suspension for a bike trailer - bit o/t

[nicks90]

dont think i'd get away with this topic in the int forum or any other - so as its a bit of fabrication i have stuck it in here.

basically i have a bike trailer / pushchair thingy for my little girl. Its quite a cool little trailer, in 2 wheel mode it has a towbar to attach to your bike like any normal bike trailer has - but that can be detached and a front wheel put on to make it a big offroad 3 wheel pushchair. Large 20" wheels + quite wide etc means its nice and stable offroad and goes over big lumps and bumps easily. Great for when Violet and the missus comes to RTVs to marshal and when we go on walking holidays to the lakes. It even has 'tree sliders' that goe round the tyres

But unfortunately the suspension consists of the squidgy tyres and thats it! So she rattles about inside. Doesnt overly bother her, but i would like to smooth it out a bit by fitting some leaf springs to the main axle.

The axle is just a solid tube that is spot welded to the frame on each side and the wheels attach into this tube via push fit connectors. The plan is to grind out the welds and remove the tube and weld little tabs to the front and rear of the frame and attach leaf springs and using small Ubolts clamp the axle tube to the leafs.

length of the springs would need to be about 20" and approx 1.5" wide and the weight of the trailer is 16kg with a max load of 25kg - so call it 40Kg overall.

I only want about 2" upwards travel and maybe 1" downward travel - so 3" overall.

Problem is where the hell would i get a small set of leafs that would do the job? Or should i just make some?

My initial thoughts were to just cut a simple bit of flat metal plate of appropriate thickness to get the "springiness" i require and weld some metal inserts from a polybush kit i have lying around to each end. Would end up being a totally flat spring and to get the upwards travel the spring mounts would obviously need to be 2" deep. But i have no idea how thick the metal plate would need to be to get the right springyness - bearing in mind the spring dimensions...

any bright ideas on making a simple leaf would be hugely appreciated and i'll post regular pictures as the project goes along.

[Boro]

I doubt fitting leaf springs would make a huge ammount of difference tbh but if you fancy playing with them i would suggest you try and find something with lightweight springs on to start such as a ford capri for eg, i cant remember whether something like a reliant robin has leaf springs on the back but if they did that wouldnt be a bad starting point and then run a single leaf each side, you can shorten them as required but i still recon they will be too stiff on something so light but try it nothing to lose

[nicks90]

i think using any sort of leaf spring from a motor vehicle would be a non-starter.

they would just be too stiff. Plus all the hassle of cutting them to length AND cutting them to width. Leaf springs are usually made from bloody hard metal and it would take ages...

The whole thing all up is 40Kg, so basically working on the principle of a peak 30kg max load each side whilst going over uneven surfaces, which would result in the desired 2" of compression on that loaded side.

So i need a leaf (probably much easier to use a single leaf rather than multileaf) that would deflect 2" in the middle with a 30kg load.

So anybody really clever on here with metallurgy who could work out what thickness mild steel would give this deflection based on the dimensions of the bit of metal?

[muddy]

You could try and find an old pram to rob for its springs?

Personaly i would use coilovers from the back of a cheap dual suspension bike.

Will.

[nicks90]

GENIUS Mr. Muddy!!!

no idea why i never thought of that.

Have a feeling they might have a bit too much curve on the leafs though and not cope with such a heavy bike trailer. But worth a gander for an old pram next time i drop some stuff off down the tip.

[muddy]

The 'too much curve' coupled with the heavier bike trailer should all come good in theory.

Somewhere like car boots, jumble sales or freecycle should have an old pram knocking about i would guess.

Will.

[david1701]

what about leather straps coach style, wouldn't do a huge amount of good but would soften down some bumps

[LandyManLuke]

So anybody really clever on here with metallurgy who could work out what thickness mild steel would give this deflection based on the dimensions of the bit of metal?

I'm pretty sure plain old mild steel will be carp, for what you want. It'll just bend, and stay bent, there's no spring in it at all.

[Night Train]

You should sign up to Cycle Chat and post there. There is lots of useful information and chat about bike related stuff.

You could canterlever the axle and use a rubber suspension by way of an old innertube as an elestic band. You can also get suspension units for MTB rear suspension, like coil wrapped dampers, which you can use. They are usually adjustable.

If you are keen on using leaf springs then for that sort of weight you can use wood laminated together. Thin strips of hard wood glued together with the grain all running in the same direction. You can then make up the curve you want and the number of laminations will give you the stiffness. You won't need an eye at each end just put the end of the leaf into a pocket that prevents it dropping out.

[DaveSIIA]

A copy of the SAE Spring Design Manual wouldn't go amiss. It goes into all the details of design, material selection and a more than a little bit of maths. Might be available through your local library.

Another option might be torsion bar, using a 110 CSW rear anti-roll bar.

[Night Train]

Another option might be torsion bar, using a 110 CSW rear anti-roll bar.

That might be a bit of overkill, 10mm rebar would be more then enough for a kiddies bike trailer. It would also be a lot lighter as well.

You could just make a trailing arm with a rubber suspension block like this.

[nicks90]

Night Train - you have pursuaded me. Rubber is the answer!

very elegant and simple solution.

i would love to go coil over - but that would mean the bike trailer would have more modern suspension than both our cars...

Also the upper mounts for the coilovers would either be up the side of the trailer (ugly + complicated) or under the frame meaning the axle would be very much lower and giving an unacceptable lift. Plus would also need trailing arms.

i hadnt thought of rubber donughts before - kinda like mini suspension. Simple trailing arms with long pin bolts holding them to the frame would negate the need for a panhard rod and rubber is easy to come by.

[rusty_wingnut]

wishbones either side, compressing a couple of rubber exhaust brackets

[Turbocharger]

I have to say, for something where weight is so important, leaf springs make a lot of sense because they give you compliance and location. I'd suspect a single leaf from a small car could give the compliance if it was mounted in the right way, eg as a cantilever beam to the centre of the axle.

Rubber is a good solution but you then need a linkage to support it all.

[Night Train]

Rubber is good and easy. Make a simple trailing wish bone so the chassis end is nice and wide for lateral location fit a wedge of rubber in between the trailing arm and the chassis and then rivit a bit of webbing strap between the two to prevent the axle dropping when you pick the trailer up. If you start with a solid block of rubber it will give the hardest ride. You can then drill holes in it to give a softer ride.

If you decide to return to leaf springs then have a look at the quarter eliptic set up where the thick part of the spring is securely fixed to the chassis and the thin end is used to mount the axle (a bit like a springy trailing arm). The down side is that it will still weigh more and put more stress on the chassis frame.

I would really avoid car parts as they are far too heavy and designed for something that will weigh up to 10 times as much .