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Technical Caliper behind axle centerline?

Discussion in 'The Hokey Ass Message Board' started by RMR&C, Mar 19, 2021.

  1. Is there a reason calipers always seem to be mounted behind the axle centerline? Do they function better in that position? Or is it an aesthetic thing?
    Asking specifically about a fenderless hotrod with a solid front axle.
     
  2. grumpy65
    Joined: Dec 19, 2017
    Posts: 920

    grumpy65

    Good tech question.........:)

    As far as I am aware it is simply a 'packaging' thing. In my opinion, front calipers look 'more correct' trailing on the rotor.
    More inportant, and I am sure you are already aware of this, bleeders to the top. This may mean a swap left/right to mount them in a leading position on the rotor.

    I did find this, but would not think it has much effect in our world.......
    "Ideally you mount them as close to the car's center of gravity as possible. So on the front axle thats the rear of the upright and on the rear axle the front of the upright.
    Its a miniscule optimization to reduce the yaw moment of inertia of the car."
     
    Last edited: Mar 19, 2021
    lippy and dana barlow like this.
  3. Wrench97
    Joined: Jan 29, 2020
    Posts: 679

    Wrench97

    Possibly for cooling the rotors?
    In the rear leaves the front edge of the rotor is unobstructed to cool air.
     
    Lepus and Frankie47 like this.
  4. dana barlow
    Joined: May 30, 2006
    Posts: 5,115

    dana barlow
    Member
    from Miami Fla.
    1. Y-blocks

    Just for those that didn't think about it much,if rotor is center flo finned,cooling air moves from center to outer of rotor. So cool airflo should go to center,not the out side or lead.
     

  5. Harv
    Joined: Jan 16, 2008
    Posts: 974

    Harv
    Member
    from Sydney

  6. Mimilan
    Joined: Jun 13, 2019
    Posts: 1,230

    Mimilan
    Member

    Only for weight distribution.
    Behind on the front and Forward on the rear would lower the "polar moment of inertia" slightly.

    The latest trend in racing cars is to have them mounted underneath to lower the CGH
     
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  7. grumpy65
    Joined: Dec 19, 2017
    Posts: 920

    grumpy65

    Hey man, hoped you would throw your oar in here...:)

    How much advantage are we talking about in a street driven hot rod? Is it worth considering?
     
  8. Mimilan
    Joined: Jun 13, 2019
    Posts: 1,230

    Mimilan
    Member

    Even a highly skilled F1 driver wouldn't know the difference with street tyres.
    It is only noticeable on lap times when racing tyres are driven at 100% of their adhesion . [a slightly lower CGH and Less Polar Moment of Inertia might gain a few 1/100's in a corner]

    Another advantage on performance road racing cars is rear mounted calipers make it easier to mount cooling ducts to the front.
    This ^^^ would be more noticeable by delaying brake fade.
     
    egads likes this.
  9. grumpy65
    Joined: Dec 19, 2017
    Posts: 920

    grumpy65

    That is the answer to go by.

    So, the theories of increased or decreased downward force on the tire contact patch due to caliper location are a myth? Just wanted to put that to bed for all who may read this thread...........
     
  10. I've always mounted my calipers behind the axles. Figured they worked better going up hill (climbing the rotor)than downhill (sliding down the rotor).
     
    stillrunners likes this.
  11. Mimilan
    Joined: Jun 13, 2019
    Posts: 1,230

    Mimilan
    Member

    It is purely rotational torque applied around the axle centreline.
    Think about the loads through the ball joints with forward loads through the uppers and rearward loads through the lowers regardless of caliper placement.

    If the upper A-arm was tilted back slightly , these forward loads try to push the ball joint downward creating front "lift" [which is what anti-dive geometry is all about]
     
    grumpy65 likes this.
  12. TCTND
    Joined: Dec 27, 2019
    Posts: 543

    TCTND
    Member

    The lowest polar moment would be achieved with the caliper either above or below , as close to the kingpin axis as possible. The optimum mechanical situation would be two calipers mounted opposite each other (or four, eight or whatever). That would cancel out any braking loads on the wheel bearings and spindle.
     
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  13. Mimilan
    Joined: Jun 13, 2019
    Posts: 1,230

    Mimilan
    Member

    That would be steering axis polar moment [also known as the gyroscopic effect]
    Don't confuse this with cornering polar moment ,where mass outside the 4 tyre contact patches tries to resist change of direction.
    Moving the mass between the wheelbase [closer to CG] improves handling.

    Having 2 or more calipers mounted around the spindle does not cancel out braking loads [ it merely acts like a radial engine around a common crank]
    Regardless of the layout, these rotational brake torque loads need to be transferred into the chassis

    The only way to increase / decrease tyre loads with brake torque is to use dive or anti-dive geometry
     
  14. lippy
    Joined: Sep 27, 2006
    Posts: 6,825

    lippy
    Member
    from Ks

    Only if you part your hair in the middle. lippy
     
    RMR&C likes this.
  15. grumpy65
    Joined: Dec 19, 2017
    Posts: 920

    grumpy65

    That's the point I was trying to make. Caliper location has negligible effect on front end loading while braking. Dive/anti-dive characteristics are related to suspension geometry. The caliper placement thing that sometimes comes up is a fallacy. Like I said earlier, the most beneficial thing you can keep in mind is to make sure the bleeders are at the top......... it really is that simple...;)
     
  16. Interesting discussion, not sure how much applies to a street driven hotrod......
    I was considering mounting my calipers in front of the axle to keep the left brake hose away from the steering drag link. Easy to swap them side to side.
     
    clem likes this.
  17. grumpy65
    Joined: Dec 19, 2017
    Posts: 920

    grumpy65

    As stated, packaging and aesthetic are all you need to worry about. It will work just fine......:D
     
  18. Mr48chev
    Joined: Dec 28, 2007
    Posts: 33,861

    Mr48chev
    ALLIANCE MEMBER

    There may be a bit of a cooling difference with the caliper mounted in front and blocking any and all air flow to the rotor. Braking before that cooling becomes an issue isn't an issue. Meaning that stopping power on a cold rotor isn't going to be any different no matter where you mount the rotor but if you do a lot of hard stops it may be a real issue if the caliper is blocking most of the air flow to the rotor.
    Think of every late model high performance rig that has built in pathways for cooling air to be directed to the rotors. then stick your rotors on your hot rod on the front side so you can block all of that cooling air that you don't already have hoses from the grill directing air to the rotors. They are just hanging on the spindles but the calipers are blocking airflow.
     
  19. grumpy65
    Joined: Dec 19, 2017
    Posts: 920

    grumpy65

    ^^^ A simple air deflector overcomes this issue, if it is indeed even an issue.....

    See @dana barlow post #4. The spinning motion creates a vortex effect and pulls the cooling air into the center area of the rotor. Even wheel choice can affect this air motion path to some extent.
     
  20. drtrcrV-8
    Joined: Jan 6, 2013
    Posts: 1,707

    drtrcrV-8
    Member

    Even if you think you have the rotor position figured out, sometimes you have to put them where available space dictates, especially when they interfere with steering components.
     
  21. Mike VV
    Joined: Sep 28, 2010
    Posts: 3,029

    Mike VV
    Member
    from SoCal

    Backward thinking..!
    Correct in dynamics, but...
    In front of the axle, the braking action is forcing the rotor/tire...into the ground...for more braking traction.
    It is a minimal action as there are many different things going on, but, it's there.

    There's also the "polar moment of inertia" action. That is, with more weight toward the center of the car, the quicker the car will turn corners..! And while the caliper would be 10" or so further forward, it's still close enough toward the rear, that the straight line traction would be negatively affected, VERY little.

    Mike
     
  22. grumpy65
    Joined: Dec 19, 2017
    Posts: 920

    grumpy65

    Hey @Mike VV , wouldn't Newton's Third Law of Motion cancel the downforce out ???
    The third law states that for every action (force) in nature there is an equal and opposite reaction. In other words, if object A exerts a force on object B, then object B also exerts an equal and opposite force on object A.
    Object A: Caliper
    Object B: Rotor
     
    Last edited: Mar 20, 2021
  23. grumpy65
    Joined: Dec 19, 2017
    Posts: 920

    grumpy65

    @Mike VV , the question in my previous post is a legitimate one.
    If there is downforce created, I genuinely would like to learn how and why. Hope you didn't take it the wrong way.
     
  24. Mimilan
    Joined: Jun 13, 2019
    Posts: 1,230

    Mimilan
    Member

    There is zero difference in brake torque between front and rear mounted callipers [brake torque is the co-efficient of friction applied at the disc radius from axle centreline]
    No matter how it is mounted the resistance /forces applied through the mounting brackets will try to apply negative caster to the spindle/axle.
    These forces can be harnessed with suspension design.

    Trailer manufacturers know this very well with Tandem rocker equalizer suspension and 2 wheel brakes.
    A backyard "cowshed engineer" will mount the brakes on the front axle. [weight transfer is the reasoning]
    This ^^^ will cause lift /loss of traction because the front springs pivot from the front [like a trailing arm]
    If the brakes are mounted on the rear axle it will increase traction because the rear springs pivot from the rear, forcing the wheel downward.
    upload_2021-3-20_20-51-45.png
    In both cases the brakes try to rotate the axle the same direction. It is only suspension geometry that changes the outcome.
    And also brake torque is far greater than weight transfer
     
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  25. TRENDZ
    Joined: Oct 16, 2018
    Posts: 386

    TRENDZ

    The only way caliper position can influence lift or dive, is to have it’s mount float radially, and it’s stop link connected to a point on the chassis.
    You can see examples of this on some motorcycles and sprint cars.
    The stop link can be positioned to make the chassis squat or separate. It can be positioned to have extreme influence on brake/chassis interaction.
    With a “standard” caliper mounting situation, there is no difference of influence to the chassis while braking regardless of caliper position.
    I can see benefits to moving caliper position for certain gains in altering the center of gravity. You see more and more drag cars with the calipers on the forward position to gain more forward weight on very quick cars. These type of cars usually have long instant center position and a lot of hardware moved forward in the car.
     
    Mimilan likes this.
  26. Jay McDonald
    Joined: Apr 6, 2020
    Posts: 116

    Jay McDonald

    there's one in every crowd lol
     
  27. grumpy65
    Joined: Dec 19, 2017
    Posts: 920

    grumpy65

    I wouldn't jump so quickly to that conclusion if I were you.
    The following is from @Mike VV 's profile page...................
    Occupation: Aerospace, Sr. Mechanical Test Engineer
     
  28. Mike VV
    Joined: Sep 28, 2010
    Posts: 3,029

    Mike VV
    Member
    from SoCal

    You all need to recheck your Vector Analysis calculations...simple as that !
    Been there, done that.

    Mike
     
  29. My car has parallel leaf springs on the front axle with the shackles at the front of the springs. Will caliper placement have any affect on handling during braking? Your analogy about trailer axles suggests it might?
     
  30. TRENDZ
    Joined: Oct 16, 2018
    Posts: 386

    TRENDZ

    Please explain how we are wrong.
    The only points that have any effect in straight line braking, are the contact patch of the tire, and the suspension geometry. The caliper could be mounted at any point radially on a spindle/axle and have zero effect on the chassis reaction under braking.
    My take on your understanding is that if the caliper is mounted in front of the axle centerline, the chassis would separate.(drive the tire into the ground for more traction)
    If this is true, wouldn’t the inverse happen with the caliper mounted in the rear? Tire would be pulled away from the road and decrease traction?
    Explain how one happens but the other doesn’t.
     
    Jay McDonald likes this.

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