turbo1889

Okay, I’ve had this thought for a while now and just found this forum a couple days ago and thought I’d ask for some thoughts critical analysis on this idea:



Basically, take an older 90’s era name brand steel frame mountain bike and do some major re-arranging of the tail end. Two smaller size BMX free style heavy duty wheels mounted low with a cargo rack built around them. Both rear wheels driven by the pedals with the forward of the two wheels having a gear spool that is one gear more then the shifter/derailer is set-up for (example: 7 gear spool with 6 gear shifter/drailer) with the same spool on the rear wheel and a slave chain on the extra (7th in the example) gear running back to the same gear on the second drive wheel. 2x3 drive traction with two wheels in the rear to provide two axles to bear the cargo load instead of just one. Not to mention the uniqueness factor.

What do you guys who have experience chopping and splicing think of the concept? Comments, suggestions, critical analysis, etc . . .

NoBrakeNate

I think you could make it work. I do like your idea about the derailer action, you would have to limit it to a 5 speed because with a chain on the 7th gear a chain wont fit on the 6th.
Or you could build off a Fixed/Free Flip flop hub and have all your gears on one side and use the fixed side for your slave chain, add horizontal drops for the rear wheel to tension the chain.
I can see in your pic that the two rears mount to a triangle and then that mounts to the frame, good thinking for bumps and such.
Then the only issue would be tight turns, one of the rears would have to skid, (like a 4x4 truck making tight turns). How tight would the turn have to be I wonder.

I say you should build it for sure.

Doug5150

It should be possible to build it (especially if you only try to drive the center wheel) but I think it would have issues with poor steering, because the two rear wheels are going to scrub laterally when turning. -That is, if it will even turn at all....


If I had to build a cargo bike like this for someone and wanted it to NEVER fail, I would use a single small car or motorcycle wheel & tire for the back end.

catmandew52

I remember seeing a 3wheel in-line cargo bicyle a looooooong time ago, but the third wheel was just a front fork in a headstock trailing the drive wheel.
check these out:
https://www.youtube.com/watch?v=W7DC4aJEBiA
https://www.cyclorama.net/viewArticle.php?id=354
https://www.youtube.com/watch?v=CJFw2La-m-o

Somehow you gotta be able to turn.
As a proof of concept go get 2 pcs of perforated angle stock(HomeDepot, Lowes, L HDW), long enough to go from trailing 3rd wheel axle bolts to rear axle bolts or even chainstays of drive bike, clamp it together with U-bolts and see if you can ride it. You might want to borrow some bikes and another front wheel from someone who wont miss them, JIC.

xenologer

What happens when you go over a speed bump?

or in a less extreme example, when the ground is not a pefectly smooth plane and not all 3 wheels can touch at the same time? ie, curbs, potholes, hills, -pretty much every place outside of an airport tarmac

is the entire cargo section with the two wheels supposed to pivot vertically to account for such situations? what would such a mechanism do to your weight distrubution? how would the drivetrain work around it?

Doug5150

I assumed the two wheels were mounted on a bogey. If that were the case, then the pair of rear wheels has a common pivot point, allowing them to separately ride up & down small obstructions.
Here is an example of tank treads using them (rather common)-
https://www.williammaloney.com/Aviati...ogieWheels.htm

It still leaves the steering problem though.

xenologer

If they are to be mounted on a bogey, why not take the bogey's pivot point, replace that with a single axel for one heavy built wheel? From the cargo's perspective the weight distribution would be identical (all the support at the one point). and this simplifies the drivetrain.

turbo1889

Not quite identical for the weight distribution, since I was thinking to build the cargo rack on the "bogey" so the only load that the pivot point would carry would be the rear portion of the rider’s weight and the weight of the frame itself. Which was sort of the whole idea anyway with all the cargo weight resting on to rear two axels which means the main frame doesn’t have to take the cargo weight load.

Don’t think its going to practical at this point though due to the skid problem previously discussed. First I built a “concept model” digging up an old set of those special Technic Legos that have gears and such:





Obviously, not exact by any means but good enough to check how much the drag would affect the steering. Long story short the more weight in the cargo area (on each side of the rear wheels with the weight being applied to the floating rear wheels assembly) the more the front wheel wants to skid and refuse to turn the bike rather then the rear wheels skidding ~ not good for a cargo bike.

After that I pulled up my CAD program and did some steering geometry analysis and figured out that with using the rough and dirty quick figures of an over all 6 foot distance between the front axle and most rearward axle with the middle axle two foot forward of the rear axle (a 4’ / 2’ total of 6’ spacing set-up) that there is a way to set up the geometry so that the bike can easily make turns as tight as five foot radius without any of the wheels skidding and make the rear wheels practically track exactly to follow the front wheels path. This would involve allowing the rear bogey assembly to pivot like a steering wheel under the rear of the frame and linking that pivot point to steer the rear wheel on the bogey a small percentage of the angular deflection between the bogey and the frame with that percentage being a function of the ratio of the wheel base of the bogey compared to the over all wheel base. The linkage itself wouldn’t be that hard, unfortunately, the same can not be said for power transfer from the pedals to the rear bogey which such a set-up would turn into a nightmare.

Long story short, I think the idea is pretty much dead at this point and a more conventional long tail two wheel set-up is in order although using a smaller diameter wheel in the rear to lower the center of gravity of the cargo is still probably a good idea.