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  1. #1
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    Default Converting a Thule cycle rack into a spare wheel carrier

    Note: To keep all the information together in a single post, I will update this original post with more images/text added for clarification. The updates will be in red.


    I've had a number of enquiries regarding the cycle rack that has appeared in at least one post of each of my past 3 cars, so put together a basic pictorial and description of what is required to manufacture your own.

    The original requirement was for a spare wheel carrier which could clamp onto the hitch of a VW Tiguan, as the car came with a retractable hitch, forcing this type of attachment. For a hitch with bolt on ball, a much simpler mechanism would have been designed, or possibly bought off the shelf.

    The carrier consists of 4 main components: Thule 970 Express cycle rack, Hitch clamping feet, Wheel carrier with clamps and Light board.


    Cycle Rack

    The Thule cycle rack attaches to the hitch by clamping the ball between two tubular sections. It has a locking facility and makes provision for adjusting the clamping force as either the hitch or the clamping feet wears away. The rack is fixed in a slightly forward position from vertical, and cannot be tilted backwards to allow opening of the hatch. This feature was provided by manufacturing different clamps.

    The rack utilizes a scissor action to clamp/release the hitch. When the scissor is open, the rack is locked and when closed, can be removed from the hitch. As part of the conversion, a midway position was created by means of a removable stopper to allow partial closing of the scissors which allows the rack to be tilted backwards allowing the hatch to be opened. The images below show the locked and fold-down positions of the original carrier.

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    A lock was adapted to limit the closing of the scissors. When inserted, it lies at the same angle the LH handle would be when it provides sufficient opening of the clamping feet to fold back the rack. This is also the anti-theft device, as if you want to remove the rack completely, this lock has to be removed to allow full closing of the arms. See image below. The 2nd image shows the lock type used initially.

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    Click image for larger version. 

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    The above is just shown for historical purposes, as due to the adjustment required of the plate position to compensate for the ball/claw wear, the lock became obscured by the locking lever, so a completely new mechanism was devised. It now works like this:

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    Click image for larger version. 

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    Clamping Feet

    The images below show how the clamping feet fit around a hitch ball. Note that it is not the neck of the hitch being clamped by the semi-circular cut-outs, but the outer surface of the ball by the inner edge of the tubular sections.

    The inside edge of the semi-circular aperture acts as an endstop against the neck of the ball to prevent it from deviating too far from the upright position should it slip when locked. To allow rearwards movement, elongation of the cut-out is therefore required, as shown.

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    Modifying the existing clamps by just elongating the cut-outs would be a cheaper option, but this reduces the clamping diameter, resulting in a lower clamping force. It was deemed safer to replace the component with a different design with a chamfered surface to provide a larger contact area and thus at least retain the standard clamping force. The image above shows this chamfered contact area compared to the ridge contact of the original.

    The new feet are machined out of 60mm diameter Al, retaining a central web for extra rigidity.


    Wheel Carrier

    The wheel carrier provides a base for three threaded studs which pass through the wheel holes which is then fixed with lockable wheel nuts. The carrier attaches to the frame of the cycle rack with hydraulic clamps, to not drill additional holes which could weaken the structure.

    Below, some of the experimental design progressions.

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    Untill we got here, via Visio, exporting to dxf to have cut.

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    And then finally, the hardware. This is the carrier plate for the XV, which also illustrates the modularity of the design. All that is required when changing vehicles, is a second (or third) plate with the relevant wheel pitch. The studs, spacers and clamps remain the same and is transferred from one plate to the other.

    E.g. the original VW plate I sold, the XV plate I made for my son's trip to the Richtersveld, the Freelander 2 and Volvo XC60, both being modified Ford Mondeos, thankfully have the same wheel bolt pitch. So the plate I have on the Volvo is the same one that has been on the LR since 2010, just with repositioning of the stud holes to improve the departure angle of the car.


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    The plate was water jet cut from 8mm mild steel sheet which was not surface treated, therefore it rusted. Paint would not work as the sliding mechanism would take it off. The next configuration – for the Freelander 2 – was therefore Gunkoted. Alternatively, the plate could be cut from Stainless Steel.

    Originally, the three wheel studs were M12 MF x 125 mm bolts with turned down heads and a counter-bored plate, so they could be flush with the surface for the clamp plates to slide over them.

    On subsequent versions the plate holes were “drilled” to 10.5mm and tapped to M12 MF. Sections of MF threaded bar cut to length were then screwed in until it was flush with the other side of the plate. The length is determined by the wheel depth.

    Wheel bolts are generally Metric Fine thread, usually M12 or M14. This is also the threads for which lockable or keyed bolts are available at most car part stores. M14 threaded bar was not available, hence M12.

    Here is what it looks like behind the wheel. The design of the carrier plate should be more clear now. The plate is fixed via the clamps to the left arm. Obviously, it cannot be fixed to the right arm, otherwise the scissors would not open. It therefore has two guides which run in slots, which keeps the plate attached and locked to the right arm when traveling.

    The 3x PE disks are there to prevent scratching the paint off the face of the wheel. It also conforms to the slight concave countersinking of the bolt holes and provides a more secure fit.


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    The forward tilt shown in the final image of this post is sufficient to hook the spare wheel over the studs and then look for the nuts without the wheel falling on your feet in the meantime.

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    Here the keyed nuts are fitted, with the adapter shown. Don't forget to pack the 21mm socket. This is where the rubber feet on the number plate board slides over.

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    Below the pipe clamps and hardware it comes with. Attached a pdf of the relevant clamp catalogue.

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    Number Plate Board
    Finally, as the spare wheel obscures the vehicle license plate, a number plate board with illumination had to be added. This attaches to the wheel via one of the unused bolt holes from the other side.

    It has been sized to fit inside the rim to avoid damaging the number plate light if you put the wheel down on that side. 8mm PVC sheet was used as thinner material – 6mm - was too flimsy and did not retain its shape, and a colour-impregnated material saves labour.

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    Here's what it looks like from the other side. The 3x rubber feet slide over the nuts locking the wheel to the carrier. A bolt is then inserted from the other side through the top hole of the wheel, which screws into the threaded Al spacer at the top.

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    General

    The standard rack is rated at 30kg capacity, but studying the Thule leaflet shows that this limitation has to do with the rearward moment two bicycles exert on the clamp. When the cycle support arms are removed, the assembled rack weighs 6.2kg and the full size spare - 255/55 R19 - 28 kg. The Centre of Gravity is much closer to the centre line through the hitch than the case of two bicycles would be. Like this:

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    A practical pull test was done showing that larger spare wheels could be carried.

    In order to qualify a product in accordance with MIL-STD-810F, it is required to subject it to an acceleration spectrum similar to what it would see in actual use. Alternatively, the actual condition that the test simulates, can be executed. This required between 3200 and 6400 km of highway use for wheeled vehicles, which was exceeded comfortably by logging in excess of 20000 km to date.


    Conclusion

    In practice the mechanism works well, as long as the clamping force is suitably adjusted. The Thule rack allows 6 levels of tensioning from nominal. After 13 years (since 2008) I have two levels to go. Once this runs out, new claws need to be manufactured, as they have then reached their wear limit.

    However, if you go over a sleeping policeman at a speed where the boot floor becomes airborne – say, 60 over a 40km/h hump - the wheel will move slightly backwards to the upright position.

    This is caused by the slot in the clamping feet, which changes the retention mechanism from friction and mechanical to purely friction towards the rear. An unmodified Thule rack won't do this. This is why the pull test referred to above was done. I'll write that up and also post here.

    Fortunately you can immediately see this displacement in the rearview mirror and stop to reposition it when the opportunity arises.

    If this is not acceptable, a different cycle rack could be used - e.g. Thule Hang-on 4 - which does not articulate on the ball but has a lift and fold function.

    Others have a lever which disconnects a friction clutch. These levers however tend to extend quite far back at the level of the ball and could suffer from ground rash offroad. Also, the base cost of these racks is probably at least 3:1 compared to the Express.


    The foam sleeves over rack tubes is an attempt to limit the damage to the hatch should someone ride into the back of you.
    Attached Images Attached Images
    Last edited by JJJ; 2021/08/04 at 10:41 AM.
    If my post insulted you, wonder where the smiley went .

    Johnie
    Volvo XC60 T5
    180kW/350Nm (1500 - 4800 rpm)

  2. #2
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    Default Re: Converting a Thule cycle rack into a spare wheel carrier

    Nice!

    How is the weight compared to bicycles?

  3. #3
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    Default Re: Converting a Thule cycle rack into a spare wheel carrier

    Good question! Had to get on the scale now with my son's NORCO Storm mountain bike and it weighs 16kg.

    The rack is still useable to transport bicycles, just without the wheel and with the carrier arms refitted. We went off to Mlilwane with two bikes on the rack Dec 2019.

    I have not weighed the Volvo wheel (yet), but I seem to recall the Freelander 2 full size spare - 235/60 R18 - weighed 24kg.

    Its on the car now, but I'll take it off during the week and weigh it.
    If my post insulted you, wonder where the smiley went .

    Johnie
    Volvo XC60 T5
    180kW/350Nm (1500 - 4800 rpm)

  4. #4
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    Default Re: Converting a Thule cycle rack into a spare wheel carrier

    Although you have upgraded the tow ball clamps, I was just thinking about what weight the frame can carry.
    Last thing you want is to hit a bump and gravity overcomes that carrier.

  5. #5
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    Default Re: Converting a Thule cycle rack into a spare wheel carrier

    Hi DeV,

    I have updated the original post with more images and also addressed your concern above. All updates in red.

    Bottom line is, some bumps do overcome the friction limits of the clamp, but it can't come off, only tilt slightly backwards untill it gets to the mechanical limit of the neck of the hitch.

    As you can see, I am within the design limits of the rack as stated by Thule, but with a better CoG.

    I would not use this rack to transport a 35" Mickey Thompson rim with a 285 size tyre on. It was specifically designed out of desperation for VW Tiguan use as there was nothing else available in 2008. I knew the Vleesbaai dune route would be undoable with a space saver, should I get a flat.

    My tyre and wheel size is identical to that of most Discovery 4s. So up to there I am happy.
    If my post insulted you, wonder where the smiley went .

    Johnie
    Volvo XC60 T5
    180kW/350Nm (1500 - 4800 rpm)

  6. #6
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    Default Re: Converting a Thule cycle rack into a spare wheel carrier

    A Grand Day Out - at the test facility
    As promised, the write-up on the testing of the spare wheel carrier clamping capability.

    For the history of this post, refer to the May 2010 issue of Leisure Wheels – Carry a REAL Spare Wheel

    Although the present format has by now travelled more than 20000 km safely, way back I was a bit wary whether the clamping force would be sufficient to keep the rack vertical on the hitch.

    One way of determining this would be a calibrated "pull test" to see at which point the clamp starts slipping. An ex-colleague of mine who then managed such a facility, was kind enough to help me out. Thanks, Sarel!

    Test
    First off, the spare wheel was removed to allow a 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 could be accommodated safely.

    In this case the ARM length was 350 mm.

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    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 attached to a hydraulic ram, clamped to the floor. The ram tensions the sling when a valve is opened on the controller.

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    Below the complete setup, with the PC in the background.

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    A different view. The hydraulic lines controlling the ram is visible here.

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    The computer read-out - pardon the sky-light reflection:

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    And the final spread-sheet result:

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    The test was repeated 3 times, with virtually identical results each time. 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.

    To calculate what mass could be potentially carried, N has to be converted to kg by dividing by g, which gives 26 kgm.

    Say a jerry can is to be carried. The container 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 at that distance behind the ball centre prior to the clamp slipping. At closer distances more, further back less, in the ratio of the arm.

    This however is the static stiction and on the threshold of grip. As soon as the car starts moving, the stiction 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 stiction can resist an acceleration of 4.3g. This is a ball-park figure for the maneuvering load applicable to light helicopters.
    Last edited by JJJ; 2021/08/14 at 09:39 AM.
    If my post insulted you, wonder where the smiley went .

    Johnie
    Volvo XC60 T5
    180kW/350Nm (1500 - 4800 rpm)

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