Soapbox Steering

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Everyone thinks that the traditional "single pivot axle with rope" steering will work. Everyone is wrong.

 

Rope and plank? just gonnae no...

 

Soapbox Steering - good

A good example of pitman arm type steering, although the steering ratio looks quite low. The single tie rod between the steering arms means steering ratio can be adjusted without affecting wheel alignment.

Soapbox steering - bad!

Oh dear lord no. This is a crash waiting to happen.

 

 Soapbox Steering Dos and Don'ts

  • Don't use "rope and plank" or "feet on the front axle" steering
  • Don't make the steering ratio too small. At least 2:1
  • Don't allow too much steering lock - you need far less than you think - 10 to 15  degrees is typical
  • Do pay attention to king pin inclination, caster, trail and ackermann angle - it will make your cartie much easier to steer
  • Do consider fitting a steering damper

Although it is fine for relatively low speeds, it becomes unstable as speed picks up because it is extremely sensitive and has no tendency to self centre. Watch the videos on the right for a example of the dreaded "speed wobble" and the inevitable result...

The most common form of steering currently in use is simple "gokart" or "pitman arm" steering. Steering racks salvaged from small cars are sometimes used.

Here are some pointers on making your steering work.

  • Don't go mad trying to make your cartie turn on a sixpence. It almost certainly won't need to be that manoeuvreable, and you probably only need to be able to turn your wheels through about 10 or 15 degrees.
  • Don't have a direct link between your hands and the steering. Given that very small movements of the front axle are required to turn the cartie, you'll need to find some way to "gear down" your steering input so that relatively large hand movements only move the axle a small amount. This is called the steering ratio. A steering ratio of 2:1 is absolute minimum, and even that is going to be  very twitchy. Anything approaching 1:1 is going to be very twitchy, and lesss than 1:1 is going to be uncontrollable and might not even be allowed in some cartie races.
  • Stiffen up the steering. Make it relatively hard to move by fitting (e.g.) a motorcycle steering damper, which can be very effective at making steering more manageable and can be readily picked up quite cheaply from online auction web sites.
  • Don't build everything at right angles to the chassis, or your steering will be very twitchy. Read up on Ackermann steering geometry and incline the king pin to make the steering more stable.

There are some great explanations of how to get the right steering geometry on the web sites shown below. Most of these sites are to do with the building of recumbent bikes and GreenPower racers, but the principles are exactly the same for gravity powered vehicles.

Further Reading :

A good explanation of Ackermann steering geometry, camber, caster, and centerpoint steering, and some really handy spreadsheets too.
Similar stuff to Peter Eland's pages, but with some good clear pictures showing all the angles.
A great way to make kingpins, using either bike steering tubes or from scratch using bushings and bolts.
Steering geometry for Gokarts - similar in many ways to the problems that need to be solved for a soapbox cartie.
More notes on caster, camber and toe. This is from the perspective of race car design, but it still contains some useful background information.

 

Theres one thing you've

Theres one thing you've overlooked with the steering info. This will give you a guaranteed 4kmh speed increase and the cart won't slow on the corners like it used to and it's a guaranteed pain in the ass to do. It's Scrub Radius. The 4 wheelers for customers I've altered have astounded the owners.

When I first started tinkering with reducing the Scrub Radius all I did was to loosen the spokes on one side and tighten the others. Hence moving the rim inwards or closer to the Kingpin. The last two carts I reset to ZERO Scrub Radius by angling the centre of the Kingpin to meet the tyre tread centre and thats where I found 4kmh. The toe out setting had to be optimized and the drivers only took about three runs to get used to the new improved steering in their carts. Check out Scrub Radius on the net. I'm converting an old head at the rear trike that I built for one race and have never used since as it's at least a cart length slower than my head first trikes. As I was saying it's being converted to a 4 wheeler (for one race too at Bathurst) and it will have zero scrub radius and adjustable roll centres. As much of the cart will be adjustable as I can make it as I've not raced down that side of Mt Panorama before and all eyes will be looking. You can view the build up www.southerntasbillycart.com ............Trikes

trikesrule | November 16, 2008 - 10:39
scottishcarties's picture

Soapbox Cart Kingpin Inclination

It's not mentioned directly, but there are links to some sites which cover steering geometry in a lot of detail. Peter Eland's explanation of steering for recumbent tricycles is really good and has some pictures that show exactly what you're talking about, and here are a couple more of my cartie.

Soapbox Cart Kingpin Inclination

The first shows how an imaginary line passing through the center of the king pin passes through the centre of the tyre where it touches the ground (more or less), and

Soapbox Cart Kingpin Inclination

the second shows how the king pin is angled back by about 15 degrees so that the tyre contact patch is behind the steering axis. This make the steering tend to self centre and makes the cartie much easier to drive.

Of course this only applies when you've got stub mounted front wheels. If you're using bicycle forks, your steering scrub is already zero and tyre contact patch is already behind the steering axis.

 

 



 

scottishcarties | November 16, 2008 - 11:55

Thats it. The pic on the

Thats it. The pic on the right is Zero Scrub Radius.

Now with the pic on the left, it shows the caster. Top of the kingpin leaning back. I've found the carts tend to transfer diagonal weight too much at 15 degrees. I set them up at 5 or 6 degrees. 

I mentioned about the diagonal transfer. I've found the ideal amount of weight transfer should only be around 3 - 4%. On a set of scales the cart transfers well beyond that amount when set up on scales and the wheels are turned. Another thing is I set the steering so the wheels can only be turned 5 degrees. This cuts down on the customers putting too much lock on when not required to negotiate a certain corner.

I see some of your tracks have really narrow roadways and supertight corners. So yes you may well need the amount of lock your building in. Our tracks over here tend to be wider and and more sweeping. ................trikes

trikesrule | November 17, 2008 - 20:49
scottishcarties's picture

lock and caster

You're right - we need more lock than 5 degrees to negotiate some of our more technical courses, and - at Catterline - to correct the oversteer when we go from the gravel to the tarmac on the first bend, but you're right that people tend to give their carties way too much lock and are surprised how little they really need.

I tend to prefer slightly more caster because the "steering jack" effect adds the the self centering tendency. Good steering is important, and a lot harder to get right when you have two front wheels.

I find that the steering arm angle is much more important than kingpin angles for eliminating wheel scrub. Spending some time getting the Ackermann angle right (see below) so that the outside wheel is pointing in the right direction when you go round a corner is time well spent.

Ackermann Angle

 

 

scottishcarties | November 18, 2008 - 09:45

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