Ackerman effect, steering ram infront of the axle?

[Steve 90]

In my view you want akerman steering. Colin Chapman was circuit racing where the steering angles are only a couple of degrees. When your driving on the limit (tarmac) the rear is slipping any way, the difference between akermann and non akerman steering angles is fractions of degrees etc, then I can see the logic in the arguement.

However, we are looking to operate:

1/. Larger steering angles

2/. get traction at larger steering angles

3/. not tear up a boggy surface (i.e. float across the top on a peaty bog as in Ladoga) when steering

Having non akerman steering will result in one tyre scrubbing when steering, i.e. usinging up some of your precious traction, tearing the ground surface, etc. Therefore if you can manage it you want 100% akermann.

Adrian

Pretty much exactly as I was thinking.

That said I am in the process of fitting hydrosteer to a volve C303 axle. I can't get 100% akermann with out compromising other issues/within the constraints of the design. I am therefore going to get as much as I can.

Adrian

Which is kind of what im looking at doing. How are you setting yours up? What is the rrelationship between king pin, rack, steer arm etc?

Steve.

[Daan]

If you drive fast, parrallel will be better, it has the effect of putting toe in on cornering, making the front end stable. Acker man does not really aply if the back end is stepping out, which is what happens when you corner fast.The reason you can turn tighter is that normally the iner wheel turns at higher angle than the outer wheel. But the inner is the limiting factor on a landrover, so parralel steering will turn the outer wheel further. I doubt you will notice a difference in traction.

Also, while you are working on the front axle, how much castor have you got? The more castor you have, the more camber you put on the wheel on turn in. This will help cornering a lot.

Daan

[RedLineMike]

The solution to this is to fit a disco 2 steering box- They give 40% higher tourque output. However this needs a 40% higher pressure from your pump - you need to fit a disco 2 power steering pump to go with it.

Adrian

i have this setup on mine & i still break the box............

D2 box's asmuch as they have a much larger sector shaft, the worm gear is made from toffee

[Warthog]

Point 3 on:

http://www.circletrack.com/techarticles/ctrp_0806_dirt/viewall.html

It would seem Ackerman has its benfits aswell as drawback.

[simonr]

The argument against Ackerman is that when you are cornering, most of the weight is on the outside wheel, and no Ackerman increases cornering stability so you loose very little by the inner wheel having a less ideal steer angle I.e. The gains outweigh the losses.

Travelling slowly however, particularly on a surface with limited traction, the weight balance between the wheels is much more equal. Therefore, not running Ackerman means you are loosing a little bit of traction as one or the other or both wheels must slip.

In practice, most vehicles do a bit of both so the Ackerman angle is set as a compromise - usually about 10 degrees. Less on faster or higher performance cars and more on slow ones such as agricultural vehicles.

There is no right ackerman angle - it all depends on the vehicle and intended application.

Si

[B reg 90]

If you drive fast, parrallel will be better, it has the effect of putting toe in on cornering, making the front end stable. Acker man does not really aply if the back end is stepping out, which is what happens when you corner fast.The reason you can turn tighter is that normally the iner wheel turns at higher angle than the outer wheel. But the inner is the limiting factor on a landrover, so parralel steering will turn the outer wheel further. I doubt you will notice a difference in traction.

Also, while you are working on the front axle, how much castor have you got? The more castor you have, the more camber you put on the wheel on turn in. This will help cornering a lot.

Daan

Thinking back to the stuff I read on this I believe that you are correct, on tarmac. Where you are not on a predictable surface, specifically an uneven surface with variable grip the effect is not relevent in my view. However slow (in relation to circuit racers) speed tight turns I think that akermann has a place.

Camber - fixed by axle.

Castor - there is zero at the minute, obviously not keeping that. Going for 6 degrees to get good self centering with the full hydro at the minute.

Adrian

[B reg 90]

Which is kind of what im looking at doing. How are you setting yours up? What is the rrelationship between king pin, rack, steer arm etc?

Steve.

Steve,

Still working on it. I have a spreadsheet that allows you to put in each key dimension and gives a graph of the akermann angle. It worked fine until I included an offset to account for the ram not being in the center of the axle. Now it gives screwy results..... I have since focused on my coil over mounts, so have not fixed this yet.

However I have some givens:

1. Ram needs to be tucked as close to axle as possible to prevent you having a scoop out front.

2/. Ram has to be on axle center line. Any lower and it is dragging on the ground. Higher and it hots the hydro pump at full bump.

3/. Wheel rium ID limits the amount you can angle out and the length of your arms. Portals make this worse as the wheel is lower. Arms need to be angled down to get any kind of decent angle and arm length (not ideal).

Adrian

[Team Idris]

From what I've seen everything from a 130 down to a home brew 80" uses the same axel on tarmac without much fuss, despite a change in acherman centre point due to wheelbase. And above 20% slip, will it have any effect at all?

Either way up, if you go hydraulic, sooner or later it will suffer the same shock-load failure, so you need one of these to get ahead of the game; http://www.flowfitonline.com/acatalog/DUEL_CROSS_LINE_RELIEF_VALVES.html .

You can drive like a mega hooligan then

Also, for those who want to protect the old box, there are ways of building spring overload clutches into the drag link. It's proper 'old school' stuff, but its an old school problem

[Daan]

I was thinking about the pressure relieve valfe, that solution has been used in rally cars for this reason.

Daan

[Steve 90]

Steve,

Still working on it. I have a spreadsheet that allows you to put in each key dimension and gives a graph of the akermann angle. It worked fine until I included an offset to account for the ram not being in the center of the axle. Now it gives screwy results..... I have since focused on my coil over mounts, so have not fixed this yet.

Adrian

Any chance you could let me know where you got the spreadsheet or did you work out your own formulas to do it?

Think I have got my head around the maths of the job now, Just waiting for some ram dimensions and I can get going on the sums but a spreadsheet would make things a little quicker im sure.

Cheers.

Steve

[Steve 90]

I was thinking about the pressure relieve valfe, that solution has been used in rally cars for this reason.

Daan

The system im looking at has a built in pressure releif for this very reason.

Steve.

[B reg 90]

Any chance you could let me know where you got the spreadsheet or did you work out your own formulas to do it?

Think I have got my head around the maths of the job now, Just waiting for some ram dimensions and I can get going on the sums but a spreadsheet would make things a little quicker im sure.

Cheers.

Steve

Steve,

It's a spreadsheet I knocked up. I need to go back to it and work out the bugs. Just tirgonometry, a tape measure/straight edge etc to measure up the axle etc.

Adrian

[discomark10]

Also, while you are working on the front axle, how much castor have you got? The more castor you have, the more camber you put on the wheel on turn in. This will help cornering a lot.

Daan

At speed yes, not at slower speeds so much