Thread: Koni shocks long travel?

whats the effect of lengthening trailing arms 300mm, factory angle just further foward of the standard position. may also fix the dreaded hops that lifted trucks get so often
david

Originally Posted by modman

whats the effect of lengthening trailing arms 300mm, factory angle just further foward of the standard position. may also fix the dreaded hops that lifted trucks get so often
david

Roll/bump steer results from the slope of the roll axis. A horizontal roll axis (parallel to ground) is needed to eliminate roll steer.

When the lower trailing arms are parallel the slope of the roll axis is the same as the angle of of the lower trailing arms (looking from the side).

Longer trailing arms can help by making the slope more horizontal for the same lift.

The dreaded hops that you are talking about, result from too much anti-squat. Longer and flatter lower trailing arms will reduce the anti-squat.

Originally Posted by Bush65

Roll/bump steer results from the slope of the roll axis. A horizontal roll axis (parallel to ground) is needed to eliminate roll steer.

When the lower trailing arms are parallel the slope of the roll axis is the same as the angle of of the lower trailing arms (looking from the side).

you sure John ?

the roll axis runs through the theoretical roll centres, in our case the point at which the panhard rod intersects with the vehicle centreline at the front, and the centre of the A frame ball joint at the rear. The lower trailing arm doesn't affect this at all.

Having the lower trailing arm horizontal to the chassis does mean that for a given amount of roll or one wheel droop or squat the wheel base is being affected less than if the arms are angled significantly downward at rest. With highly angled arms like on a lifted Landy, the wheelbase is being shortened or lengthened to a far greater degree for any given suspension movement and increasing the rear steer.

Longer trailing arms can help by making the slope more horizontal for the same lift.

absolutely.

The dreaded hops that you are talking about, result from too much anti-squat. Longer and flatter lower trailing arms will reduce the anti-squat.

I'm betting this could be fixed with good shock tuning. Data logger and LVDT to measure the frequency of the hop and valve to suit. I'm guessing it's a pretty low frequency, so decent low speed rebound valving may fix it.

Originally Posted by rick130

you sure John ?

the roll axis runs through the theoretical roll centres, in our case the point at which the panhard rod intersects with the vehicle centreline at the front, and the centre of the A frame ball joint at the rear. The lower trailing arm doesn't affect this at all.

Yes I am.

The roll axis that you have described is the roll axis of the body.

For roll steer, we are concerned with the roll axis of the rear axle (the roll axis of the front axle is not usually a problem because the driver compensates for it).

The roll axis passes through 2 points that do not move when the axle articulates.

To eliminate roll steer, those fixed points should be at the same height (roll axis is horizontal).

For the suspension with an upper A-frame, one of the fixed points is the ball joint.

For control arms (eg lower trailing arms on rovers) the the fixed point is a point where the lower (or upper) control arms converge.

When control arms angle in, it is possible to arrange the geometry so that they converge at the same height as the other fixed point.

When the control arms are parallel (eg rover trailing arms) they converge at a point parallel to the arms at infinity.

For suspension with a panhard rod. one of the fixed points is where the panhard rod intersects with the vehicle centreline.

So the roll axis of the rover rear suspension, with A-frame and parallel lower trailing arms, passes through the A-frame ball joint and is parallel with the lower trailing arms.

The rear roll centre is the point where the rear roll axis intersects a vertical plane through the centreline of the axle. Only if the A-frame ball joint is vertically above the centreline of the axle, is it at the roll centre. On rovers, it is close enough for our purposes.

The point at which the front panhard rod intersects with the vehicle centreline is not the front roll centre, but lies on the front roll axis. If the angle of the front roll axis is small this point will be close for determining the height of the front roll axis.

Originally Posted by rick130

Having the lower trailing arm horizontal to the chassis does mean that for a given amount of roll or one wheel droop or squat the wheel base is being affected less than if the arms are angled significantly downward at rest. With highly angled arms like on a lifted Landy, the wheelbase is being shortened or lengthened to a far greater degree for any given suspension movement and increasing the rear steer.

absolutely.

Originally Posted by rick130

I'm betting this could be fixed with good shock tuning. Data logger and LVDT to measure the frequency of the hop and valve to suit. I'm guessing it's a pretty low frequency, so decent low speed rebound valving may fix it.

It may, but is treating the symptom, not the cause.

With lower trailing arms at a steep angle, the anti-squat is high. The traction force at the tyre contact with the ground is transferred to the chassis through compression in the lower trailing arms. Because of the angle of the arms there is a horizontal component (of the compression in the arm) equal to the traction force, and a vertical component, which provides the anti-squat force.

When accelerating (up a hill is when hop is more noticeable) the inertia (through the centre of gravity) transfers load from the front to the rear suspension (causing squat). The vertical component of the compression in the lower trailing arms apposes the transfer of load (anti-squat), so the rear springs are not compressed as much (and can even lift, with suspension geometry with more than 100% anti-squat).

During an off road hill climb, the compression in the trailing arms changes as traction is lost/regained, and changes the load on the springs.

Truly bad examples of rear suspension geometry have rising anti-squat as the suspension droops. This is why the length of the upper control arms (or A-frame) are shorter than the lower trailing arm - so the anti-squat reduces as the suspension droops.