I picked up a '93 Marin Bear Valley SE from a vintage store and spent time restoring it (surface rust, old components etc). I was pleased with how it turned out and enjoyed riding it, nice old steel frame. I then decided to convert it to electric with the Bafang BBSHD 1000W kit and have the motor already installed. Here is my slight dilemma; It's 1x7 speed (replaced front 3x chain-ring with BBSHD 1x chain-ring). I spent the extra money buying the BBSHD 1000W kit as I wanted the extra speed but don't want to be restricted with only 1x7. What to do?? My plan currently is to upgrade to 9 or 10 speed..but that involves a new rear wheel as my old freehub body only takes 7 speed..so new rear wheel, stretch out rear triangle, new rear derailleur, new shifter, new chain, maybe an upgrade to disk brakes? Or should I simply revert it back to normal and buy a new mtb already with 10+ speeds and convert that instead? I like the idea of having one bike only though. Please help! How difficult/expensive to upgrade drive train? Thanks
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What even makes you think you'll need any more than 7 gears with that motor? A 1000W motor should be quite capable of driving a bike at any sane speed up any hill even without any pedalling and just a single fixed gear... Whether that's a good thing is of course another matter. I agree with the answers saying that what you want seems to be a motorbike, rather than an electrically assisted bicycle.– leftaroundaboutCommented Nov 12, 2021 at 12:36
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What is the range of the existing 7 speed freewheel?– AffeCommented Nov 12, 2021 at 17:39
2 Answers
The cheapest update you can do is a bigger chainring. You have the BBSHD chainring (how many teeth? it exists up to 52 teeth), but there are alternatives:
- third-party compatible parts (google for "ChainRing for the BBSHD")
- adapters to regular BCD chainrings (I found some of them via googling "BBSHD to BCD adapter"), then you may find large or very large chainrings (I know they exist for recumbent and for small wheels track bicycle ... both are exotic bicycles, so do not expect to find them easily and of good quality).
However, a bicycle is made to go as fast as a bicycle. I guess you would like to go as fast as a moped. I have the feeling you are pushing the frame (and the fork!) at the edge of what was built for. The 1000W motor is already a quite a powerful one. You are adding weight, torque ... and speed, to a frame that was not designed for these additional loads.
Ride safe, respect the local laws (especially of physics, well, they will act anyhow with or without you paying attention) ... and compare the forks size/thickness of small mopeds with the fork of your nice steel frame ;) .
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3Banging some rough numbers into a power/speed calculator, ignoring rider input, 1000W is good for 55km/h on the flat not wind etc. You would need to spin a 44/11 at 110 rpm cadence. 50/11 will get you down to 100 cadence at 55km/h. Engine would not be road legal in many jurisdictions around the world (typical limits are 250W-350W and/or speed limiters).– mattnzCommented Nov 12, 2021 at 8:55
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1@mattnz thanks a lot for the rough numbers, very interesting, I was thinking only in relative terms. I would add that "Road legal" may not be an issue. 4-5 years ago I was regulary watching videos on youtube of people (mostrly from the US) building ebike with motors with 1'000 or even 10'000 Watt power, then riding in forest roads and the like. I was very impressed how unreliable these contraptions where (if you "cycle" fast and wild 20 miles out on a road forest, and suddenly the battery catches fire ... it is a long walk back home), and how much people were attracted by raw high numbers.– EarlGreyCommented Nov 12, 2021 at 9:02
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1Quick note in case it affects the OP: I believe they're converting an MTB, and it may have some limit on max chainring size. In particular, I'm not sure that a road size chainring (e.g. 50t and above) will fit. Commented Nov 12, 2021 at 14:06
So you have a 7 speed bike, with presumably a cassette of 11 though 28 teeth, and a single chainring of 40/42/44 teeth.
Your rear wheel will be a 26" or 559 according to modern nomeclature, and your rear tyre is probably between 1.5 and 2 inches (37-52mm)
The motor will propel you faster, until either
- The assistance limiter kicks in,
- The maximum speed of the motor is reached,
- Aerodynamic and rolling resistance forces balance the power input by the motor plus the rider's efforts.
So the only way to get a higher top-end speed will be to move a factor stopping you. That may mean disabling a software-based limiter.
You can get more aero by using a thinner tyre, a more supple tyre with lower rolling resistance, a tyre with ideally a smooth tread, and increasing the air pressure in your tyres within-reason.
You can alter the ratio of input to output by increasing the size of the chainring, and decreasing the tooth-count on the cassette. 11 is the common smallest size, so if your bike has a 12 or 14 tooth smallest cog then consider something with a smaller smallest cog.
The cost here is that as top speed goes up, power and torque drop off. It is possible to go faster on a bike but take longer to get to that speed, so it is always a compromise.
You can go for a larger radius wheel too, but that also trades power/torque for top-end speed, and is likely such a swap would require a different frame.
You can additionally add more power by pedalling harder - a mid drive does not replace the rider's effort, and from a safety point it is important to keep your legs turning over all the time.
Every one of these changes could cost additional money.
Ultimately, if you crave speed without the required personal input of effort, then a bicycle might not be the right tool for you, and you should look onward to other powered transport options.
Specs for this kit are taken from https://bafangusadirect.com/products/bafang-1000w-bbshd-mid-drive-ebike-motor-kit
