Front Suspension Geometry

[scooterrebel]

One of the things that really concerns me is the front suspension.

I like the idea of using parts from a small car, but the frame attachments is the touchy part.

Strength, alignment, safety how are these elements addressed by builders. My fear is that a few millimeters off and you have something that needs to be taken all apart and remade.

Is a good ruler, level and a square going to cut it for a trike you want to go highway speeds?

When I was a kid, my father built a sand bug that was street legal, he used a model A front suspension it had a shimmy at certain speeds. He was a mechanic and had a buddy who was a auto body repair man, so it was built at the auto body shop and the two of them should have been able to get it pretty well made. Their skills were beyond mine anyway.

[srx660]

That is the reason i like the idea of using a Mazda Miata front suspension for a RT. You just cut the whole front subframe section out of a miata, then measure the mounting points and make mounts at the exact same place on your frame. Take the suspension apart and mount the miata a frames to the chassis. I would not worry about millimeters as most suspensions use shims for exact alignment.

A Quick Guide to Front Suspension Geometry

Everyone knows that having the cars tracking done is vital for good handling and even tire wear but what is it and what are all those other figures you get back after having the car set-up?

When it comes to front suspension geometry there are three terms often used, Castor, Camber and Toe but what exactly do they mean?

Toe – Can be expressed as in or out and is measured in either degrees or mm. It relates to the leading edge of the front wheels and whether it is further in than the rear of the wheel (toe-in) or further out (toe-out).

Toe-out is very rarely (if at all) used on road going cars as it can cause wandering and instability under hard braking.

Toe-in gives the car straight line stability and the steering its self centering ability. Having said that, some FWD cars run 0mm toe, wheels straight ahead parallel to each other.

Toe is set by adjusting the length of the trackrods, on the steering rack, by winding them in or out of the track rod ends.

Camber – Is either negative or positive and usually expressed in degrees. It is the angle at which the top of the wheel sit in relation to the vertical, if it is inboard of the vertical this is negative camber and outboard would give positive.

Generally a small amount of negative camber is required to give the car stability while cornering.

Camber is not always adjustable, on some double wishbone suspension the upper balljoint can be moved (or rotated) to give some adjustment, an adjustable top mount is needed for coilovers/Macpherson struts, or camber bolts, a bolt with a lobe on it’s shaft, used in place of one of the lower mounting bolts to give some adjustment.

Caster – This is the angle at which the steering axis sits in relation to the vertical when viewed from the side of the vehicle, the steering axis being either between the top and bottom ball joints on double wishbone suspension or bottom ball joint and top of the strut on Macpherson/coilover equipped cars. Again measured as an angle and is always leaning towards the rear of the car (positive). Caster gives stability in a straight line as it helps the wheel straighten (think of it in terms of how a shopping trolley wheel
straightens, as long as it's not one of those wonky ones) and stability through corners as the angle creates more camber as the wheels turn.

Caster rarely needs changing from the factory setting but can be done with adjustable camber arms or spacing the upper wishbone further forward or back at it’s chassis mounting point.

That's really all you have to be concerned with on front suspension's. Find the factory setting, set the chassis up with factory settings, and then adjust the suspension once you get it up and running.

SRX660

[scooterrebel]

Thanks,

I was just looking at the Suspension for Gilroys Thistle, it looks very period to the Morgan, but I am concerned about its abilities. the idea of using a transplant form a modern car is certainly a easier way out of the problem but would be less convincing for a replica build.

When I finally get around to starting this, I may change directions but knowledge is valuable.

[kolibri]

This is a German three wheeler page they sell a JZR RT Kit with frame and frontsuspension maybe its a way for you to start.

www.jzr-threewheeler.de/sale.htm

Cheers Kolibri

[jim99]

If you are using a transplanted front end form a modern auto most of the work is already done, just use factory settings, but possibly a little less camber, as your total weight should be less.

One thing not to overlook if you build your own suspension is Ackerman. That is the angle of the steering arms to the center line of the trike.

Ackerman is the steering geometry that causes the inside wheel in a turn to turn more than the outside wheel. Without Ackerman one tire will tend to "scrub" in a turn.

Yours, Jim

[Deleted]

Jim is right on the Ackerman. Even when using the Miata front end, you still need the intersecting lines of the steering arm hook up point and the ball joint / kin pin to intersect at the centerline of the drive axle. So wheelbase on your trike design may need to be adjusted or the track width adjusted from the factory width.