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...
;-)

 

[Lons]

Chris, there's nothing to stop you welding or bolting a rectangular base on to the existing trailer which would be far better for strength and stability, wont affect the boat if you place it right. The trailer in it's current state isn't suitable imo remember a boat only rests along the centre bar so as Dabop says you need to look at weight distribution.

However there are several things to bear in mind. You need to know the weight of the trailer as it stands now, it looks quite beefy, you also need the weight of any extra metal, timber etc you add whether fixed or temporary and need the combined weights of the bikes, you mention two or three. All these have to be added together and remain below the 750kg weight limit for an unbraked trailer or you risk a fine. 750kg sounds a lot, it isn't!

 

[Inspector]

Hard to tell from the picture but if the width of the trailer allows you could put the bikes across the trailer. One behind the axle, the second forward and if needed a third (or one bike on its own) over the axle. Maybe a little more windage that shouldn't be too much of a burden unless you are going very long distances.

Just a thought 90º out of the box.

Pete

 

[Dabop]

Chris, there's nothing to stop you welding or bolting a rectangular base on to the existing trailer which would be far better for strength and stability, wont affect the boat if you place it right. The trailer in it's current state isn't suitable imo remember a boat only rests along the centre bar so as Dabop says you need to look at weight distribution.

However there are several things to bear in mind. You need to know the weight of the trailer as it stands now, it looks quite beefy, you also need the weight of any extra metal, timber etc you add whether fixed or temporary and need the combined weights of the bikes, you mention two or three. All these have to be added together and remain below the 750kg weight limit for an unbraked trailer or you risk a fine. 750kg sounds a lot, it isn't!

I gave the weight of a fairly common 125cc bike up above (a quick check shows most are actually a few kg lighter) so two bikes would be about 280kg, leaving 470kg for the trailer- I know my own boat trailer (which is a center beam plus 2 shorter side beams, triangulated at the front- with the suspension mounted on a seperate rectangular section that can be loosened at its ubolts and slid back and forth to achieve balance) weighs in at only 170kg!!!

Not the best shot (I happen to have its compliance plate stored on the computer for the insurance, along with all the other plates off the other trailers) but you can see in the shadows the center beam and two triangle beams in this shot

His existing beam and axle would have to be solid bar stock to make up THAT much of a difference!!!

Actually I wish mine had a higher ATM, as it is currently only 450kg- which means when I have the 40hp on, and a full tank of fuel for it, it is VERY close to its max legal!!!
(back when this trailer was made, the rego cost more as the ATM rose, so people would make it as light as possible to save money on rego here)- now its the same price for anything under 750kg here, so a full 750kg on it would cost exactly the same as my current 450kg... (that 170kg trailer is long enough to fit my 4.2 boat on btw, so its not a small boat or trailer)

On thinking about it, I might get it reclassified to 750kg, it would mean a new rego inspection and a new weighbridge ticket for the DMT, but would add peace of mine when the boats got the fridge and other junk in it...

When I bought the boat, it still only had a 9.8hp on it, and my 40hp is considerably heavier (I can carry the old 9.8hp by myself, the 40hp took two of us to lift it on, and we were struggling with it!!!), which is why its so close to the limit- as it was I had to slide the axle back further to actually get downforce on the ball!!!- When I first put the 40 on, it was sitting on its jockey wheel, and it lifted the wheel off the ground until the motor hit the ground!!!

Not good!!!

I also swapped out the little wheels for a set of 14" car rims and tyres, as I towed it long distances (sometimes thousands of km on holidays) and I have seen WAAAY too many of those small wheels spin off over the years (remember they turn a LOT faster than your car tyres do to cover the same distance as their diameter is smaller)

 

[Chris152]

I gave the weight of a fairly common 125cc bike up above (a quick check shows most are actually a few kg lighter) so two bikes would be about 280kg, leaving 470kg for the trailer- I know my own boat trailer (which is a center beam plus 2 shorter side beams, triangulated at the front- with the suspension mounted on a seperate rectangular section that can be loosened at its ubolts and slid back and forth to achieve balance) weighs in at only 170kg!!!

Not the best shot (I happen to have its compliance plate stored on the computer for the insurance, along with all the other plates off the other trailers) but you can see in the shadows the center beam and two triangle beams in this shot
View attachment 189162
His existing beam and axle would have to be solid bar stock to make up THAT much of a difference!!!

Actually I wish mine had a higher ATM, as it is currently only 450kg- which means when I have the 40hp on, and a full tank of fuel for it, it is VERY close to its max legal!!!
(back when this trailer was made, the rego cost more as the ATM rose, so people would make it as light as possible to save money on rego here)- now its the same price for anything under 750kg here, so a full 750kg on it would cost exactly the same as my current 450kg... (that 170kg trailer is long enough to fit my 4.2 boat on btw, so its not a small boat or trailer)

On thinking about it, I might get it reclassified to 750kg, it would mean a new rego inspection and a new weighbridge ticket for the DMT, but would add peace of mine when the boats got the fridge and other junk in it...

When I bought the boat, it still only had a 9.8hp on it, and my 40hp is considerably heavier (I can carry the old 9.8hp by myself, the 40hp took two of us to lift it on, and we were struggling with it!!!), which is why its so close to the limit- as it was I had to slide the axle back further to actually get downforce on the ball!!!- When I first put the 40 on, it was sitting on its jockey wheel, and it lifted the wheel off the ground until the motor hit the ground!!!

Not good!!!

I also swapped out the little wheels for a set of 14" car rims and tyres, as I towed it long distances (sometimes thousands of km on holidays) and I have seen WAAAY too many of those small wheels spin off over the years (remember they turn a LOT faster than your car tyres do to cover the same distance as their diameter is smaller)

Having read your posts, I got to worrying about trailer regulations and inspections - it seems in the UK the only stipulation (on older trailers, anyway) is that they're 'roadworthy' and meet pretty basic rules (lighting/ weight etc). Same goes for insurance (we're with Admiral).

 

[Dabop]

Same here, a new trailer has a lot of new regs to meet, but an old one like my boat trailer only has to meet the regs of when it was built...

(I actually game the system a bit here- building a new 'homemade' trailer is $$$ these days with engineers certs required, fees etc etc (I could buy a new factory built for less $$$)- so the last two trailers I got for my hire company, were old rusted out box trailers for $25 and $30- basically a completely new built trailer, using the old drawbar (literally the only thing left from the original trailer lol) and compliance plate, all the rest was new...)

Bit like grampas axe- its had three new heads and five new handles- but it's still his old axe lol

By rebuilding an older trailer, I get a new trailer at about a third of the cost of a factory made one, its just been 'rebuilt and reregistered' lol

 

[Chris152]

Bit like grampas axe- its had three new heads and five new handles- but it's still his old axe lol

It's 'Trigger's broom' here:

 

[Dabop]

It's 'Trigger's broom' here:

god, haven't seen only fools and horses for ages...

 

[stuart little]

Same here, a new trailer has a lot of new regs to meet, but an old one like my boat trailer only has to meet the regs of when it was built...

(I actually game the system a bit here- building a new 'homemade' trailer is $$$ these days with engineers certs required, fees etc etc (I could buy a new factory built for less $$$)- so the last two trailers I got for my hire company, were old rusted out box trailers for $25 and $30- basically a completely new built trailer, using the old drawbar (literally the only thing left from the original trailer lol) and compliance plate, all the rest was new...)

Bit like grampas axe- its had three new heads and five new handles- but it's still his old axe lol

By rebuilding an older trailer, I get a new trailer at about a third of the cost of a factory made one, its just been 'rebuilt and reregistered' lol

Just like Trigger's broom!!!!

 

[stuart little]

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.

View attachment 189084

I'd say the axle is too far back for that, & could put too much weight on the hitch. I'd also replace those 'castors' with something like 10" mini wheels?

 

[Chris152]

We haven't tried, but I think the axle is held down with the three U-bolts and can be moved forward or back. The side rollers sit onto the wooden 'stringers', which are strong - previously it had rollers that ran parallel to the direction of the boat, and were onto the GRP which pushed it up, which created stress.

 

[Dabop]

We haven't tried, but I think the axle is held down with the three U-bolts and can be moved forward or back. The side rollers sit onto the wooden 'stringers', which are strong - previously it had rollers that ran parallel to the direction of the boat, and were onto the GRP which pushed it up, which created stress.

Remember that a 125 in particular isn't a big bike and if their COG is like my old one- its pretty much in the center (or even slightly rear biased)- so really, even a 30cm 'rear overhang' behind the axle would be more than ample- basically the bikes would just be sitting right at the back (which is actually a good thing- the weight should basically be right on the back axle, or just slightly forward from it)

Going from that picture- you have more than ample rear overhang already to do the job (especially as you have such a long drawbar in front of it, that will lessen the effect of moving the bikes weight slightly forward over a shorter one- as contradictory as that sounds)

 

[Dabop]

For those who might be thinking 'why would a long drawbar make it LESS of an effect- a long lever makes it have MORE mechanical advantage right???'

Another quick sketch lol

Remember that a trailer has THREE points of support (for a single axle trailer)- the two tyres, and the hitch itself...

In the top two, the bike moves forward say 30cm, on a short 1m drawbar- you have effectively moved the weight forward 30% of the total drawbar length...

In the bottom two, the bike moves forward the same 30cm, but this time on a say 4m drawbar- thats only moved forward 7.5% of the total length, and so the weight increase as a proportion of the total length is much reduced- to get the same proportional increase, the bike would have to move WAAAY forward ie 1/3 of the way along the trailer towards the front or over a metre further forward...

(thats why 'short drawbar' trailers are so sensitive to load placement, and why so many newer box trailers are coming out with much longer drawbars than the older ones did... rather than the '3ft' drawbar often seen on the older ones, new ones may be as long as the body of the trailer itself!!!)

As opposed to this which is more common in the older trailers...

 

[Chris152]

I was quite happy to take your word for it, Dabop