not important, its where there is space to fit it

and regards calipers, again its on what is deemed feasable, hell those crazy french but the Handbrake in the front wheels aswell!!!, on some cars

regards split calipers, if your refering to the Lexus/toyota ones, thats purely a manufacturing process....its easier to drill and manufacture 2 halfs that can be bolted together than manufacture a solid caliper

some manufacturers even use double calipers per disc (aston martin)

i think your just looking into the process to much tony.

at the end of the day putting the caliper leading, allows for more room for the steering arm, which is placed at the rear as thats where the steering rack is, its placed at the rear underneath the engine because its easier to connect to the steering column

Edited February 4, 2006 by Monster-Mat

[Tony-Bones]

The torque I am talking about is the effect of the caliper gripping the disk, I only refer to the front brakes as the majority of braking is on the front.

The disk is effectively connected to the road and the caliper to the car so when you brake the caliper tries to rotate the car around the disk, hub and hence wheel contact point on the road.

This applied torque lifts the back of the car meaning that more of the cars weight is being supported by the front suspension hence the front suspension compresses. An extreme example of this is a motorbike doing a "stoppie" or someone going over the handlebars on a pedal bike.

Any steering effects caused by all that will be more related to the change in geometry of the front suspension rather that the brake assembly. So IMHO the position of the caliper relative to the disk and/or rack would not be significant.

Conversation relating only to the front......

Kinetic and inertia ver fwd/rwd... caliper/steering position.

Observation shows most fwd cars have the caliper in front of the disc and the steering rack behind and the opposite can be said for rwd.... on new performance marques rwd they have split caliper positions at 9/3 respectively.

I understand the principles offered but i feel there is more to the positioning....

Example: fwd- caliper front- steering behind/ under breaking will convert inertia directly down on the tyre, compress the suspension and steering via the rolling resistance, and as you say there will be inevitable 'dipping' of the nose.

whereas: rwd-caliper rear-steering front/ under breaking will convert inertia directly up toward the suspension, then in reply the suspension will 'dip'/compress and splay the front steering rack into a negative position.

So why have 'split systems'...... seemingly this will belay both effects?

My question was... on this developing car.... rwd caliper front... steering rear... looks wrong! i feel there are deliberate reasons for the positions of the caliper and the steering rack due to the forces involved.

In debate construction seemingly positions these elements deliberately, does anyone know why? and can the forces be measured mathematically.

[MacRS200]

Understand what you mean by the upward and downward forces depending upon caliper position.

But this would only apply if the caliper was directly connected to the car body which is sprung weight, it is not, it is bolted to the wishbone which is unsprung. So any forces generated by the caliper are translated directly into trying to twist the wishbone an hence the the torque effect. This causes the suspension to compress not the caliper trying to go up. So the caliper position is not significant.

Obvoiusly the postition of the rack will have an effect on what happens to the wheels, if the rack is in front the wheels will "toe out" as the track rods lift and vice versa for rack behind.

Like Mat said its more to do with where you got room to fit the bits.

[Tony-Bones]

as far as i am aware, and ive worked on many cars, 99.9% of steering racks are behind the wheels.

some manufacturers even use double calipers per disc (aston martin)

i think your just looking into the process to much tony.

at the end of the day putting the caliper leading, allows for more room for the steering arm, which is placed at the rear as thats where the steering rack is, its placed at the rear underneath the engine because its easier to connect to the steering column

This is my problem.... rwd like BMW MERC and have a configuration of caliper rear/steering front.

To solve this problem for my customer developing a new car i felt the configuration was wrong.. manufacturing positions is part of the deal and not a issue but the reasons are..

Mat you may be right i do 'over Analise' but if i am wrong this will cost someone £20.000 or so! in development loss, so appreciate i am a little tense with this one.... the geometry positions (by construction) have been born almost, each move by degrees is another casting and construction...... the major factor here is i cannot conclude the overall geometry performance with this caliper/steering position... i feel it's wrong!

As inadequate as it reads i don't know why!.... i need to understand why there is obvious disparity between construction and drive type.

I can conclude three Kinetic areas

1:fwd/down

2:rwd/up

3:split 0

Simple option is to opte for 3 but this will belay other areas like suspension/steering compression.

If it helps here are the stats

Manufactured kit car pre assembled.

RWD

initial geometry positions were

Camber front: 1 degree negative

Castor: 6 degrees

KPI: 4 degrees

Toe: 20' minutes

I wrote....

Camber: 1 degree negative

Castor: 5 degrees

KPI: 11 degrees

Toe: 20' minutes

Simple maths.. as things were the Castor was way in front with a near vertical kpi..... like an LDV, the wheels splayed \ / without resistance and made no attempt to steer forward..... adding the kpi controlled this, but there is an unstable attitude when breaking....hence my desperate plea

[tdiplc]

Tony, we can help you on this - email me pls

[Tony-Bones]

Tony, we can help you on this - email me pls

Done.... i have asked several other forums FWD/RWD and the replys are interesting but not conclusive.

It would seem i have opened a can of worms.....

There is a real need to solve this position... my neck is on the block here... thousends of pounds rest on the result (final development positions) ....

TDI....... i need your help!!!!!

[710]

What MarcRS200 says is completely correct.

The position the caliper is mounted has no effect at all on the braking effect.

The position on the steering rack, however, is important.

Behind or in front, different suspension effects toe-in on bump or vice versa, as Mark says.

So if a manufacturer decides to put the steering rack behind, then he puts the calliper at the front. This is so it doesn’t get in the way of the steering arm.

And vice versa.

The manufacturer will decide to put the rack behind or in front depending on:

On cheap cars it’s the room and packaging that determines where they put the rack.

On performance cars it’s the effect of the bump steer and other suspension effects that determine where they put the rack.

caliper up or down:

Down: Brake calipers are not usually put in the lower position because they collect mud etc. and they are hard to bleed. But it is better for centre of mass. F1 cars do this.

Up: The calipers will usually get in the way of a strut or something if mounted upward. Plus they are hard to bleed.

Here is a way to imagine it.

Pretend you are a calliper. You are sitting/strapped down on a really big suspension upright. The car is moving forward. You reach over and clamp the disc with your right hand. You squeeze your hand, get friction on the disc, and the force is trying to pull you off your seat on the upright in a certain direction.

Then you try squeezing with the other hand. Same thing; you will feel the same force and same direction.

Then you move your hand around the disc while squeezing, from front of the caliper to the back of the caliper. Move your hand all around the disc while squeezing.

You will be pulled off your seat always in the same direction no matter where you put your hand.

It doesn’t matter where you put your hand and it’s the same with the calliper.

peter

[star a]

apparently, the front brakes on the volvo 245 had a caliper at the front of the disc on one side, and the other sides caliper was at the rear of the disc. so it can't make much difference where it goes, except maybe for cooling reasons on high performance cars, perhaps the 'calipers at rear setup', allows more air to pass over the discs.