Caster wheel Design angle off vertical

[Lionelgee]

Hello All,

Is there a specific angle off vertical between the horizontal pivot point and the offset of the wheel axle to be considered when designing a caster wheel arrangement? I would like to be able to use existing wheels and make my own caster arrangement for some specific purposes. I have been looking for a while and no commercially available unit seems to fit my requirements. It looks like I will have to custom make something. Knowing some design tips and tricks about casters would be much appreciated.



In the diagram the red line is the centre line for the main horizontal swivel point for the caster that passes vertically down
The yellow line is the vertical point passing through the wheel axle
The blue line represents the angle or rake which passes between the horizontal swivel point to the axle of the wheel
The brown lines represent the horizontal distance between the horizontal swivel point and the centre of the wheel axle
The top horizontal dimension line represents the distance between the centre point of the horizontal swivel point and the centre of the wheel's axle.


Do all these 5 factors go together to form an effective and efficient caster design?

Kind regards
Lionel

[JDNSW]

As far as I am aware, the only factor that matters is the distance between the two vertical lines.

But what is ideal depends on a variety of factors.

1. Planned running surface relative to the tyre width and the wheel diameter - if the wheel width is great compared to this distance castering will not work except on surfaces smooth compared to wheel diameter.

2. Wheel diameter relative to planned running surface - castering effect may be lost if surface is rough enough that the point of contact for a wheel is ahead of the pivot point.

3. If the distance is too great, wheel shimmy will probably result except at very low speeds, but again, suppressing this will require sufficient tyre width for friction to damp the shimmy.

To summarise - there is no magic formula that will give the right answer - "it depends"!

[RANDLOVER]

As John says the determining factor in castor is the pivot point has to lead the axle, by your drawing and I expect most trolley wheels, the difference seems to be the same as the radius of the wheel.

[Lionelgee]

Hello John and RandLover

I have a heavy duty three point crane jib that is way too big for my little Yanmar four wheel drive tractor. I was going to use the three point linkage side of the jib to sit a trailer axle and wheels under. Then directly under where the long reach hydraulic ram goes - close to the jib frame's pointy end of the triangle, I was going to mount a similar arrangement to the front wheels of an old Farmall H tractor. See the photograph.

My sister's boyfriend is an aircraft frame mechanic - aviation engineer, during a visit here he ticked my project off as being both feasible and practical. His one comment was I should think of incorporating a caster angle into the copy of the Farmall H pivoting wheels. The whole unit will be mounted via a tow hitch to my tractor. This arrangement allows for the crane unit and its load to be self supporting as it is a towed unit. Not relying at all on the weight of the tractor to counterbalance the load. The wheels and tyres will be something from the wrecker with the donor as a Holden or Ford where the hubs are readily available from any trailer place or auto parts store.

I think the idea of the Farmall H style front wheels being arranged as a caster configuration is so it turns around by itself with changes of direction. Unlike the tractors the front wheels will not be controlled by a steering arrangement.

Instead of having the 'legs' of the caster wheel being external to the wheel - like the Farmall H there will be a central king pin column. Then a triangle shape made out of thick plate and a steel hollow section frame in the shape of a triangle to create a caster angle and mount the two wheel hubs and stub axles off. The Farmall H had a central "P-Wheel" bow shaped frame that went in between the wheels. Instead, I will be using hollow section steel and round bar instead to form the pivot and hub assembly. More like the older 1936 F12 Farmall tractor's front wheel arrangement.

The terrain traversed will be out in a flat grassed - okay weeded - paddock and towed either onto a concrete slab or a recycled bitumen road-base mix hard pad.

Farmall H tractor photographs accessed 11th August 2025 from, Farmall H - Wikipedia (https://en.wikipedia.org/wiki/Farmall_H#/media/File:Farmall_H_tractor_VA1.jpg)
Farmall H parts diagram accessed 11th August 2025 from, 302 Found
(https://avspare.com/catalog/case/56592/53702/676081)1936 International F12 Farmall tractor accessed 11th August 2025 from, https://www.gumtree.com.au/web/listing/farming-vehicles/1335732577


Yes, it is a lot of work and there are no doubt better options available. I would like to do it just for the challenge and the fun. Get the jib out from just leaning idle against the wall in my shed and have it out doing some work. I was going to buy the front wheel arrangement off a spares tractor - until the bloke told me how much money they wanted for the parts. It went a long the lines of extortionate and 'stand and deliver'. I walked away instead.

Kind regards
Lionel