Soapbox Steering
Everyone thinks that the traditional "single pivot axle with rope" steering will work. Everyone is wrong.
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. A car wants to travel in a straight line and, if you take your hands off the steering wheel the front wheels will naturally move back so they point straight ahead. A push bike will do the same thing. A traditional cartie, however, will not do this. It will have all the directional stability of a shopping trolley and if you let go of the steering rope it will veer off sharply to one side or the other. Watch the videos on the right for an examples.
There are some links at the end of this page to web sites which explain in detail all the issues involved, but first - a few notes on turning circles. The London Black Cab has a turning circle of 25ft (7.6m), and is one of the tightest turning vehicles on the road today. An ordinary car would typically have a turning circle of between 10m and 12m.
To turn a typical "rope and plank" steered cartie with a wheelbase of, say, 1.2m and a track width of 1m inside a 7.6m circle, you would have to move the front axle through just under 10 degrees. Assuming you've attached the rope as far away from the pivot as possible, that means you'll have to move your hands by less than 9cm. Think about that - from straight ahead to the equivalent of full lock in a Black Cab with just 9cm hand movement. Now think about how far you have to move your hands to get to full lock while driving a car, and perhaps you'll see why traditional "rope and plank" cartie steering is so very sensitive. This, coupled with the fact that "rope and plank" steering does not self centre, makes it practically impossible to steer.
Although the temptation is to make a cartie that can turn pretty much in its own length, the truth is that a turning circle of 20 to 30 feet is going to be more than adequate.
So here are a few pointers on ways to make traditional cartie 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 about the minimum. 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.) bungy straps, bike inner tubes, springs, etc.
- Don't build everything at right angles to the chassis, or your steering will be very twitchy. Think "shopping trolley". 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. | |
| Some notes on how to make traditional "rope and plank" steering less alarming. | |
| 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
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.
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
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.
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
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.
Moment Arm
You may have have read I'm building a 4 wheel cart. This is a first for me. Built plenty of trikes and I know how to make them rip down a track but this world of 4 wheelers is new to me. A lot of things are the same as a trike but as I'm building this machine I've come across something(s) that may well effect the way most of us think when building a 4 wheeler.
The Moment Arm is the distance or length between the Roll Axis (RA) and the Centre of Gravity (CoG).
The Roll Axis is the line connecting front and rear Roll Centres (RC).
S can you draw a line like you did with the SR thru perhaps the same cart to give an example.
Actually I've seen quite a few wheels collapse on 4 wheelers because the distance between the two was way too much. If it's to little it causes to much side load on a bicycle wheel. Which causes the cart to slide. This is common in carts where the driver is really low and the wheels are mounted at axle height.
So I thought I might ask around and see what others are running so I don't have to cut and shut like I did with my yellow Supercart. Time is precious at the moment.
.........thanks Trikes
Roll centres
Have a look at some go kart
Have a look at some go kart pics from front on. Because they don't have suspension like a car the Roll Centre is much easier to see. Have a look at this cart. It's BLOWFISH from Perth Western Australia. Owner is Wooley
[URL=http://img107.imageshack.us/my.php?image=bathurstbillycarts028if6.jpg][IMG]http://img107.imageshack.us/img107/3323/bathurstbillycarts028if6.th.jpg[/IMG][/URL]
See how the arms sort of point down to the ground under the cart. Where they meet is the Roll Centre.
On the rear the arms are level. Meaning the rear RC is located at axle height. This guy potato chipped a 20" wheel at his first race meeting at Bathurst Mt Panorama. He reckons to this day it was his diving and some of it probably was but I can tell you he has way too much split between the heights on his RC'. This cart will always be 'loose' as in wanting to slide the back end out. If he lowered the rear RC about 75mm the cart will accelerate harder out off the turns at a greater rate than gravity would allow. But he has to ensure he stays with the wide track as the narrower the track the higher the RC'.