Do Nano and Swytch electric upgrades threaten forks?

[ExBrit]

I was watching a YouTube video about the Brompton Electric and they mentioned that Brompton has strengthened the stem and front forks to allow for the extra torque from the 250W front motor. The Swytch and Nano upgrade kits have similarly powerful front motors but they are designed to be added to existing Bromptons without the strengthened forks. Does anyone think this is a potential issue?

 

[steveindenmark]

I have not read anything about them and I think Swytch has probably sold a lot more units than Brompton.

 

[cougie uk]

I don't see a huge risk. I'm not the heaviest rider and don't push the Brompton to the limit so there would be a lot of margin there.

 

[T4tomo]

Brompton forks are steel (or titanium), I think they are plenty strong enough.

 

[berlinonaut]

Brompton forks are steel (or titanium), I think they are plenty strong enough.

You should by all means NOT mount a motor to a Brompton Ti fork. I do know personally of one case where the ti-fork broke after a retrofit motor was mounted. It is simply not designed to take that force. On the P-Line (the current ti model) Brompton themselve do use a steel fork on the Electric version instead of the ti-fork on the bio-bike version.

 

[berlinonaut]

I was watching a YouTube video about the Brompton Electric and they mentioned that Brompton has strengthened the stem and front forks to allow for the extra torque from the 250W front motor. The Swytch and Nano upgrade kits have similarly powerful front motors but they are designed to be added to existing Bromptons without the strengthened forks. Does anyone think this is a potential issue?

Front wheel motors have been mounted to Bromptons since roughly the mid 2000s. I do not know of any steel fork that broke. So in practice it is probably safe (or a neglectable risc). Still, I heavily doubt that any of the kit-manufacturers has done stress tests or anything like that in a laboratory. They simply do a bet (and most customers are not aware of that). Worth noting that at least in Germany if you retrofit a motor to an existing bike the person or company who mounts the motor (not the company selling the kit) is from thereon legally considered to be the manufacturer of the whole bike and takes the full accountablity for everything, may it be motor-related or not. The original manufacurer (in this case Brompton) is freed from any obligations. Thus, if you mount a kit yourself, you are personally accountable for everything that happens. If you let a workshop do it it's the workshop. In case of trouble or an accident this can make a huge difference... I do not know how this is handled in the UK.
Brompton have on the Brompton Electric strengthened the forks (which is directly visible if you compare the forks of the Electric side by side with the non-Electric) and they have also strengthened the main frame. They will also have tested the frame to the various ISO-norms (as they have to) and there is probably a reason why they strenghtened it. Especially the original steel forks may and do work with a motor, but they were neither designed nor tested to take the forces that a motor applies.

 

[berlinonaut]

I have not read anything about them and I think Swytch has probably sold a lot more units than Brompton.

Swytch is a very low end kit and a mass seller. The kit has to be self mounted and they are a very sales driven company, not a technical driven company and not a quality driven company. I heavily doubt that they did care or look into the topic at all.

 

[bonzobanana]

Steel is the right material for a hub motor, typically the way a hub motor damages a fork is twisting within the dropouts but that is more an issue for softer/weaker aluminium or carbon fibre where it hasn't been strengthened for force to be applied in such a way. These little hub motors that go into Brompton ebike conversions I don't think are that powerful. Yes the forks are being used a little beyond their expected use but steel is a material that is very abuse-able.

 

[berlinonaut]

Steel is the right material for a hub motor, typically the way a hub motor damages a fork is twisting within the dropouts but that is more an issue for softer/weaker aluminium or carbon fibre where it hasn't been strengthened for force to be applied in such a way. These little hub motors that go into Brompton ebike conversions I don't think are that powerful. Yes the forks are being used a little beyond their expected use but steel is a material that is very abuse-able.

Maybe I am a bit German here but I do not consider this a valid statement and am also wondering on what foundation you think that hub Motors for a Brompton are not that "powerful". The Brompton fork is not designed to take any torque at all as on a human powered Brompton you would not have torque on the fork. You have a huge range of different motors from Crystalite direct drives to geared motors like the Tonxin models, that are used in most retrofit kits today. They do have a nominal power of "just" 200-250 Watts but can vastly go beyond that in certain situations. The amount of torque they produce is most of the time not even layed out in the datasheets of the motors.

Steel forks on the other hand are vastly different, too: From a massive fork of a BMX bike or a cargo bike over slim elegant models on Racebikes to cheaply build forks on bikes from a hardware store. There are dozends of different steels, each with their individual properties. There are well crafted and badly crafted forks. There are different fork designs. Just because the fork ist made of steel this means exactly nothing.

Do you have any experience with a retrofitted motor in a Brompton? Are you an engineer with experience in bicycle fork design and specification? Most retrofit motors just use the existing dropout to keep the motor's momentum unter control - clearly nothing that the Brompton dropouts were designed for. There is a reason why torque arms exist: https://ebikes.ca/product-info/grin-products/torque-arms.html
And on the other hand there is a reason why forks for disc brakes are stronger and differently designed from forks for rim brakes. There is way more to consider than just the dropouts as forces from a motor (or a disc brake) onto the fork are obviously completely different from no forces onto the fork...

So generalisations do not work here, neither with motors nor with forks. There are a lot of bikes with steel forks where I never ever would fit a motor.

Practice seems not to have shown massive issues - still this is a bet because probably no one ever really checked the Brompton forks regarding the use of a motor from a solid engineer's perspective including tests, measurements and reviewing desingns and materials. Apart from Brompton themselves - and they changed the design of their fork for the electric version. As this rises cost this says something: They probably wouldn't have done it w/o a reason. So it is probably a valid assumption that current retrofit motors typically do not overload the fork under normal circumstances (that's what the practice shows but no one takes warranties for that) but that on the other hand the fork is probably used beyond it's spec, technically within the stretched safety zone buffer and this seemed not to be safe enough for Brompton themselves (thus they designed a stronger fork for their Electric).

 

[berlinonaut]

Here you see i.e. the data from Grintech's motor simulator for two different hub motors on a Brompton (a Crystalite direct drive and a geared motor). Both motors do nominally have 250W max. If you use the same motors but different cotrollers you'll end up with vastly different data. Also if you use different motors or change parameters...

 

[T4tomo]

Did anyone expect...

 

[berlinonaut]

Did anyone expect...
View attachment 665352

Well, it's you bones that break, not mine if you fit a motor to a Brompton ti-fork. I am always eager for real world experience reports. But there are some experiences, that I prefer not to make personally...

 

[ExBrit]

Here you see i.e. the data from Grintech's motor simulator for two different hub motors on a Brompton (a Crystalite direct drive and a geared motor). Both motors do nominally have 250W max. If you use the same motors but different cotrollers you'll end up with vastly different data. Also if you use different motors or change parameters...

View attachment 665350

So if I read this correctly, the torque while riding up a steep hill at the slower speed would be about 16Nm on a Brompton. Accelerating from a stop would also generate significant torque. That's quite a lot for a system that hasn't been designed for it. I'm sure steel can handle a lot, but it also fatigues over time.

I think my takeaway here is that Swytch systems cause front forks to fail rarely if ever - otherwise they would have been sued into oblivion by now. But "rarely" is not good enough for Brompton.

 

[T4tomo]

Well, it's you bones that break, not mine if you fit a motor to a Brompton ti-fork. I am always eager for real world experience reports. But there are some experiences, that I prefer not to make personally...

I wasn't planning to, I was merely pointing out someone else was going to do it, steel is probably the best material fork to do it on, and the most common fork material on Brommie

 

[berlinonaut]

So if I read this correctly, the torque while riding up a steep hill at the slower speed would be about 16Nm on a Brompton. Accelerating from a stop would also generate significant torque.

In this example scenarios. With a different controller torque can be much higher:

It really depends from the individual setup and this is a simulation anyway.

I think my takeaway here is that Swytch systems cause front forks to fail rarely if ever - otherwise they would have been sued into oblivion by now. But "rarely" is not good enough for Brompton.

Would agree. "Not good enough for Brompton" could also mean: They did the mandatory ISO test with the original fork and did not pass. It could also mean: They just wanted to be safe, even that there were no issues during the test. It could even mean: They wanted a differentiator from the competition an thus enforced the fork though there was no technical reason for that - with the benefit that potential retrofit kit buyers would feel unsafe and rather buy a enforced original Brompton Electric instead of retrofitting a kit.

We don't know what the real reasoning was for Brompton.

BTW: With my 1st Gen. Swytched Brompton set to full power power would rise to close to 400Watts, even in the flat when starting over, but not beyond that. I think the 1st generation's Brompton motor was "just" 200Watt rated max, not the 250W legal limit.