Chassis Build Help!

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Ok, so we may have made our kart design a little Big. Unfortunately, we are stuck with the design now as the rules for the Red Bull soapbox states the exterior look of the vehicle must remain the same. The chassis can be changed, but overall length/ width has to remain the same.

The first drawing is an overhead view of the basic layout of the floor chassis, seating arrangement, and somebody bits. Thanks to your advice from an earlier post, wheels will now be 14'' pit bike wheels instead with 15mm axles. Stub axles we plan to use. Cant show the steering parts as I’m not sure of their dimensions fully yet.

The base of the design and the main dimensions. The overall length of all the sections added up here comes in around 20metres in length.

A side view drawing we plan to add a full roll over bar also:

We thought about doing the base floor of the chassis in hollow box mild steel. Unsure first if we should go with 1.5mm thickness or 2.5mm. Also unsure what overall dimensions we should use e.g. 20x20mm, 30x30mm, 40x40mm etc The total weight of the vehicle needs to be no more than 80kg - not including the driver/ passenger. Together we weigh roughly 160kg and are sitting in the centre. The old how much weight can you put on a ruler before it bends and breaks!

I’ve calculated the next part just for the mild steel base chassis frame part alone.

50x50x2.5mm is 3.71kg per metre
Total weight = 74kg Price £160

40x40x2.5mm is 2.92kg per metre
Total weight = 58kg Price £131

30x30x3mm is 2.51kg per metre
Total weight = 50.2kg Price £115

30x30x2.5mm is 2.14kg per metre
Total weight = 42.8kg Price £101


I'm unsure of the weight of the pit bike wheels, breaks, seats, steering, roll over cage, and light weight skeleton frame of some sort to support the body will need to be added on top of the that weight too. The body will be made from thin lightweight plywood, either covered with printed graphics or spray painted.

Should we be doing the chassis as it is above as a simplistic design, or should we make it out of circular pipe/ tube with thin walls, create a space frame chassis similar to this:

The space frame option above would be more work, but believe it will be a lot lighter and more ridged based on our overall dimensions? What size tube and thickness would we then need to create it like that?  


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Hi, if you use know chassis

Hi, if you use know chassis and engineering principles. a triangulated chassis using 25mmx25mm 16SWG (1.6mm wall) is more than adequate. This is the standard for Lotus 7 type kit car chassis i.e Caterham, Westfield etc.

You should be able to build a non suspension cart which is reasonable rigid and still get below the 80kg .

Round tube may be lighter, but you have to add in additional build time, which will run away quicker than you would think.

The basic triangulation for the sides could be as simple building creating the triangulation from the top of the roll bar to points as close as the axle lines to reduce flexing.

The axles will need to be sturdy with a reasonable diameter (30 - 40mm and a minimum of 2.5mm wall. 

If buying in bulk, you could probably reduce your steel costs if you go to direct to a company like Hub le Bas, but know the amount of steel you only need to pay the delivery costs once. 

When looking at wieght, always allow for the bits you forget, welding, bolts, seats etc. It is surprising how much they can add. As an indication, our first cart, even though a single seater, weighed in at about 50kgs, when we first went to Cairngorm in 2009. The current one with suspension and much stronger rollbar comes in at just under 100kgs. 

With regards to the steering, keep it simple and light. If you are going with solid axles, go for kart steering rather than a steering rack.

Have a look at our website, especially at the earlier chassis, which may have been simpler, but still took a couple of hefty knocks.

Hope this helps and good luck at Red Bull 


azuma | May 3, 2013 - 21:10

Hey, thanks for that

Hey, thanks for that reply, changed our chassis way of thinking completely!

Our new chassis design pictured below.


I wanted to ask, would 25x25x1.2mm square box be ok to use?

We’re using 3 long 25x25x1.6mm square box down the centre. One on each end and one in the centre. In between these will be using the 1.2mm thick square (same idea ladder design)

Would these be ok to mount the seats on to? The Axles on each end will be in 40x40x2.5mm square.

The top part of the chassis things becomes more fun. Square we will be using to speed up the time as suggested.

Where they come down/ weld on the floor chassis part we have lined them up to the square going across the floor. We plan to use 1.2mm thickness for all the top part also.

I listened to your advice about connecting the highest part as close to the axles as possible, but from the roof of the design it was difficult as it would have needed to go through the exterior design itself.

Hopefully at the window line it should be ok?

Also, want to double check with you on the rear axle as to which you think would be better?

The 40x40x2.5mm box is welded on at the bottom to add support. The grey box on top is 25x25x1.2mm. A hole is cut into it as shown. Red line is the extended spindle. Goes through both of the grey boxes with a washer/ nut holding it on. In the centre between the boxes can be bolted town with a clamp too.

Or would this one be better?

Cut a square steel  end plate. Cut a circle in the centre.  Spindle goes through it. Bolt and weld it on the other side then weld the whole thing on the end of the 40x40x2.5 box.
autobots | May 12, 2013 - 19:05
scottishcarties's picture

soapbox chassis design

@autobotsuk #soapboxrace I think I'd go for 25mm x 50mm x 2.5 for the main rails running down the centre, mounted with the 50mm side vertical. You've got some long sections there, and 25mm box migth be a bit too bendy.

As for the wheel mounting - the single plate option would work if the plate is thick enough, but might be a pain for maintenance. The plate would need to be quite chunky too. You've got a heavy chassis and two people onboard - it's going to take a fair old hit when you come off the ramp.

Sorry to be so late in replying - hope this is still useful.

scottishcarties | May 24, 2013 - 10:51

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