I thought it would be intersting to re-visit these posts made in July by Gauis49 from SadFrancisco and Merlin the manufactuer of the Aero Cycle Car.
Gauis49 first:
So, I've done some more reading and I have a few considerations to post. The front geometry is bad for several reasons including:
1) The steering rack is too wide for the car, requiring short tie rods instead of longer ones. When the suspension goes up or down, the short length of the tie rod compared to the longer control arms causes the end of the tie rod to pull toward the chassis more quickly than the ends of the control arms. This in turn causes the steering arm on the upright to be pulled towards the chassis faster than the upright, which causes the upright to rotate about the kingpin axis. As this happens, the wheel's toe substantially changes.
The usual way to avoid this problem is to use tie rods that are about the same length as the control arms and to locate the steering rack so that the tie rod is parallel to a control arm.
2. The kingpin axis doesn't even come close to intersecting the tire contact patch. More precisely, the axis of the kingpin does not pass through the plane of the wheel at ground level. This means that there is essentially a lever arm giving the tire leverage over the steering. So when you hit a bump, the wheel is pushed backwards, this in turn creates torque around the kingpin which manifests in a sudden, violent jerk in the steering wheel. Basically the offset of the kingpin is dead wrong. This sort of error is quite literally a text book mistake,
"It is disastrous, as some of the pioneer designers discovered, simply to move the steering axis (the kingpin) smartly sideways [inwards]. That makes room for the hub and permits the swivels to be moved further apart, thus reducing the stresses on both. Sadly it also means that every road shock tries to twist the wheel about its steering axis - not exactly the kind of feel we were looking for. Far from each wheel being naturally self0centering, each will be trying to twist itself around its steering axis in a state of constant conflict with the other. Such permanent tensions are bad news in any kind of engineering, but especially in a system as safety-related as the steering."
-Car Suspension at Work: Theory & Practice of Steering, Handling, and Road Holding. Jeffery Daniels 1988. Page 77-78. (
http://www.amazon.com/Car-Suspension-Wor...pension+at+work )
I'll post more on this later. I've been very busy working 10-12 hour days at a new job, so my free time to post is sparse. I hope this helps explain whats going on with our beloved trikes. They are wonderfully fun, and I would enjoy mine even more if it didn't occasionally try to rip the wheel out of my hands and kill me when I hit bumps on the freeway (75mph+).
Good night everyone.
P.S. The exhaust heat shield mount tore out of the muffler last night, leading to quite a lot of noise and some improvisational welding. Enjoy ->
http://www.youtube.com/watch?v=m5PbUq8lvwwAttachments
kingping inclination.jpg
front suspension illustrated.jpg
And from Merlin, who make the Aero Cycle Car:
I have no bump steer on my Aero Cycle Cars either. The problem is that in essence Morgan have gone for a sliding pillar look with their upright. The camber is fixed, it cannot be adjusted, and it looks excessive, anyone know what it is, it looks in excess of 3 degrees, when 1-1.5 degrees is the norm.
next take a look at the wishbones, almost similar length, so arcs of travel are out. The precise location of the steering rack and angles in both planes of the track rod arms/ends is absolutely paramount, and is fundamentally wrong. They've used Mini/Metro ball joints top and bottom of the upright, however by using one at the top, there is no ability to adjust camber. Any vehicle built using Chapman/Lotus unequal length wishbones has used a top joint from a Ford Transit (drag link joint) either with a 20mm thread or an 18mm thread, thus allowing for adjustment....this is standard practice.
It is impossible to get exact "centre point steering"/"scrub radius"....ie extend the line down through both top and bottom ball joints on the upright to the ground alongside the tyre, then measure the lateral distance to the tyre wall. It can be achieved of course by the wheel having an offset, as of course modern cars always do now. If using motor cycle wheels (centre laced, or MWS MG TA outer laced wheels (as Morgan do) and these wheels are 2.5 inch rims shod with 19 x 4.00 tyres...and the wheels have no offset 9they clearly don't) then the extended king pin line will never intersect the centre of the tyre, but will come to within 25-30mm of the tyre's inner edge.
I've no idea why Morgan have exceeded even this...Triking and I manage within reason to get the closest, 25-30mm....we manage this by being tiny tiny one man band businesses, yet Morgan employ how many....over 180!?
My guess is having looked very closely at the suspension is that they've used or modified standard bolt on splined adaptors, rather than go to a specialist company called Orson Equipment to have had made combined splined hubs that could easily have achieved a far better centre point/scrub radius.
The wishbone length issue has not been thought out properly, they've tended to have gone for aesthetics rather than standard mechanical theory.
I drove their factory demonstrator around Blyton Park's track last October and found the front end extremely heavy. the overall experience was good on a track with no bumps of course. The following week the offside wheel came off this very car, as we know. Morgan have subsequently improved their wishbones apparently, but when both my fabricator and I looked closely at the top wishbones in particular, the welding looked rather lumpy and done rather cold, thus the "thick to thin" welds were incorrect.
There is room for a lot of improvement....so come on Morgan, do so!
I can't comment on the bevel box issue, but this is made by Quaife too, as well as the "cush drive" attachment between engine and gearbox.
So, to re-iterate, bump steer is primarily an issue of rack location and angles of the track rods and steering arms, and is so fundamental.
A very good book to read on front suspension is by Des Hamill, and has some easy to understand diagrams and text...it is a must.
I feel they both summerize the problem well, and it seems that if it is to be improved or fixed, more needs to be done than just raising the height of the steering rack tie rods with a spacer, or repositioning the steering arms on the uprights as the factory appear to be experimenting with.
The rack is too wide making the tie rods too short, additionally the king pin inclination is fundamentally wrong, giving kick back through excess leverage. Until these fundamental faults are rectified, I think that problems will not be solved, improved a little yes, but not solved.
It appears that MMC have more or less taken the design of the front uprights,wishbones, and steering arms direct from the Liberty Ace, with little in the way of modifications. The problem being, that they moved the steering rack behind the engine, and it had to be higher, therefore the tie rods had to droop, and the special Suplex shox were not available to hide the problem, and they did not have the time or inclination to redesign the front end geometry.
We will have to see where they are going with it, but until then, it is a matter of waiting patiently [or not].
