Noob frame builder here. I’m building a v2 of my electric cargo scooter pictured below (the main frame wasn’t built by me but by the awesome @colinreay).
I realize this forum is mostly about bikes but I’m hoping electric scooters that blur the line between bikes and motorcycles might be okay
The main problem my v2 needs to address is lateral rigidity. This is generally a nuisance with most cargo bikes that people just deal with. Its a slightly bigger problem with my bike since this thing is designed to go fast (25mph) on very very bumpy SF bay area roads. With kids on the rear rack (~120lbs) I can basically send waves down this frame by shaking the front a little.
Right now the bike has a familiar planar bicycle frame. What are my options? Frame weight currently is around 20lbs with the rear subframe - am okay with increasing it to 40lbs.
Simply use wider tubing (its 1-3/8 for most of the frame tubes right now, 1" for the sub frame)
Different geometry / design while staying planar to resist torsion / flex
Lighter version of a perimeter / double spar moto frame.
Something else?
If there is some material I can read about testing for flex in solidworks FEA that’ll be great - how do you guys who make cargo bikes deal with this?
Front wheel missing from the CAD. I use a double crown DH fork to make the front end more rigid:
Frame members should be as wide as your knees will allow. Either giant round, or maybe more practical, ovalized, or even better, a space-frame.
Oh right you don’t pedal, so just make it super-wide, maybe with padded knee rests.
If I see that on a bike path I will yell at you, “no motorcycles!” I’m grudgingly accepting e-assist or pedelecs on bike paths and MUPs, but anything with a throttle that lets you pass me without pedaling at all evokes fantasies of my pump in your spokes. That reminds me, gotta start work on my hardened Cr-Mo pump…
Kidding obv, anything that replaces a car trip is fine with me.
Without kids I ride regular bikes so i hear you I used to have a electric assist Yuba for the kids but you quickly realize that on flat ground you are simply providing at max 10% of the energy and on hills you are putting your kids at risk by crawling along the road annoying cars. So I chose to remove the driveline, drop the COG by like a foot and make it safer for them on the roads. Mostly I am on road battling cars going 45mph on narrow roads
Where do you get ovalized chromoly tubes from?
By space frame do you mean like a motorcycle frame? Trying to avoid that since I don’t want it to look intimidating but maybe there’s no option.
my understanding, is that part of the reason the rear triangle of a bicycle works so efficiently (in terms of the problem you’re having) is that a) the seat stays make a nice triangle in the plane you’re worried about, so the rear axle can’t simply parallelogram over one way or the other; it has to twist the seat tube ( which is very had to do, and the seat tube is a really responsive torsion spring) and even then it pivots rather than strafes… and b) the rear wheel assembly weight isn’t really that substantial.
I might be wrong, but it doesn’t look like anything that supports that rear-seat-combo is triangulated in the plane that would resist it swinging left to right (it can simply ‘parallelogram’ by flexing at the joins right?) , and the weight of it (once there’s a kid or three in/on there) is probably quite a bit.
so when the bike tilts or corners, the structure is encouraged to flex ALOT by the kids flopping about, and there’s really nothing there to effectively resist that.
TLDR; put a triangle on it laterally.
NB; if you hunt around bike and especially motorbike communities, there’s a bunch of well know anecdotes that making things stiffer actually made them worse, ( that one fire-blade? anyone? ) but if its already so bad its making you consider a redesign I think you’re probably safe to weld some struts in there and see what it does.
edit: when I say “parallelogram” I mean to flex left/right without any of the members needing to get longer or shorter, like a cardboard box when you open the top & bottom.
also, a more general thought is to double-check you’re not hitting bumps big enough to momentarily go negative trail on that thing once its sagged; its probably just the photo/angle but it looks like quite a steep steering angle for the fork offset, and wheel size.
ive seen a few of cargo bikes (both home made and factory) reliably get the big-scary-wobbles in swoopy downhill corners or over bumps, which in the end needed Angle-sets.
I had the same question and Stephen Wood kindly offered this picture of his ovalizing process. Smashing the tube from side to side, bit by bit in the vise, keeping it between the pieces of wood to keep the phase.
Lateral triangles are the key, as @crowe-molybdenum said. A lot of production bikes don’t do a good job here and make the rear racks heavy instead – including popular ones like my wife’s Tern GSD. Look at Tubus rear racks for a good example of how to use lateral triangles to get stiffness, and just extend that down the length of your rear rack. Here is a picture showing a v1 Tubus Cargo from the back and you can see how good the lateral triangulation is:
In comparison most rack designs have completely vertical frame members with a lid on top, and without the triangles they are counting completely on tubing stiffness to avoid lateral loads.
I replaced the stock rack on my Benno mid-tail with a custom one and it both saved weight (my steel rack was about 60% of the weight of the stock aluminum one) and increased lateral stiffness through the used of triangles. Benno’s stock rack tried to have lateral triangles, but they provided too much tire clearance (more than the frame) and the triangles weren’t wide enough to be effective.
I disagree with you on the effectiveness of pedelec cargo bikes. At bike speeds (15-18mph – at/over the speed limit of MUPs here) my motor is consuming about 100W (so output is more like 60-70W) on flat ground. On steep hills the consumption spikes and motor power consumption goes above 1000W (output at the wheel again being lower), but maxes out at about 3X my human output. My school commute with the kids has a few blocks of a 10% grade hill and I’m carrying 120-130lbs of kids and kid gear on the bike (plus me at ~200lbs and the bike at ~70lbs). We go up that hill at ~10mph (slower is fine, but not very efficient on my direct drive Grin All-Axle hub – it looks like you are using the same motors).
The rear suspension of your design makes it trickier to get the lateral triangulation because you need to leave room for the suspension rear triangle. Have you considered making a rigid rear end, it would make this a lot easier? It would also remove any flop in the pivot bushing which also increase lateral flex.
Thanks for taking the time to explain this. To understand - what you are saying is that it can move sideways since there are no cross braces? I should’ve given you a better CAD angle in which there is a lateral brace on the top. A lateral brace at the bottom part would ofcourse help but it would be in the rear wheels travel path.This might help give you a better understanding:
Is the fix to make that center brace into a sort of X that’ll resists parallelogram-ing?
Yes this is definitely an issue - the frame isn’t made for this much travel at all. And yes headshake is definitely an issue due to this. This is another thing I intend to fix in v2 - have a more relaxed trail. Initially I did think that’s the only problem to fix - but the wiggle happens even on totally flat ground.
Gotcha - thanks this is really good reference. So what you are saying is that this angled piece from dropout to the top cross brace gives these racks rigidity:
I think this is the main thing for me safety wise and is very dependent on where you live. In Oakland I am climbing single lane mountain roads with pot holes and 30mph traffic. Doing this will cause a pileup of angry drivers behind me - and angry driverss are dangerous. I’m using the same grin all axle motors - just 2x of them . You should be able to go a lot faster if you add a throttle (and want to!). These motors are beasts and don’t cook till past 90C
Yes - but that’s the big reason why this custom build exists. I want a nice comfortable and safe ride at 25mph. You definitely need suspension at that speed around where I am (norcal) or it gets old quick.
Yes, that’s right about the bracing. The triangles formed by it are being compressed or stretched when the rack is twisted, and triangles are good at resisting distortion.
If your swingarm prevents good triangulation then I’d try corner gussets and larger diameter tubing for the rack.
I think what you are building is more of a motorcycle than a bicycle. There is nothing wrong with that, I think electric motorcycles are cool. But I do think they deserve a different class of hardware, especially braking systems. Even the best e-bike brakes aren’t designed for 400+lb vehicles going over 30mph. Neither are most suspension forks.
The All-Axle has plenty of power but is inefficient and has low torque at bicycle climbing speeds. If I start a steep hill at 20mph then it can sustain it, but it has basically no acceleration on steep hills.
The way that your rear rack and frame integrate also looks poor for stiffness. I’d look at making a triangle from the outer front corners of the rack to the back of the seat tube.
This bike actually weighs around 70lbs which is in-line with most cargos and less than many RM bikes. 400lbs is a very heavy super bike Definitely needs good brakes though - luckily 85% of the braking is done by regen on the All axle. Fork stiffness is definitely a concern - but most ebike/cargo forks (like the suntour cargo your tern is using) are okay for upto 300lbs (usually the fork will only see 50-40% of the total weight).
Its a ultra-light motorcycle - technically a electric scooter as far as California is concerned. My idea is to blur the line between ebikes and scooters (which is already quite blurry). I personally think the future of urban mobility is going to be ultra-light scooters like this in the US as batteries/motors gets lighter.
What motor controller / wheel size are you on? I’m on 20" wheels with the slow wound all axle and at max amps (40A @ 72V) I can start on 10% grade no problem and cruise upto 20mph. The motor can produce around 96N.m which is a decent amount of torque specially for a small wheel. The motor will heat up after 30 minutes of this though. Yes its inefficient when doing this but it’ll definitely work.
Yeah that’s a much better idea - also maybe weld it instead of bolting it which is what’s happening now.
I meant 400lbs with bike, rider and passengers. A guess but probably in the ballpark.
I’ve ridden the All-Axle on 20” wheels and it has way more torque due to the shorter lever arm. I have it on a Benno Carry-On with 24” wheels and the torque feels very different. I have a PhaseRunner controller.
Our Tern is a 1st gen and doesn’t have a suspension fork. I’m glad for that, those Suntour Mobie forks are crazy heavy.
Ah sorry I misunderstood the weight thing. I have given thought to using motorcycle / custom front forks but seems like tern did the work for me - glad to hear they are super heavy that means they are stiff
Hmm still shouldn’t be too bad with 24” wheels. Can help debug it if you want.
