Modifications & 'How to' Guides

[jcubed]

Hi All,

I have looked into a number of options as to the possibility of making the Spare Wheel Carrier available in Australia. One option that was eliminated straight away, was making it here (South Africa) and shipping it, as the transport cost would kill it.

All the parts, with the exception of the wheel plate, could be either purchased off the shelf or machined by someone with access to a lathe and milling machine. I therefore enquired locally what the manufacture of the plate would cost - this has to be laser cut - but regardless, the shipping for this part only, would still be in excess of $100.

Plan C, was to get a quote from a Melbourne-based company to do the laser cutting, thereby limiting shipping to inland parcel post cost, and this resulted in the plate itself costing similar to what only shipping from here would be. However, to get them at this price, I need to order a minimum quantity of 5.

The Australian end of the manufacturing and assembly would be managed by Transporter (Adelaide), and he has already reserved one of the plates for his own use. I need an indication from other readers whether they would be interested in such a device, bearing in mind that the application is not limited to a Tiguan, but will fit ANY vehicle towbar. The different standards in number of car wheel bolts, diameter and PCD, as well as the fold-back distance for your particular rear door, will be adapted to order by Transporter.

I don't want your money yet, but purely an indication of the interest in the device. For now, I need 4 more takers. This intent will then be used by Transporter to source and price the remainder of the bought-out items and machining cost and to derive a selling price from this. This will then be communicated back to you via the forum and could either be reasonably priced or we all come to the conclusion that it is not practical.

I know of no other comparable device world-wide, so Transporter would have to use his common sense and knowledge of the Australian market to guide him here. But as he is in the motor industry, he should have a good idea.

One of the concepts is to market the device as a kit to the mechanically minded - a giant USEFUL Meccano set - and also as fully built up (by Transporter) for the less practical.

I am awaiting your response with anticipation.

Regards

Johnie

[jcubed]

Hi softroaders and engineering types,

As I will shortly be driving a vehicle with a full size spare wheel, I have been thinking of different applications for my spare wheel carrier, e.g. carrying two jerry cans.

Although the present format has travelled more than 10000 km safely, two jerry cans are somewhat heavier than a spare wheel, to the tune of 50kg vs 30kg. One way to determine whether the rack would stay on the hitch with the extra mass - or rather the moment caused by it - is to execute a "pull test" to see at what point the clamp starts slipping.

An ex-colleague of mine who manages such a facility was kind enough to help me out earlier this afternoon. By way of pictures, I thought that some readers may be interested in the results.

First off, the spare wheel was removed to allow the strap to be attached to the wheel plate so that the pulling force would be applied from where the wheel centre would be. It is important to pull from a known point, as the distance from here to the centre of the ball - the ARM - is crucial in order to calculate the moment at which the clamp friction is overcome. This moment is used in additional calculations to determine what mass of any number of items, e.g. jerry can, larger wheel, etc can be accommodated safely. In this case the ARM length was 350 mm.



Next, the other end of the sling was attached to a load cell. The load cell measures what force is exerted on it as the sling is tensioned, this information being fed to a PC. The other end of the load cell is clamped to the floor, which has threaded inserts arranged in a square grid.



The complete section looks like this, with the PC in the background.



The load cell is attached to a hydraulic ram, which can be extended or contracted. The complete setup is shown next, with the hydraulic pump on the right, with the pipes feeding the load cell.



Here is what the computer read-out looks like - pardon the sky-light reflection:



And the final spread-sheet result:



The test was repeated 3 times, with virtually identical results everytime. The graph is interpreted as follows. The point where the clamp begins to slip, is where the graph levels off. This is at around 0.72 kN.

Multiplying this value: 720N by the ARM (0.35m), gives a torque of 256 Nm. Compared with the max engine torque of a 1.4 TSi engine (240Nm), you could therefor use this coupling between the engine and transmission without it slipping.

To now calculate what mass could be carried, Nm has to be converted to kg by dividing by g, which gives 26 kgm. A typical jerry can is 200mm deep with the centre of gravity halfway between the front and back, i.e. at 100mm. This is the distance of the CG BEHIND the wheel plate, which is directly above the hitch, and in this example constitutes the ARM.

Divide 26 by 0.1m = 260kg can be supported THAT FAR BEHIND THE BALL CENTRE. Closer more, further back less, in the ratio of the arm.

This however is the static loading, and on the threshold of grip. As soon as the car starts moving, the friction will be overcome, and the clamp slip.

However, if a sensible mass of say 60kg is supported, this will give a safety factor of 4.3, or in different terms - the attachment can withstand an acceleration of 4.3g. This is a ball-park figure for the manoevring load applicable to light helicopters.

So yes, the cycle rack could carry two jerry cans.