Caliper mounting position and its effect on handling

[thehelix112]

Started this thread to stop more of a hi-jack from this thread: http://forums.hybridz.org/showpost.php?p=712084&postcount=28

 

In response to

mounting the calipers as low as possible on the spindles(clocked around to 5 or 7 o'clock to keep center of gravity as low as possible...

 

 

I am not sure what you mean about mounting the calipers `low on the spindles'. Are you just talking about the 5/7 o'clock mounting position?

 

Some random thoughts: while you are correct in that this lowers the centre of gravity of the strut assembly, as this is sprung, it does not affect the centre of gravity of the car at all; and thus its only affect is in determining the magnitude of the moment placed on the chassis by the strut assembly when in a turn.

 

This moment on the inside wheel will attempt to pitch the car undesirably (out of the corner), but on the outside wheel it will attempt to pitch the car into the corner. I would hazard a guess that these two moments would all-but cancel each other out, and you're left with no impact at all.

 

 

Unless I have missed something incredibly obvious. This is something I have never completely understood, and am keen to do so. Though it might be better in another thread?

 

Dave

[bjhines]

I remember an article several years ago that went into this in great detail... It seems that the clocking of the calipers can affect more than just the center of gravity... there was something about the application of braking force and inertia as well...

 

I have been looking for the article... cant find it yet...

[thehelix112]

Application of braking force: not too sure what you mean there, will have to wait for the article unless you can remember and be bothered expanding?

 

Inertia: Depends what sort you are talking about. It will definitely affect the moment of inertia to turn the strut(wheels). This will be at a maximum at 3/9 o'clock + degrees castor, and at a minimum at 12/6 + degrees castor. I can also imagine it affecting the inertia of suspension travel as it affects the centre of gravity/mass of what has to move, but my brain is a little fuzzy this morning.

 

I look forward to you finding said article, and on smarter brains than mine chiming in. JohnC?

 

Dave

[JMortensen]

I remember something from my motorcycle days about a decade back that has to do with this. In particular, it had to do with the swingarm suspension at the back of a motorcycle. It seems to me that the deal was that if the caliper was mounted at the back of the rotor that applying the brakes would compress the rear suspension more than a caliper mounted at the front of the rotor on top of the swingarm. This is all really fuzzy memory talking here.

 

In car terms I think the idea would be that a caliper mounted in front of the rotor would not tend to compress the suspension, whereas one mounted in back would.

 

Seriously vague memory on this subject, but maybe that will get the ball rolling on that part of it anyway...

[zguy36]

All forces that interact between the caliper and rotor are all internal to the knuckle that holds the wheel bearing and the caliper mount. No matter where you mount your caliper, it will NOT compress your suspension.

[JMortensen]

That's kinda what I thought at the time, but this guy who was telling me was pretty sure of himself. I remember who it was too. It was a co-worker of mine, and he was relocating the calipers on his Ducati 888. Not that it makes his story any more likely to be true...

[jt1]

I think it's true of swingarm suspensions. If the caliper is to the rear, the force is upward on the arm. If it's on the front, it's downward. I'm not sure it affects a strut at all, still thinking about that.

 

Mounting the calipers inboard would slightly reduce the PMOI of the car, but the amount would be very small.

 

jt

[JMortensen]

I just remembered something else. I know that semi-trailing arm suspension is effected differently, but in that type of suspension stepping on the brakes compresses the rear suspension. My impression there is that using the brakes causes torque at the hub, and that makes the arm want to stick straight out from the chassis. So for most cars with semi-trailing arm this makes the rear squat. I don't think that this effect translates to our strut type suspension though.

[ZDreamin72]

All forces that interact between the caliper and rotor are all internal to the knuckle that holds the wheel bearing and the caliper mount. No matter where you mount your caliper, it will NOT compress your suspension.

 

this is true, the sspension is mouted perpendicular to the rotating force of the wheel and rotor, so on a car it has no effect, it may have an effact on the suspension on a motor cycle because of the parralel nature of the suspension with the rotor and wheel, although the second you put any force of braking into the car the brakes ahve nothing to do with anythign anymore, ats all about the chassis, jsut mount the caliper where it is easiest to get at and chages pads

[jt1]

OK, I'm beginning to think that if the caliper is mounted to the rear, at 3:00, it exerts a moment on the strut which is resisted by a longitudinal force at the top of the strut, exerted by the unibody. The caliper also exerts an upward force which whould be resisted by a vertical unibody force, which would tend to compress the strut.

 

If the caliper was at 9:00, the force would tend to compress the tire.

 

Maybe.

 

Edit: My clocking is looking at the car from the drivers side.

 

jt

[thehelix112]

jt1,

 

As zguy has said, the caliper only exerts a moment on the strut assembly.

What could be happening however, is that due to the castor of the strut tube, the strut tower/body is required to exert a force that is not perfectly horizontal to cancel this moment. If you have positive castor, then the moment on the strut will be attempting to lift the end of the strut, fairly clearly this will result in compression of the suspension.

 

Can anyone see a flaw in my thought process?

 

That the moment on the strut caused by the caliper is likely to result in suspension compression seems (to me, at the moment) undeniable.

 

What IS questionable however, is whether the location of the caliper on the disc changes this suspension compression. I can't think of a reason why it would on strut suspension.

 

Dave

[jt1]

I was wrong when I said the caliper exerts a moment on the strut/spindle assembly. It only exerts a force. If the caliper is to the rear of the strut, the force is going to compress the strut, if the caliper's on the front it will compress the tire.

 

I don't think camber affects it. When you increase camber, you change the angle of the strut/rotor/caliper/tire as a unit. Changing the inclination angle of the strut would increase the compression.

 

jt

[thehelix112]

jt1,

 

That was a typo, I fixed it to say `castor'.

 

Can you explain why you think the caliper will exert a force rather than a moment? Are you thinking along the lines of centripetal forces, as thats what it sounds like, but I don't see how it applies.

 

Dave

[LS1 240Z]

well almost all front brakes are located at the 3 oclock position, reguardless of year, make etc, some rear brakes are mounted at 3 oclock and some are mounted at 9 oclock. just speaking from "common sense" i would think that mounting the rear caliper at the 9 oclock positon would help nose dive somewhat. if it was at 3 oclock it would seem like it would want to pick the rear end up..

[cygnusx1]

No, No, No, No, squeezing the pads against the rotor produces a moment about the center of the pads. That moment is completely checked by the wheel bearings as a vector force radial to the spindle and perpendicular to a line between the spindle and pads. In summary, there are NO forces transmitted to the suspension from the interaction of the pads with the rotor no matter where the caliper is clocked on the rotor or how hard you hit the brakes.

 

All the forces that the suspension sees and that are large enough to effect handling are from the tire contact patch and the cars center of gravity acting on the suspension, NOT the caliper.

 

On the other hand a couple of very small, almost negligable forces to the suspension effected by caliper position do exist:

1) In a dynamic condition example with the wheel off the ground spinning fast, apply sudden braking, the suspension will shudder/jolt as the rotating momentum of the wheel is converted into vectors on the spindle. These sudden changes in wheel speed are not usually seen on the street or track unless you lock up a wheel. The "direction" of the jolt will be determined by the position of the caliper on the clock.

 

2)When the suspension hits a bump, the entire unsprung suspension, including caliper is accelerated upwards. If the caliper is at 9oclock or 3oclock (or trig fraction off either angle) it will impart a slight twisting force to the suspension due to momentum. It is a VERY small force compared to what the suspension is designed for.

 

Go ahead and teach me something new if you think I am wrong.

[bjhines]

I am pretty sure what I saw on this was from the designers at Porsche...

 

The discussion was centered around that last point about the inertia acting on hard lock up.. It had something to do with their ABS sytem on a race car...

[cygnusx1]

bjhines, you are correct about the ABS being the culprit of this semi-myth. The ABS would be converting the wheels momentum into vector forces because of the fact in my point #1. Those "shudders" ARE effecting the way the tire meets the road. For agressive ABS especially, with racing suspension and very rigid tires, position of the caliper could become a factor. For the rest of us with soft suspensions and squishy tired commuter cars, the "shudder" is mostly absorbed by the soft tire and not transmitted to the contact patch.

[jt1]

So far as the start of this thread, caliper location affecting handling, there are lots of things to think about there, some maybe significant, some not.

 

I think caliper location can definitely cause compression of the strut, here's why.

 

First lets define force as an linear influence acting on a body, with a direction and magnitude. A moment is a rotational influence, caused by a force multiplied by a radius. Or, a force is simply pushing on a body, with a known direction and load. A moment is when you tighten a bolt with a wrench, you apply a force to the end of the wrench, and this force acting on the radius, the length of the wrench, results in a moment, a rotational influence, being applied to the bolt.

 

Now lets look at the strut as a free body, in equilibrium. Lets assume it's vertical and looking at the car from the drivers side, it's the front strut and the caliper is mounted at 3:00, to the rear of the strut.

 

When the caliper grabs the rotor and tries to move, it applies a force to the strut. Since the caliper is at 3:00, the force is straight upward. The strut is in equilibrium, so all forces and moments must be equal. The reaction to the force comes from the unibody, straight down, and it is this force that compresses the strut.

 

Since the force applied by the caliper creates a moment, there will be other forces applied to the strut to resist this moment, by the unibody and the ball joint, but it is only the vertical force that causes compression of the strut.

 

If the caliper was at 9:00, the force would be downward, and would be resisited by an upward force on the spindle, applied by the hub. If the caliper is at 6:00 or 12:00, the force would be horizontal, and would not compress the strut, just load the unibody and LCA/TC rod.

 

The key is that for the strut to be in equilibrium, all vertical and horzontal forces must be equal, and all moments must be equal. There are lots of other forces acting on the strut, but I am only discussing the calipers influence. Recognize that the caliper is not exerting a moment on the strut, only a force. If you were tightening the bolts holding the caliper on, then you would be exerting a moment on the strut.

 

And I don't think it makes any real world difference, since we are pretty limited as to how we mount our caliper to the strut, with only a limited range allowed by the bracket.

 

John

[thehelix112]

It seems to me that you will NEVER have just a force unless it is being applied at exactly the centre of mass. If you apply a force anywhere else, you generate a moment about the object's centre of mass.

 

Simple analogy: Get a pen on your desk right now and place it vertically away from you, flick it from the side at approximately the middle. The whole pen moves. Now flick it at one end. The whole pen spins about its centre of mass.

 

This is exactly what is happening with our struts.

 

For the `flick' given to the strut (about its centre of mass) not to rotate the strut, it must be reacted by something else capable of exerting a moment on the strut (strut tower, ball joint/LCA, castor rod, steering arm).

 

Now the interesting part is that as far as these `something elses' are concerned, they are exerting only a force, which then becomes a moment using radius as the distance from application to the strut's centre of mass.

 

Now here is where I get fuzzy, and is why I started this topic, this means that as far as the strut tower is concerned it is only exerting a force vectored in the opposite direction to that applied by the caliper.

 

If the car is rolling forwards then the moment applied by the brakes to the strut is going to be anti-clockwise (when viewed from the drivers side), this means that the reactive force will be clockwise, which means for example, the strut tower will be pushing backwards/down?

 

Bah, I am always open to correction, but I need more convincing that the brakes are no generating a moment on the strut about its centre of mass, which seems to be the main point cygnus and jt1 are making.

 

Guys?

 

Dave

[jt1]

The force the caliper applies to the strut generates a moment, but the caliper itself doesn't apply a moment.

 

I'm not fully convinced my above post is correct. I've been thinking about the wheel/hub/tire as a freebody, and the forces it exerts on the spindle. It's possible the caliper force is being resisted by a force on the spindle, not the top of the strut.

 

Are Johnc, katman, and Cary all on vacation?

 

John