Trailer conversion for motorbikes?

[Chris152]

OK, the trailer for our dinghy is now all good after some bits have been replaced. The trailer from hitch to back is about 4.5 metres, and it's about 1.6 metres wide.

My next question is - is it reasonable to fabricate a base, to clamp (with U-bolts) to the existing trailer, to transport motorcycles? We're thinking of a rectangular frame with two or three runners for the wheels of two or three bikes to stand on, maybe an upright or two to strap to, and a simple rear frame to attach the light board. I'll do a drawing of that isn't clear.

Clearly, the trailer is too long for the bikes, but if we can avoid having to buy a separate trailer for the bikes it'd be good,

Thanks for any thoughts, C.

 

[Sideways]

What sort of bikes ?
Compared to a dinghy, there are plenty of big heavy bikes that could impose a much bigger point load on the frame and potentially bend those square section tubes if you hit a pothole.
Bikes might carry their weight higher up too, so you'll need to arrange good anchor points, good tie downs and potentially double them up. Having a strap fail and dropping a bike off the side of the trailer doesn't bear thinking about.

If we're talking a pair of 125cc trials bikes, I'm sure this must be doable

 

[Dabop]

Have you seen the 'skeleton' motorbike trailers- that kind of thing would be perfect for your bikes for two bikes (three would be an issue, because of the dinghy's rollers in the middle)

Get some C-channel wide enough to fit the bikes tyres, fit two 'sideways beams' to go from channel to channel under the channel to support them, and use U saddles to connect that to the existing center beam of the trailer towards the rear and towards the front (I would undersling them myself to lower the COG and match the height of the axle so it would then become a 'center support beam' for the channels)- the biggest issue is your two vertical hull supports on the axle itself- maybe see if you can make them easily removable, and then leave the bike runners on basically 'permanently' ie they become part of the structure of the trailer- put hull supports in for boat, take off for bikes...

(maybe a square box section just bigger on the inside than the external size of the tube on the uprights with some holes and pins to secure them in place when needed, pull the pins for removal for the bikes...)

Ideally, I would take those hull support rollers bases off, and thats where the 'channels' would sit on top of the axle, cut holes in the channels at that point to drop the supports bottom end into, and see about making up a new support bracket that both holds the channel in place permanently and supports the vertical tube of the hull supports as well, with provision for securing the supports with easy removal...

(at this point- i would go the final step and triangulate the fronts of the two ends of the beam supporting the front of the channels back to the existing center beam of the trailer towards the hitch- that would stiffen it up considerably and reduce flexing)

Although I would check the load ratings of those little tyres and of their hubs- many of them don't have much of a load rating, and motorbikes can be heavy...

If you can't quite visualise what I meant- look at the picture of that bike trailer above- yours with two bike capacity would look almost identical from the rear- but your existing trailers center 'front to back' beam that is the 'backbone' of your trailer would be in the exact same position as that center bike channel and would be its 'replacement' in that picture... the new 'front and back' support beams out to the channels would be under that existing 'backbone' like the center bike channel is in that pic, and your axle is a third 'channel support' beam in the center

 

[Dabop]

I was thinking something like this (see if it makes it clearer) existing trailer is in blue (light for axle, dark for 'backbone' beam of the trailer)
Two new support beams (basically copies of the existing axle- minus the wheels of course lol) mounted the same way and underslung the same way- shown in red
Two channels mounted on top (so basically in the same position on top as the 'backbone' beam) in orange
The two (now removable) hull support rollers basically exactly where they are now, but going through the C channels are obvious...

In yellow- 'optional extras' an extra support beam to 'make the spacing's all match, and two 'triangle brace beams' to triangulate the entire thing- which would stiffen the trailer up immeasurably!!!

Currently the axle is very poorly supported- and hitting something with a tyre could see it put under severe twisting force- say there a big pothole in the road (and for those little tyres- EVERY pothole is a big one lol) one tyre continues on straight ahead, with the entire weight of the trailer pushing at it from the 'backbone' beam- while the other tyre tries to stop 'dead in its tracks'- and all that twisting force is currently taken in a very small area by two ubolts...
(have you had issues with those center ubolts working loose over time??? I suspect so... from every little bump in the road working at them- especially as it has no suspension...)

This would make the trailer a permanent 'combo' and stiffen it up considerably into the bargain...

 

[Chris152]

Thanks both, that's really helpful.

Sideways - it'd be for 125s at the moment, and limiting to two would be fine, more often just one. I guess a total of about 280kg with two bikes onboard. The steel used for the box section is fairly hefty, but point taken about weight/ pot holes, no shortage of those round here.

Dabop - We'll read through your design together later today and try to make a plan. The wheels on these little trailers have always looked small to me, but it seems to be pretty standard. We replaced them when we got the dinghy as the cost of wheels including tyres was about £5 more than just buying the tyres, and we're keeping the speed down anyway. The rollers and stems lift out from the cross beam, so no issue there.
I guess one question in the design drawing is where the weight is relative to the axle - maybe the centre of weight of the bikes needs to be over the axle, or we could end up with too much nose weight - so we'd just shift the bike frame further back? It's really kind of you to have taken the time to do that design!

 

[Dabop]

LOL- only took a minute or so in LibreDraw...
Most bikes I have ridden and transported were actually pretty much neutrally balanced ie not that much difference in weight between the front tyre and the back, so having the 'balance point' of the bike slightly in front of the axle of the trailer is where you want it (although with such a light trailer/load, it really wouldn't be that much of an issue except on the tiniest of cars towing it... as long as there is 'down force' on the trailers tongue, its good (up force is BAD- that causes 'trailer swaying' and likely a rollover!!!)

So basically- yes you do want the bikes back quite a way but it looks from the picture you would have enough rear overhang to achieve that (especially with the lighter bikes like 125's- I probably wouldn't load a pair of Goldwings on it though LOL)

You can find the balance with a set of bathroom scales (most I have seen go up to at least 150kg, many 250kg- weigh the front tyre first, then the back tyre- the combined weights is the weight of the bike, and the ratio between the two is the 'front/back' balance which will give you the 'COG of the bike (approximately, but good enough for this)- measure from the front to the back axles on the bike, and split that distance by the same ratio- thats the point you want just a bit forward of the axle on the trailer...

 

[Dabop]

As an example- using a KTM125- it weighs 137kg- if we weighed it and got say 65 kg on the front wheel and 74kg on the back wheel on a set of bathroom scales- we get 139kg (so pretty close to the listed weight of the bike) and we have a 0.94:1.07 front to back weight split... 1:1 would be exactly half the weight on each wheel)

(divide the measured weight of the bike by 2, then divide that figure by the front tyre measured weight and that gives the front half of the ration, then that same 'half weight' by the back measured weight to give the back half of the weight ratio)

Measure the distance from the front axle to the back axle- and do the same ratio on that distance...

That measured point is the COG front to back of the bike (ie where it has equal weight on both sides)

A final test after construction is to measure the tongue downforce of the trailer- put the jockeywheel onto the scales and measure the weight- check it has downforce, and measure its amount, against the total weight of the trailer and load (you may need to take it to a weighstation to get this- if you can find one that can measure this light lol)
The tongue downforce should be between 10% and 20% of the trailers total weight including load... (as long as it doesn't exceed the tow vehicles tongue downforce limit- not likely in this case lol)- this can allow you to 'finetune' the loading by shifting the bikes back and forth until it is 'perfect'

Or skip the weighbridge and just make sure that there is down force on the tongue and its less than the towbars downforce rating...
;-)