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I have a cheap 20" folding bicycle. Its specs say that the max rider weight is 105kg. The frame is steel (vaguely defined as "HiTen" steel).

I want to convert it to electric, so will be adding approximately 12kg more (mid motor, battery, etc.). I also want to carry panniers, let's say another 20 more kg. My weight is 100kg. So with some reserve, the bike needs to carry approximately 150kg of weight.

I assume that the weak point will not be the steel frame itself, but some components, like bearings, rim material, spokes and so on...

The question is: what components are recommended to change (in general) to get a more sturdy folding bike - i.e. which components have the greatest impact on the load capacity of the bicycle?

I assume that the exact type of bicycle is irrelevant, but let's say something like this.

Edit

Just found this article about the stress analysis of the folding frames. Really interesting reading. https://www.irjet.net/archives/V6/i10/IRJET-V6I10207.pdf

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    "I assume that the weak point will not be the steel frame itself, but some components, like: bearings, rim material, spokes and so on..." - not a safe assumption Mar 10 at 7:18
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    @AndyP if it was a gas-pipe diamond frame the OP would be right - those frames are bomb-proof. So the OP's assumption is wrong in the sense of inapplicable, rather than in the sense of absurd
    – Chris H
    Mar 10 at 9:08
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    Do you need it to fold still? If not, consider selling the folder and getting a frame without a fold mechanism.
    – Criggie
    Mar 10 at 9:41
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    @Criggie yeah, must be a folder. Need carry it many times in trains or buses and/or even when hitchhiking. :D
    – jm666
    Mar 10 at 14:00
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    @jm666 sounds like your intended build is to be an electric motorbike more than an electrically assisted pedal bike. Remember the pedals are the main power, and the motor is just there to help. If you intend the motor to do all the work while you just sit still, then it needs a lot more battery, cooling consideration, duty cycle rating, etc.
    – Criggie
    Mar 10 at 18:01
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tl;dr: if you want a sturdy folding ebike, you need to have a sturdy folding bike.


Even if the bicycle statically/dinamically can stand more than the declared weight (at your own risk), the (expected) tolerance on the various parts and the quality of all the parts (both in terms of absolute quality and relative quality, relative to quality control) will result in a continuous failure of parts, such as:

  • excessive stress to brakes, that then may results in pooor braking conditions or in the rims being rapidly consumed;
  • chain, cassette, sprockets, all performing poorly, with the only availbale gear being the smallest one, so when battery is dead ... better be ready to push;
  • hubs, spokes starting to demonstrate helio- and hub-centrism is only a zero-order approximation of real life physics;
  • Unexpected combo of failure, like the motor pushing the chain, which does not engage, caught by surprise you brake, braking make the front rim toast, you fall head on (disclaimer: I am not judging you nor advocating for compulsory helmet use)

Cheap, reliable, light, worry-free. Pick two, if you want a motor: pick one.

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  • Okay - so, if I change on the bike the things you mentioned e.g. 1) wheels(spokes, rims, hubs), 2) cassette 3) derealluier 4) chains - in theory could get a bike with 150kg load capacity? How someone can "judge" the real quality of the (steel) frame itself? (Asking because myself saw many products (even highly priced) what was a piece of overpriced junk.)
    – jm666
    Mar 10 at 15:08
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    @jm666 In theory yes ... regarding a folding bike you have always the weakest point: the hinge. How to judge quality: it is a difficult call, I agree on the price not correlating linearly to quality (my opinion: cheap things are still too expensive for their quality, expensive things are often too expensive for the quality improvement over a moderately priced bicycle). Unfortunately the best tool to check for quality is time: I guess by scouring the internet you can find enough report about which folding bikes has been reliably used for long trips and you can extrapolate from there.
    – EarlGrey
    Mar 10 at 16:43
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    @jm666 and don't forget that if you invest in high quality components (especially the wheels), in case the frame fails you, you may be able to carry them to the next frame.
    – EarlGrey
    Mar 10 at 16:45
  • GOOD POINT about the components. Thanx.
    – jm666
    Mar 10 at 17:04
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On a non-folding bike, I'd look at the wheels, and probably replace them with 36 spoke touring wheels and reasonably wide, tough tyres (e.g. 35mm). The tyres absorb some of the shock loads.

However a folding bike has made some compromises affecting the structural strength, as well as introducing weak points. Two major ones are the main hinge and seat post. The latter is probably OK as your added load isn't on it, but the main hinge will be under added stress from the extra unsprung mass. It's probably not replaceable or serviceable, and obviously overloading the specifications would give the manufacturer an excuse to get out of warranty repair. The risk here is that the frame won't last as long as it should.

Adding a mid drive motor may not be as easy as you think - there's not much room given the need to not affect the folding, combined with the small wheels (short chainstays and reduced ground clearance). A kit for a different folder might help, but might be very specific in its mounting.

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    The motor looks promising, but that's a lot of power (750-1000W) to put into a potentially weak folder (and also illegal in many places, like the whole EU). It might be useful if you've got steep stuff, though it's more than you'll need on the flat, and applying that power from a smooth motor rigidly coupled to the drivetrain is a bit different to stomping on the pedals, which would flex the frame
    – Chris H
    Mar 10 at 9:03
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    I'm not worried about the steel itself, but the workmanship on the joins, and in particular the hinge. Even in the best folders this is a weak point, not helped by being near the heaviest load (rider, but a mid motor only adds to this, as presumably does the battery). Going over bumps in the road while sitting almost above the hinge puts a huge stress on it. A rider can unweight the saddle to reduce this, but a static load can't
    – Chris H
    Mar 10 at 14:28
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    1. What makes you think aluminum alloy is fragile? 2. The frame you've got, of "hi-ten" (high-tensile) steel, is the cheapest grade of steel found on bikes. The next grade up of steel that you typically see on bikes is 4130 CroMo steel.
    – Adam Rice
    Mar 10 at 16:13
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    This is absolutely the right plate for "More Spokes" - but 36 is rare on a 20" rim. I have a rear with 32 spokes, and the nipples are awkwardly close together. 36 would be tight.
    – Criggie
    Mar 10 at 17:53
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    @Criggie at least there's not much unsupported rim between the spokes on a small wheel, but in a 20" the stress of a shock load is distributed over less spoke length, which might make a difference
    – Chris H
    Mar 12 at 15:45
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My main concerns would be spokes in the short term; but wheels can easily be exchanged.

The second and graver concern is the frame. When I was riding about 100km per week, including some cobblestone and curbs, but nothing crazy, even "normal frames" (on your your standard average 500-1000$ bike) started to break; my weight then was around 80kg. (Anecdote: When I started to absorb the impact from going down a curb partly through the handlebar, instead of just through the legs, at some point the handlebar broke!) I never had such trouble when I was lighter; 80 kg seems to be some kind of threshold. I would be concerned about the longevity of my folding bike if I rode it a lot (e.g. an hour each day) with a body weight of 100 kg, no matter what the manual says — without any extras.

That said, you can mitigate the impact of the additional weight by putting the batteries directly over the back wheel, e.g. by putting them on or under a bike rack like in this image. That will not increase the load on your frame much, and neither will your banisters.

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    Good point - I simply need loss weight. :D Thats true - hopefully the long-range (over many weeks) trip will help also in this... :) :) :)
    – jm666
    Mar 10 at 17:13
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    @jm666 The less you use the motor, the more the weight comes off! That said, if the motor gets you using it, then that's certainly more exercise than not using it! :) Mar 10 at 19:14

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