crashbox455,

Simply put: yes but as you might expect there are a great many other things influencing the placement as well.

Most motorbikes as you have probably noticed have the caliper situated behind the fork leg.

The idea being as the brakes are applied the caliper applied an upward force to somewhat counted the dive of the front forks due to weight transfer.
As an added benefit keeping the caliper within the wheelbase assists in the sprung  to unsprung mass ratios.
Same applied to cars but ir is easier to see on bikes.

Placing the caliper ahead of the wheel centre line as you mentioned above does indeed do as you point out and add to the squat of the chassis as the reaction is applied.

In the Citroen DS (1955 until 1975) the calipers were at the "six o'clock" position and rigidly bolted to the final drive housing which also helped to lower the centre of gravity a little. There was a metal duct to direct cooling air under the bumper to the caliper and disc. The handbrake was also on the front wheels (as was around 60% of the running weight of the car)and located behind and above the main calipers somewhere around 4 o'clock.

From an owners point of view the calipers were forever being littered with road debris, mud dust small stones etc and the park brake pads took about a full day to change even after some practice.

Technically elegant it was but not the most practical design about even at the time.

One of the "many other things " mentioned earlier!

Hope this is of some help.

Cheers, Pete.   
 

If considering the 6 o'clock caliper position, there is an additional disatvantage further to the ones Pete listed on the DS....... If your brakes are outboard, you can get pad knockback. I know that at least one F1 team have used this position because of the low COG advantage but abandoned it because of severe pad knockback when running over kerbs........ An extreme example - nevertheless I like the advantages and design purity of the 6 o'clock position. Not good when fording rivers either though.

Pete, can you explain ? :

"As an added benefit keeping the caliper within the wheelbase assists in the sprung  to unsprung mass ratios."

Gents,

Am I missing something here? The caliper has to be mounted to a structure which support the bearings which in turn support the disc. As such, the only force that the caliper can transmit to it's mounting is a couple about the disc's axis. The location of the caliper around this axis has no effect on the couple generated when braking. The couple force can be used to generate ant dive force through the geometry of the suspension linkage. Motorcycle calipers are usually mounted behind the fork legs to keep their mass close to the steer axis, thus reducing angular momentum.
 I have seen an anti-dive set-up on a motorcycle which used a link from the caliper to the lower fork yoke. The caliper is mounted such that it is free to rotate about the wheel spindle, and braking loads are carried by a pushrod directly to the lower yoke. The location of the pushrod attatchment to the caliper can be altered to control the degree of anti-dive.
 The location of the caliper on cars is controlled by packaging constraints, cooling requirements and mass distibution among other factors.

Pete.

"Motorcycle calipers are usually mounted behind the fork legs to keep their mass close to the steer axis, thus reducing angular momentum."
A similar effect has been claimed for "forward axle" M/C forks, and touted as helping to reduce headshake.
Since car wheels when steering pivot on a "king-pin" line starting at the ground somewhere in the contact patch and tilting mostly inward, the 6 o-clock position might be the winning caliper position for cars.  

But, since shimmy and tramp with IFS is pretty much unheard of, I'm satisfied to leave my calipers where the mfr put them.

Peter 7307 wrote

"The idea being as the brakes are applied the caliper applied an upward force to somewhat counted the dive of the front forks due to weight transfer.
As an added benefit keeping the caliper within the wheelbase assists in the sprung  to unsprung mass ratios.
Same applied to cars but ir is easier to see on bikes.

Placing the caliper ahead of the wheel centre line as you mentioned above does indeed do as you point out and add to the squat of the chassis as the reaction is applied."

PTwizz, I agree, I do not see how the calliper o'clock can possibly affect the antidive, or, more generally the forces in the wheel or the body, in a conventional setup. I also fail to see how it will change the unsprung mass, except for the trivial effect due to the change in bracket weight etc.

The inboard disc brake is a whole spearate subject, not directly relevant tot he original question.

 

Cheers

Greg Locock

I also fail to see how total unsprung mass can be changed, or how it can really have any antidive effect if it is solidly mounted to the spindle/axle housing.

The only effect I can visualize, is on the wheel bearing loads. Cooling, debris, water ingress, bleeding, and steering clearance are all very valid factors though.

There is also the very real effect of steering feel to be considered. Forward mounted calipers tend to add to the steering force felt by the driver while rear mounted ones tend to lower the steering torque feel. This is simply due to the moment imparted by the caliper's mass center (and all the supporting mass) about the steering axis from cornering centripetal force. It's easiest to visualize on plan view. Of course at six o'clock, this effect is nil (or nearly nil depending on caster.

I agree with those above that caliper placement has no effect on vertical dynamic loading. It's just a moment that the rest of the suspension sees.

Ramon

Caliper location may have a small effect on unsprung mass, where the SVSA is short, and locating the caliper closer to the instantaneous centre reduces it's moment about that point.

Pete.

Pete - After a bit of head-scratching, it appears that this effect is more or less related to caster gain.  But other than inducing some fore/aft forces on the ball joints in order to generate the rotational acceleration of the whole business about the Y-axis, what else shows up as an end result?  Or are these forces of significant enough magnitude to, say, affect bushing deformations?

Norm

Am I missing something here?

Imagine one corner of an IRS with three lateral links and a drag link, and a spring above the wheel centre.

The calliper is bolted to the spindle. The wheel bearing is fixed to the spindle. The wheel is free to rotate about the y axis only, and a tangential force is applied at the calliper to the wheel.

In this system, look at the forces aaround the wheel centre. The only longitudinal force reacted to the body is via the longitudinal link, and is equal to the braking force at the CP. There is also a vertical force at the long link-body bush, whose magnitude is given by the ratio of the length of the link to the rolling radius.

That's it. The free body diagram of the spindle CANNOT show any component at the spindle to link joints due to the o'clock location of the calliper.

I am 99% sure that the same would apply to any suspension where the brake calliper forces are reacted directly into spindle, assuming a rigid spindle etc, which is not a bad place to start.

Cheers

Greg Locock

Yup, have to agree with Greg.

If you mounted a brake in a sealed box, with only a shaft coming out of one end, there is no way you could determine where the caliper was inside the box. In fact it could be a drum or band brake, there is no way you could tell.

Oh, I cunningly ignored the vertical force at the contact patch/road spring, which is drawn on this here diagram I'm holding up to the screen.

 

Cheers

Greg Locock

Greg - This isn't a brake torque reaction per se; rather it's forces induced by Y-axis rotational accelerations of the upright + caliper + brackets, etc., about the SVIC that would generate equal and opposite fore/aft forces at the two ball joints under bump/rebound suspension movement.  If, say (and ignoring chassis pitch), the UBJ follows an upward and rearward path while the LBJ moves upward along a vertical path, the spindle assembly will be made to rotate as seen in side view along with the vertical motion.

Although I suspect that it's a rather small effect (my brain is starting to hurt trying to figure out what any consequences might be), I would expect it to be a relatively larger effect in a really lightweight street car with big brakes and iron calipers than in a mass-market 3000+ lb 5-passenger sedan.

Norm

The effect I was referring to has nothing to do with braking forces. It is simply the mass of the caliper, and its radius of gyration about a virtual centre (SVSA instant centre) with suspension displacement. I fear I may have deviated so far from the original question that I have caused confusion.

Pete.