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How do you start building a custom lightweight e-bike? (Brompton)

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How do you start building a custom lightweight e-bike? (Brompton)

Old 07-13-22, 04:49 AM
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alex1234
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How do you start building a custom lightweight e-bike? (Brompton)

Hi!

I am a total noob in this field. I have seen many inspiring posts on custom Brompton builds and I am thinking of building a custom Brompton bike with the aim to make it as lightweight as possible so that I can offset the e-bike conversion added weight.. That would mean that I would have to use mainly Titanium parts.

I know how to get the obvious main parts such as: titanium main frame, forks, seatpost, handle bar, crankset, wheelset, etc

However, for all the other tiny parts that are necessary to complete the build, how would one go about them? Would you just buy a used / broken brompton for parts and start from there? Or does it make sense to buy every single thing separately ? The second option sounds very hard to implement mainly because you'd need a complete catalog of every little component, plus instructions on how to plug everything together.

Any thoughts on how to start with such a project?

Thanks!
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Old 07-13-22, 07:37 AM
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To start with, realize that it usually does not make much sense to custom build something that factories make. Brompton now makes a titanium bike. Yes, it is expensive, but building your own clone will be expensive too. Looking at the Chinese titanium frames on AliExpress, their main frame diameter is sized wrong. They are the same diameter as a steel Brompton. This tells me they are either heavier than they need to be or more flexible than they should be. Finally, buying parts separately is the most expensive way to do it.

It would only make sense if you found a very cheap Brompton parts bike with a mangled main frame. When you are finished, it won't be that light or ride as well as a working Brompton
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Old 07-13-22, 07:41 AM
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Thanks, that's very useful info! As an alternative, I was thinking of buying a Brompton P Line (10kg) and trying to reduce the weight as much as possible.
How many kg do you think could be saved by replacing parts on the Brompton P Line? Do you think it would be possible to reach a final weight of 8kg or even 7.5kg ?
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Old 07-13-22, 08:12 AM
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The economics will not work out to start from scratch. The only reason to do so is to have a bike spec'ed with exactly what you want based on your personal taste or particular eccentricities to satisfy a particular goal. In a situation like this cost is often a secondary concern.

It's an e-bike. From a performance perspective, weight is essentially meaningless in the first place.

I see what you are doing here. You want portable transportation to extend the last mile public transport does not serve. Adding 7kg of motor kit ideally subtracting 7kg of bike...

You are working at cross purposes.

People often lighten Brompton's at high dollar cost because they are too heavy for public transport or for climbing many stairs. People electrify bikes at great weight expense to ease the work load of locomotion.

You want both...& it is indeed a worthy goal. But it is going to be a challenge even with infinity money & resources.

I can appreciate your desire & I wish you luck. My 100% leg powered Dahon weighs 15kg. Surely you can beat that.

Last edited by base2; 07-13-22 at 08:30 AM.
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Old 07-13-22, 08:21 AM
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Thanks for the reply! The reason why I need a lightweight bike is not for performance but for back health. I found that when I tried a Brompton Electric (around 17.5 kg) for a couple of days my back suffered whenever I had to pick the bike up (which happened more often than I expected). Especially when, due to hastiness, I didn't assume the proper position before lifting it. I also love the Brompton folded form factor that allows me to store it in trains, restaurants etc.
So, I believe it is possible to have a pretty lightweight electric Brompton, given that the P Line + the new Swytch Kit (2.6kg) would weight around 12.6kg. That's not too bad as a starting point (I know, the swytch kit is not as performant as the heavier Arcc or the original Brompton Electric, but would do the trick).



Originally Posted by base2 View Post
The economics will not work out to start from scratch. The only reason to do so is to have a bike spec'ed with exactly what you want based on your personal taste or particular eccentricities to satisfy a particular goal.

It's an e-bike. From a performance perspective, weight is essentially meaningless in the first place.

I see what you are doing here. You want portable transportation to extend the last mile public transport does not serve. Adding 7kg of motor kit ideally subtracting 7kg of bike...You are working at cross purposes.

People often lighten Brompton's because they are too heavy for public transport or for climbing many stairs. People electrify bikes to ease the work load of locomotion.

You want both.


I can appreciate your desire & I wish you luck. My 100% leg powered Dahon weighs 15kg. Surely you can beat that.
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Old 07-13-22, 08:38 AM
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Brompton just released the new P line electric. According to the email I received yesterday it is out and should be available to purchase.
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Old 07-13-22, 01:08 PM
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Originally Posted by Pahana View Post
Brompton just released the new P line electric. According to the email I received yesterday it is out and should be available to purchase.
Thanks for the reply. Yes, I have seen it but I still find it too heavy for my purposes. I'd rather go for a converted P Line, even if it's not up to the standards of the official P Line Electric, as I will be saving roughly 3 kg
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Old 07-13-22, 02:54 PM
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Have you considered a Carbo folding bike? Personally, I have no experience with ebikes.
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Old 07-13-22, 09:59 PM
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Originally Posted by alex1234 View Post
Thanks for the reply. Yes, I have seen it but I still find it too heavy for my purposes. I'd rather go for a converted P Line, even if it's not up to the standards of the official P Line Electric, as I will be saving roughly 3 kg
I'm going to trust that the engineers at brompton know better than you. Even if you discount the weight of the motor, bikes with motors need to be heavier in order to better withstand forces placed on the frame by the motor itself.
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Old 07-14-22, 04:32 AM
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Originally Posted by Schwinnsta View Post
Have you considered a Carbo folding bike? Personally, I have no experience with ebikes.
thanks! I have test driven it, and it is pretty good. However the light one (model x) is single speed only which is a deal breaker for me, and their model s which is multispeed weighs around 15.3kg which is a bit too heavy for me
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Old 07-14-22, 06:50 PM
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My suggestion is look over readily available options first and not just stick to Brompton.
There are plenty of folding or small form factor electric bikes from China (Taobao, AliExpress, etc ).
One of those might already suit your needs.
A Brompton, because of steel and parts is already over 10kg to start.
A Dahon/Tern/or clone 16" alu frame is already a lighter starting point.
Then you have that OLD, so you will need a motor wheel that fits that.


Just a random (ugly looking) example that I pull off a store here. (which obviously is a re-badged "something" from China )
DYNAMIC MINI 16 Electric Bicycle - LTA approved foldable e-bike | PAB (mobot.sg)

Or this off AliExpress.
ONAN Chinese 250W 36V 5.2AH Brushless Aluminum Alloy Mini Folding Electric Bike (alibaba.com)
(Which BTW, has suspension which is actually safer at high speed and small wheels when hitting bumps )
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Old 07-14-22, 07:55 PM
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Originally Posted by pinholecam View Post
Or this off AliExpress.
ONAN Chinese 250W 36V 5.2AH Brushless Aluminum Alloy Mini Folding Electric Bike (alibaba.com)
(Which BTW, has suspension which is actually safer at high speed and small wheels when hitting bumps )
I looked at something along that line that a friend got herself. As far as I could tell, the first time some essential proprietary component breaks, and there were quite a few of those, the adventure is going to be over.
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Old 07-15-22, 04:12 PM
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Originally Posted by alex1234 View Post
Thanks for the reply. Yes, I have seen it but I still find it too heavy for my purposes. I'd rather go for a converted P Line, even if it's not up to the standards of the official P Line Electric, as I will be saving roughly 3 kg
The Hummingbird is a featherweight electric folder,... It's expensive, but it might be what you're looking for.
​​​​​​https://hummingbirdbike.com/products/electric
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Old 07-17-22, 06:43 PM
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Originally Posted by 2_i View Post
I looked at something along that line that a friend got herself. As far as I could tell, the first time some essential proprietary component breaks, and there were quite a few of those, the adventure is going to be over.
Most dedicated designs will be in this sort of category. (ie. potentially hard to get broken part )
There are a few pluses (at least from my end of the pond),
1. There is usually a local seller who provides some level or service/replacement
2. They are kind of cheap so its not too much a pain to just get a new one

That said, I think something more reliable in part replacement will be an existing bike, modded with a electric motor. Something also the lines of a Bafang motor kit or something.
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Old 07-18-22, 02:23 AM
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Originally Posted by tds101 View Post
The Hummingbird is a featherweight electric folder,... It's expensive, but it might be what you're looking for.
​​​​​​https://hummingbirdbike.com/products/electric
Yes, its lightweight but its the Zehus integrated hub motor with a single speed. The E-assist is very limited (very small battery) and if you need to pedal, its a single speed which isn't efficient.

The Electric P-line is relatively heavy and heavier than some third party solutions mainly because it has a relatively big battery capacity.

I fully agree with Schwinnsta: the best solution for a lightweight Brompton is now the Brompton T-line and its possible to save some more weight on it by replacing the brake calipers and levers which are the same heavy ones as on all Brompton (Ridea has excellent lightweight brakes designed for the Brompton, its also possible to mount the Cane Creek Eebrake on the Brompton which are the best caliper brakes on the market, very lightweight and powerful).
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