# Single wheel, suspended kids trailer?



## Steve71 (Mar 15, 2004)

Guys, I'm looking for some feedback on this project. Basically I'd like to design a trailer similar in function to the tout terrain single trailer (see links below). Too ambitious for an inexperienced welder?

http://www.tout-terrain.de/2/products/trailer---kids-bikes/singletrailer/index.html

In action






1) I have no welding skills, but I figure after building about 3-5 prototypes the final build's welds should be sound? Plausible?

FWIW all prototypes will be trashed and bashed with a dumy load of sand. No live pasangers until I've proven the trailer is 100% safe.

2) Cheaper lower grade steel for the proto's and practice welds - $100-200 for steel for 3-5 protos seem like a good estimate? Another $100-200 for the final build with higher quality co-mo steel?

3) Welder - $300-$500 used of Craigs list? What type OA, TIG? I would like to learn from books as attending a class would be a PITA - l'm a stay at home dad and I live over an hour from a major city, however I usually get a 2 hour break each day I could devote to this project. Any book suggestions?

4) Tube bending? Tight bends (12" dia) plausible with a cheap harbor freight tube bender?

5) Fittings? This will be mounted via the seatpost so I'll need to source a suitable seatpost clamp and ball joint or pivot system with suitable range of motion, load capacity, weight and material. Where does one source this stuff?

6) Jig - I guess alignment is far less important in a trailer than a frame, however the damn thing still has to track straight. The protective 'cage' just needs to look OK and be structurally sound. Does it seem feasible to build a straight trailer chassis with a home made jig?

7) Target weight would be under 25lb with air shock and wheel - seem realistic?

FWIW the singletrailer is over $3K, two of them would be $6K! That is why I just don't save myself the trouble and buy one/two.


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## rustola (Jan 15, 2008)

Steve71 said:


> FWIW the singletrailer is over $3K, two of them would be $6K! That is why I just don't save myself the trouble and buy one/two.


If the main motivation here is saving some money, I'd say you'd be better off buying one and saving yourself a LOT of time and frustration. This is an ambitious project, both in design and fabrication.

Sorry to rain on your parade - it looks really cool and I'm all for DIY ingenuity, but it sounds like you're not super aware of the scope of what you're proposing.


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## Steve71 (Mar 15, 2004)

rustola said:


> If the main motivation here is saving some money, I'd say you'd be better off buying one and saving yourself a LOT of time and frustration. This is an ambitious project, both in design and fabrication.
> 
> Sorry to rain on your parade - it looks really cool and I'm all for DIY ingenuity, but it sounds like you're not super aware of the scope of what you're proposing.


Thanks for the input rustola. If they were cheaper, say $1K each they I'd just buy one/two, but come-on $3K is simply ludicrous.

Design shouldn't be too much of a problem, the suspension doesn't have any braking/pedaling inputs to worry about, so a simple trailing arm will work. Just need the ideal leverage rate and I can always get a custom tune at Push. I'm an EE so I'm a very design orientated person. My brother is also an Aeronautical engineer, so I can always bother him if need be. Besides I have a number of ideas that would be (hopefully) a significant improvement over the singletrailer.

But when it comes to the actual building part I'm at a loss. I know it's not going to be easy - I'd say 200-400 hours of building/designing. I could always work with ply wood until the design is finalized and then have Walt or DFW weld something up for me, but I'd honesty rather learn myself and doing the protos in steel would make things more interesting.

However, if the reality is that you need to spend 200-400 hours welding a specific steel before you have enough experience to trust your work, then I understand.


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## Yogii (Jun 5, 2008)

If welding is the problem, either find some else to do it. Ever think about using aluminum and just screwing it? You mentioned plywood, they make airplanes out of the stuff. You can glue it and screw it! Seems like with your background and skill set, steel is not real.

With your brother's background, why don't you make it out of CF and make something really neat....when you are done $3k will seem cheap.

How handy are you?


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## rustola (Jan 15, 2008)

My point is just that saving money shouldn't be the leading edge - but it seems to me you have some other motivations besides money:

1) interest in design
2) possible improvements to the product
3) interest in fabrication?

If so, awesome, go for it. But doing the math here on your own numbers ($3k, 200-400 hrs), you're talking about paying yourself between $7.50 and $15 an hour, before you've even spent a dime on tooling, raw materials, or components. Once you factor those things in you're talking about much less (half?). If you're an EE, I'm guessing you could earn more than that in a heartbeat. Heck, you can earn more than that at Starbuck's.

But it sounds like a cool project, and if you're into it and ready to shed some sweat, do it. 

As far as the reality of the metalwork, it largely depends on your background and you as a person. I teach TIG welding, and there's a lot of variation in my students on how much practice they'd need before starting to have confidence in a welded tubing joint. But on average it's probably more in the 50-100 hours of practice range. Helped *immensely* by personal supervision. Learning from a book alone is really really tough.


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## Linnaeus (May 17, 2009)

You can probably score a torch and some tanks cheaper than a TIG welder and just fillet braze it together. Either would work though. 

I would focus on just building proper joints for practice, and then make your finished product once you are proficient. That would save a lot of $$ and time. Copy the existing model you linked too as much as possible (except the single sided swing arm). If you *need* to make a prototype, do it with wood, or better yet, CAD.


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## Yogii (Jun 5, 2008)

Where did you get the $3k figure from?


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## Yogii (Jun 5, 2008)

Why do you need 2?


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## Blaster1200 (Feb 20, 2004)

Since this is generally a mountain bike forum, I'm guessing the objective is to take your child offroad in the trailer once it's complete. I don't know about you and what you think of your child, but I'm a little reluctant to put a child that close to the dust being kicked up by my rear tire. This brings me to my next point. What are you going to do about the canopy? The two-wheel child carriers I've seen are relatively wide with a clear windshield. The sides open up for air, although I'm sure it lets in a lot of dusty air. Have you thought about your cockpit design, too?

Edit: I just watched the youtube video with the EMPTY trailer. Even they're smart enought not to ride like that with a child in the trailer.


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## Steve71 (Mar 15, 2004)

Yogii said:


> If welding is the problem, either find some else to do it. Ever think about using aluminum and just screwing it? You mentioned plywood, they make airplanes out of the stuff. You can glue it and screw it! Seems like with your background and skill set, steel is not real.
> 
> With your brother's background, why don't you make it out of CF and make something really neat....when you are done $3k will seem cheap.
> 
> How handy are you?


I guess Alu could work for a proto, but I don't think it would be rigid enough long term. I think ply would be cheaper and easier though.

My brother lives in Australia and works on re-designing failed alu parts on fighter planes - no experience with CF.

And to answer your question, yes I believe I'm reasonably handy. I've designed and built a set of PC sim racing pedals out of alu. I've also designed and built some huge horn loaded speakers and sub out of MDF and ply. Best of all I enjoy building & designing stuff.


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## Steve71 (Mar 15, 2004)

rustola said:


> But doing the math here on your own numbers ($3k, 200-400 hrs), you're talking about paying yourself between $7.50 and $15 an hour, before you've even spent a dime on tooling, raw materials, or components. Once you factor those things in you're talking about much less (half?). If you're an EE, I'm guessing you could earn more than that in a heartbeat. Heck, you can earn more than that at Starbuck's.
> 
> But it sounds like a cool project, and if you're into it and ready to shed some sweat, do it.
> 
> As far as the reality of the metalwork, it largely depends on your background and you as a person. I teach TIG welding, and there's a lot of variation in my students on how much practice they'd need before starting to have confidence in a welded tubing joint. But on average it's probably more in the 50-100 hours of practice range. Helped *immensely* by personal supervision. Learning from a book alone is really really tough.


Yeah I hear you.. once you factor in your time, DIY doesn't look so good. But I'm a stay at home dad, so while I can go down to the garage when my daughter is asleep, I can't really leave her and go work at Starbucks for two hours. 

50-100 hours is a lot of practice though. I think my best bet is to do a proof of concept out of ply and then re access things. It should only take a couple of week-ends to come up with something.


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## Steve71 (Mar 15, 2004)

Linnaeus said:


> You can probably score a torch and some tanks cheaper than a TIG welder and just fillet braze it together. Either would work though.
> 
> I would focus on just building proper joints for practice, and then make your finished product once you are proficient. That would save a lot of $$ and time. Copy the existing model you linked too as much as possible (except the single sided swing arm). If you *need* to make a prototype, do it with wood, or better yet, CAD.


I would need lugs to fillet braze, right? Do lugs come in all shapes, sizes and angles? I would have though using lugs would be very restrictive, but I'd love to be wrong about that.


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## Steve71 (Mar 15, 2004)

Yogii said:


> Where did you get the $3k figure from?


Their are one or two people on MTBR that have them, they said the cost was $2500-$3000.

I would need two when we have a second child. My wife rides and she can tow the little one.


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## Steve71 (Mar 15, 2004)

Blaster1200 said:


> Since this is generally a mountain bike forum, I'm guessing the objective is to take your child offroad in the trailer once it's complete. I don't know about you and what you think of your child, but I'm a little reluctant to put a child that close to the dust being kicked up by my rear tire. This brings me to my next point. What are you going to do about the canopy? The two-wheel child carriers I've seen are relatively wide with a clear windshield. The sides open up for air, although I'm sure it lets in a lot of dusty air. Have you thought about your cockpit design, too?
> 
> Edit: I just watched the youtube video with the EMPTY trailer. Even they're smart enought not to ride like that with a child in the trailer.


My design would have the child rear facing so no worries about dust and stones etc.

The trailer chassis would have a strong roll cage, and use fabric to offer protection just like the zillion other trailers out there. The child would be held in place with a 5 point harness.

FWIW the singletrailer has around 8" of travel that is tuned very softly and (supposedly) can take a curb at 20mph without harm to the passenger or trailer.

While I love the more full-on steep and techy trail we have in CO, I also live near a lot of mellow trails (that I've been avoiding for years  ) that would be great to ride with a trailer.

Some singletrailer action pics in this thread.

http://forums.mtbr.com/showthread.php?t=371309


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## smdubovsky (Apr 27, 2007)

fillet brazing requires no lugs (thus the 'fillet' part. (pronounced fill-it *not* fill-A for this application.)) If you consider yourself handy, go find a used oxy torch setup off craigslist and have at it. This stuff isn't rocket science. Getting beautiful results w/ little/no post-rework takes *LOADS* of practice but just putting something together reasonably straight that will hold together is far easier.


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## Steve71 (Mar 15, 2004)

smdubovsky said:


> fillet brazing requires no lugs (thus the 'fillet' part. (pronounced fill-it *not* fill-A for this application.)) If you consider yourself handy, go find a used oxy torch setup off craigslist and have at it. This stuff isn't rocket science. Getting beautiful results w/ little/no post-rework takes *LOADS* of practice but just putting something together reasonably straight that will hold together is far easier.


Thanks man, that's what I needed to hear! It doesn't need to look super pretty, just (as you said) be reasonably straight and hold together!

I still think I'll do one proof of concept out of ply and take it from there.


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## Smokebikes (Feb 2, 2008)

Here is a great resouce for what you may want to build it out of............you may consider square tubing since it is a little easier to work with as far as cutting and such. Also, another option for this project could be a wire fed welder, if you know someone who would loan it to you they are quite easy to use on thicker materials (not bike tubing). Lastly, I just gotta say be careful carting around precious cargo...........crashing should not be an option. Good luck! :thumbsup: 
http://www.aircraftspruce.com/menus/me/steel.html


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## dr.welby (Jan 6, 2004)

I can't believe no one has said it yet, but this is the perfect opportunity to go to your local community college and take their intro to metalworking and welding classes.

You'll learn how to at least MIG, gas-weld, and fillet braze, you'll have access to experts, every college has a big material bin that you are entitled to use with your lab fees, and they will have all the little tools and geegaws that you don't realize you need yet. And of course they have the big ones that cost more than the $3k for one trailer.

You can build your trailer as your final project, and many colleges are pretty liberal about "open shop" time.

Some general design thoughts:

The trailer looks like it has two main parts - a spine and a roll cage. For building the spine you'd probably want something like a "keel jig" - search for motorcycle frame jigs and you'll see how they do it. I'd build something like that out of angle iron to keep the spine and pivot aligned.

The roll cage portion doesn't need much precision and since it's smaller tubing you can easily bend it to fit. I'd use MDF as needed for jigging sub-assemblies. You can draw out your centerlines on a sheet and just screw down little blocks on the sides of where the tubing goes.

If you smartly overdesign the tubing dimensions and are a new welder, your joints may be the design's achilles heel. When you design it, take time to think "what will happen if this joint breaks?" You may wish to avoid joints when possible - for example the spine looks like it could (and maybe is) made from one long piece of tubing with several bends. If you go this route your local muffler shop might be able to do the bends for you if your design matches their smallest dies.


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## Steve71 (Mar 15, 2004)

Smokebikes said:


> Here is a great resouce for what you may want to build it out of............you may consider square tubing since it is a little easier to work with as far as cutting and such. Also, another option for this project could be a wire fed welder, if you know someone who would loan it to you they are quite easy to use on thicker materials (not bike tubing). Lastly, I just gotta say be careful carting around precious cargo...........crashing should not be an option. Good luck! :thumbsup:
> http://www.aircraftspruce.com/menus/me/steel.html


Thanks for the link. I figured that square tubing was my only option.

The only time I ever crash is DHing at the resorts. The more mellow XC trails + trailer shouldn't be an issue.

It will be fun testing the handing/dropping/jumping limits with a dummy load. 

I'll look into the wire fed welder.


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## Steve71 (Mar 15, 2004)

dr.welby said:


> I can't believe no one has said it yet, but this is the perfect opportunity to go to your local community college and take their intro to metalworking and welding classes.
> 
> You'll learn how to at least MIG, gas-weld, and fillet braze, you'll have access to experts, every college has a big material bin that you are entitled to use with your lab fees, and they will have all the little tools and geegaws that you don't realize you need yet. And of course they have the big ones that cost more than the $3k for one trailer.
> 
> ...


Thanks for all that useful info dr.welby! A fail safe design where possible is a good idea.

I mentioned in my OP that a welding class isn't really an option due to my schedule and location. However if it's that or nothing I guess I could look into a week-end class.

Once again thanks to everyone for the help, much appreciated!


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## puncturerepairkit (Aug 18, 2007)

Edit: My mistake, didn't realise this was a necro.


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## mickuk (Jul 6, 2007)

I went through this loop about 7 years ago - ended up making a long wheelbase / low c of g / full sus kiddy carrying bike. Did plenty of singletrack with it including the rideout at SSWC (2007 I think). There is a picture here that I can't link to. http://www.atomiczombie.com/gallery/tandems2.html

Might also be worth considering an axle trailer attachment like a BOB Yak (it then leans better with the bike, lower c of g and braking on downhills will not tend to lift the back wheel of towing bike like seatpost mount does). You could buy a second hand YAK copy and use the axle mount hitch and chassis etc. Then do your own seat, cage and larger rear wheel / suspension etc.

I don't really care that mtncullen may / may not have an association with Cavercab trailers - it is something new that I've never seen before and hardly big business - plenty of framebuilding pros use this forum to show off their products.


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## patpend2000 (May 11, 2004)

How about cannibalizing the rear end off of a full susp bike, I'm thinking an older cannondale super v. simple single pivot design, would include an air shock, you could then design the trailer part around the swingarm.


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## Walt (Jan 23, 2004)

Thread resurrection - did you ever build this? I am considering a similar project.

-Walt


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## mickuk (Jul 6, 2007)

Hi Walt

Is this a project for Junior? If so, the important bit is to finish before they grow too big 

I _might_ have a friend who could help with an outer covering (very tough microwave welded PVC with zips, windows etc) - pm me if you want me to investigate further.

My suspended non-trailer creation ended up a bit rushed and ratty because of rapidly growing child deadline (I really wanted to do a suspension trailer but it ended up quicker just to modify an existing scrap frame with suspension, small back wheel and stretched wheelbase). We went some crazy places with that bike.....


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## calstar (Mar 26, 2013)

very cool pics. looks like the little guy already has back seat driving down, pointing the way in the second pic.

Brian


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## TroyS600 (Mar 29, 2007)

Mickuk, 
Way late here, but do you have more pics of that rig? That's pretty cool! I've been trying to figure out how to get the kiddo out comfortably on some singletrack.


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## mickuk (Jul 6, 2007)

I really wouldn't do a direct copy - I was working around an ancient old frame that was way too small, 1" head tube, elastomer fork etc etc.

A modern version with a good fork, disc brakes, wide range 1x10 with clutch mech, platform shock etc would be awesome.

The 2 stage drivetrain / rear swingarm on bb shell pivot was done because I already had the rear triangle from a folding bike project (I had less frame-build experience back then and a finite timeframe to get running before kids grew too big!). It did however work quite well (no issues with chain forces / bob).

Doing it today, I'd use a much simpler swing arm and drivetrain, probably with some kind of mid point idler guide to control the chain from flapping and run the chain forces through the pivot point.

The main bit to get right is the kid seat position. There are trade offs between how low you run the seat, wheelbase, heel clearance, c of g position etc. The attached crappy CAD drawing shows an approximate starting point for you to work from.

Best of luck - show us what you build! If you want some detail photos then PM me (passenger is now 11, riding his own custom 26" hardtail so the suspension frame is gathering dust in the attic)


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