Electric power steering on a Beta

[peteracs]

Thanks for the update, you sparked a subject which I have thought needed to be aired and resolved. The top mounts which are currently available from memory have not had a good reputation for reliability. It would be interesting to explore actually having a batch of top mounts made especially if they can be made at sensible price by modifying an existing mount.

Are there any suggestions where to start on this?

Peter

[mangocrazy]

Hi Ian/Peter,

Thanks for the correction. I think I'm starting to get my head around it now. If this were motorcycle steering geometry it would hold no terrors for me, but the addition of a pair of wheels confuses me greatly...  

Ian will probably be able to confirm or deny this, but what you are doing by increasing castor angle on the front wheels of a car is equivalent to increasing the rake on motorcycle forks; i.e turning your sports bike (steep steering head angle) into a chopper (shallow steering head angle). This gives slower, heavier steering but with a predilection for stability and going in a straight line.

Graham

[WestonE]

Hi Graham

As a Mountain biker and holder of a motor cycle License unfortunately the comparison does not work well because bikes steer mainly by response to leaning except for slow speed parking. Cars use turning wheels around an axle point. Caster introduces twist for pressure on the contact patch and changes the wheel angles through turning to introduce negative camber which works against roll to maintain the contact patch. There is a complex relationship between toe in/out / camber and caster before we get to Ackerman angles. There are conditions we want to avoid like bump steer, sudden changes in camber through suspension travel or bush movement or wheel turning. We want the steering to bite into a turn (toe in/out setting & miss matched front/rear track ) with a flat tyre contact patch if possible and maintain strong grip and lead the car through a turn despite body roll.

You need to do some reading because this is a complex inter linked subject. Chad from The Monte Hospital commissioned an analysis of the Monte design from an Aston Martin Suspension engineer before building some excellent modifications to dial out the mistakes

Lancia were pioneers in balanced responsive handling including miss matched front to rear track and wishbones mounts that change load through the wheel travel, but had to build to a budget and ensure comfort. No Power Steering meant compromising the front suspension design. Front wishbone location was/is terrible under load and the cars squat under power (lack of anti squat geometry and little rear spring pre-load). If you want to see some of the design fit a strut with no spring and move the wheel through different positions with a trolley jack changing the strut compression.

Years ago I built in rear spring pre-load with short strut rods, had Barry Waterhouse nylon wishbone bush supports and added 1.5% negative camber after converting to Power Steering. Caster gives a bigger win than just negative camber because it maintains the tyre contact patch better. I deliberately built my car with twin adjustable rear links, fully adjustable coil overs, stronger wishbones with fully supported bushes and multi position front top mounts. This was so I could adjust toe camber and caster. I have gone for an educated guess on spring rates and the platforms on the struts allow adjustment of rake (some nose down attack maybe) and corner weighting. Because of the sliding rear anti roll bar you can even fix 'set back' on a Beta. That is when one side of the car is shorter wheel center to wheel center which creates a desire for the car to steer toward the shorter side.   

Ian knows what he is doing with the adjustments he is trying and it is valuable insight. I will only be able to test again when the car runs!

Eric     

[mangocrazy]

Hi Eric.

OK - I was grossly over-simplifying things I now realise. Time to take a crash course on 4 wheel vehicle steering geometry and suspension dynamics, I think...

Graham