Cartie Design – taking things too far!

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by bigmac

Duncan McIntosh Trailers CartieThere was a lot of interest in the Duncan McIntosh Trailers Cartie at the Brechin race so I thought I’d share a few of the technical details. Having never raced, or even been to, an actual cartie race before I started to design from a clean sheet of paper. Maybe this was an advantage or maybe just naïve, we will see when it is tested at longer more demanding races.

The starting point for the design was the Thule Roof Box. It was used as a base for two reasons; 

  1. I realised that although we have access to good fabrication machinery and tools, bodywork and the complex curves they require would be difficult, and
  2. As a Thule distributor we wanted to advertise our wares somehow. I can’t think of a more aerodynamic packaging solution “off the shelf”- ideal!

A quick email to Thule acquired us an ex-demo model. Huge thanks to everyone there for the help.

Next came the suspension. Now not all Carties have suspension, but I was determined to use another product that we sell- small trailer suspension units. I could have made things easy for myself by not having suspension, but hey, where’s the fun in making things easy for yourself?!!! These particular units are Peak Flexiride fully bonded rubber-in-torsion versions. Thanks to all the folk at Peak Trailers for their help with suspension rates, etc. I have used one unit at each wheel to give fully independent suspension.

Soapbox Cartie SuspensionI didn’t want the suspension sitting out in the airflow but rather inboard, inside the bodywork. I decided to use a swingarm and linkage design, not unlike the rear of a motorbike albeit turned 90 degrees to the wheel. The linkage was a simple male and female rod end/rose/heim joint threaded together- this also gives ride height adjustment! A big thanks to R&M bearings in Dundee for their help.

This is when things get tricky. I realised that Carties differ from road/race cars regarding suspension in a major way- there are no major acceleration forces dictating pivot locations. Although we can ignore acceleration forces this still leaves braking forces having an effect on the suspension, so the design I chose is a semi-leading front and a semi-trailing rear. The swingarm (green lines in the photo below) pivot round a virtual axis (shown red) joining two important points; A- the neutral braking force point, and B- the camber change virtual pivot.
 
Point B was chosen as I felt it was the best compromise for maintaining almost zero camber with the road whilst cornering (since there is no anti-roll bar) whilst minimizing any bump-steer. I know what you are thinking - “It’s just a bloody cartie for goodness sake!!!” -I am the first to admit that I have taken this build FAR too seriously!

Soapbox Cartie SteeringSteering-wise, the Cartie has an Ackermann set-up allowing the outside tyre to turn a wider radius than the inside one. This is coupled with an inclined (in both caster and camber) virtual kingpin, again using rod ends as pivots. The kingpin inclination in camber is aligned so that it gives a tyre scrub radius of “almost” zero (actually around 10mm - see pic below)
 
Kingpin inclination in caster is a little bit more extreme - sitting at 15 degrees. Although this is extreme for a car it is not extreme for bikes, where Downhill bikes can have as much as 27 degrees of caster. I did this for increased stability at speed but only time and testing will tell if I am right!

Soapbox Cartie Stub AxlesI knew wheels would be the key ingredient to the cartie so I went for 16” (305) rims on mountain bike disc brake hubs. These were a little like hen’s teeth but I found the right ones at KMX Karts. Only problem was that I didn’t think that the ½ “ axle size would be adequate for the job. I wanted to increase the inner bearing to a larger bore one to give a stronger stub axle. This proved to be difficult as the outer size of the standard bearing was 28.575mm (1 1/8”). This is when I am extremely lucky to have a Dad that can turn parts on the lathe! We (the Royal We!) turned out the inner face of the hub to accept a 30mm bearing that had a bore of 17mm- much better. A few more nights on the Lathe and my Dad had produced four lovely stub axles to fit these new hubs. Cheers Dad- you’re a star!

Soapbox Cartie ChassisThe Chassis started out as a simple ladder frame of small box section steel but this was subsequently beefed up as it was found to be flexing down its length. The new triangulated truss frame isn’t flexing anytime soon! The front and rear swingarrm were also beefed up and triangulated as they too were susceptible to flexing. The aluminium treadplate seat is, despite its looks, relatively comfy!

So that’s it - the product of much thought and many, (too many!) late nights. We’ll have to wait and see if it is at all competitive at next year’s races - Whatever the result, I am sure I will enjoy it immensely!

Ian McIntosh.

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