Brake anti-lift affected by length of tailing arm?

[stownsen914]

I am trying to understand whether brake anti-lift is influenced by the length of the rear trailing arm on a semi-trailing arm suspension. I understand that anti-lift generally increases as the side view instant center rises. Also that it is desirable to have the distance from wheel center to the side view instant center not be too short, or (at least with beam axle type suspensions) you can get brake-induced wheel hop. Is brake-induced wheel hop a concern for a semi-trailing arm suspension too? Can this be mitigated by making the semi-trailing arm longer? I am trying to visualize this, but it's not working!

Thanks.

Scott

 

[BrianPetersen]

It is absolutely affected by the length of the trailing arm. In fact, the location of the trailing arm pivot point is the main governing factor.

Take a look at any of countless IRS arrangements on front-drive cars. Most of them have a trailing arm of some sort involved, and they generally don't have wheel hop issues under braking.

 

[stownsen914]

I can see how the vertical placement of the trailing arm pivot impacts brake induced wheel hop. But why does the length of the trailing arm matter? (Not disagreeing, just trying to understand ...) I've tried reasoning through whether there can be a lever effect involved that would vary with trailing arm length. However if we assume the anti-lift geometry is similar and the damper and spring are located near the wheel hub, it seems like similar force would be required to cause upward wheel movement regardless of trailing arm length.

It may be worthwhile to explain my application ... I am designing a new rear suspension for a Porsche 914 used for road racing. Rules dictate that I need to keep the semi-trailing rear suspension, but geometry and dimensions are otherwise open. So I am trying to optimize the suspension to, amongst other things, reduce or eliminate wheel hop that I have experienced under threshold braking.

Help me understand .... thanks.

Scott

 

[BrianPetersen]

When you apply the brake, the brake caliper etc is applying a torque to the thing that it's mounted to ... the hub. It tries to drag the hub along with it.

If you have a trailing arm type suspension, the forces that oppose that torque, are between the tire contact patch and the pivot point of the trailing arm.

If you are going forward and you apply the brake, the torque reaction tries to pull the tire contact patch up and it tries to pull the mounting point of the trailing arm down. This is in addition to the tire contact patch pulling back (braking) and that force in the fore/aft direction also gets applied to the trailing arm pivot, so the height of that pivot matters, too.

The closer these two points are (i.e. the shorter the trailing arm), the greater the forces are. The further apart these two points are (i.e. the longer the arm), the smaller the forces are.

The downward pull on the trailing arm pivot location acts against the unweighting of the rear end due to braking. If you get it right, the downward pull can be made to fairly closely offset the unweighting.

If you have a semitrailing arm, project the pivot axis outward to where it crosses the centerline of the wheel, and that's the effective length of the trailing arm for purposes of this calculation.

I do not know why beam axle suspensions seem more prone to wheel hop under braking than independent trailing-arm-based suspensions. You never see beam-axle suspensions with anywhere near as short an effective trailing-arm length as most IRS designs and yet they still hop.

 

[stownsen914]

Most helpful Brian. Thanks for explaining that in detail.

Scott