Why retrofitting factory built ebikes for performance is not usually recommended

[paxtana]

Summary

Generally the best solution if a bike does not have the performance you need, is to sell it and get a different bike, or do an ebike build with a kit using a non-electric bicycle as the starting point, typically a mountain bike.

This is especially true in instances where there is a high load, or where there are lots of hills.

Defining performance

Performance is related to several factors:

• Weight
• Current, i.e. how much power the controller that is being used is pulling
• Voltage of the battery
• Capacity of the battery
• Max peak for the BMS in the battery that would be used on the bike
• Balance / Handling

In depth

Current
This is amperage, and has a direct correlation to how much torque you get. The more current you pull, the more torque you get. You want high torque in a bike with low weight and good handling.

Max peak for the existing BMS
The stock battery is built for specific output of power, both in the cells and the integrated battery management system (BMS). Typical usage is already probably near peak for the stock controller, because it was purpose built for that factory built ebike. which means you not only need a new motor/controller, you also need a new battery. At that point you will be replacing components that cost the majority of the value of the bike and the end result is a worse bike than if you built from a regular MTB because you still have extras like battery mounts and so on, versus just selling it all and using that money on the kit.

Capacity
Even if the max peak was within spec for the upgrades, any new upgrade to the controller and/or motor is going to burn through that capacity much faster and reduce your range. So again, you are in a position where you need a new battery anyway.

Voltage
This is relating to top speed. Typically you need more voltage to get more top speed, as more current usually just adds more torque. If a significant increase in speed is your goal then you need a higher voltage battery, there's no way around that.

Balance and handling
This is related to how high the weight is, how you brake, how you corner, whether the front of the bike goes up or down when you go over a jump and so on. Often if a factory ebike is retrofitted certain design choices need to be made such as putting batteries higher than they would optimally be, or not centering the weight. For weight that is high, this leads to a load transfer which reduces the effectiveness of rear brakes. For weight that is not centered, such as putting it on the rear, this can cause rear slideout upon hard braking as the inertia in the rear causes the back of the bike to want to keep going similar to how a semi truck can jackknife on the highway when braking hard.

Weight
Often folks who ask about converting existing ebikes want to keep whatever is already on the bike and just add an entire kit plus battery. Sounds good in theory right? Problem is it adds a lot of weight. The bike does not feel like a bike anymore, is harder to move around, harder to put on a bike stand, harder to put on a bike rack on your car, harder to service, and it handles differently. These issues may not seem that big a deal but it can make the bike less enjoyable compared to the bike you would have if you did not have two separate ebike systems on the same bicycle.

Conclusions

By now the takeaway from this should be clear that we are usually looking at an end result that will often be heavy, handle poorly, and/or have various non-optimal design choices made in the retrofit that would not be necessary if simply starting off with a more ideal donor bike. And even if you remove all electrical components from the original bike, stripping it down to just the bicycle itself in order to start over just to build it back up using a kit, end result is you have a bunch of unusable expensive parts left over with little resale value. Alternately if you sell the factory built ebike you may be able to get some good value out of these parts by selling it on something that is usable, thus saving money over the course of the project.

There are exceptions of course, but they tend to be expensive and/or time consuming. If a purpose-built upgrade kit exists or is developed for your particular bike, it would likely comprise a controller and battery upgrade at a minimum, and lithium is not cheap, nor is development costs. And not many aftermarket companies are going to be willing to put resources to adding value to a competing product versus developing their own product lines since that does not make much sense from a business perspective, so these sorts of upgrade kits are relatively rare.

[tklop]

I have been asked this question myself.

I appreciate this posting--it's a very thorough explanation of the whys and wherefores. I will refer back to it--the next time I'm asked.

Seems worth also mentioning that the factors which make upgrading a factory-built e-bike a poor idea, do not all apply to upgrading a regular bike--that was previously-converted.

Sure--for the same exact reasons, pretty much any upgrade will likely require also a battery upgrade. But after that--if a rider is willing to stay within the same style (or brand) of system, they might be a little less likely to have to "start over from scratch".

I can also see where in some cases, yes--everything will need upgrading. But even when starting over again "from scratch" there's a lot of pretty-much "plug-and-play" systems (like the TSDZ2) which may be viable (depending of course upon the original selection of components).

Many conversions use the KT-LCD3 displays--for example. That display is meant to run either 36V or 48V systems--and is often paired with both motors and controllers, which too can operate at either voltage. For someone with one of those systems who is running a 36 volt system--if they want an easy upgrade, all they'd really have to do is swap to a 48V battery, and update the voltage-readout settings in their display.

I don't have one--but I'm also willing to bet that's at least potentially true with Bafang systems. Maybe the displays aren't interchangeable, but I bet a lot of the components would be. So, going from a BBS02 to a BBSHD won't probably be a total "do-over"---and I can't imagine that it'd alter a converted bike's balance much--if any at all--going from one Bafang mid-drive to another. I suppose this might also apply to "factory bikes" made with Bafang mid-drives--but since I've yet to own a Bafang--I'm just not certain. [edit--see post #3 below for expert Bafang clarification]

Naturally, not every upgrade would work out that way. The next-bigger Cyclone (from the 3KW to the 4KW) requires both a new controller, and a wider mounting-bracket. You could reuse the controls, but other than that you'd find yourself pretty much starting over. Even so--if already you've done a Cyclone kit once, re-doing one isn't going to be that difficult for you.

Overall weight, balance, and weight-distribution, handling, etc., would be things (I assume) the rider would already be used to--on their already-previously-converted bike. And yeah--an upgrade in power or capacity might make a little difference to these properties--but probably not too much.

Just my two cents.

Unlike for a Factory-Made e-bike, I believe that re-doing, or upgrading a previously-converted bike--is actually not a bad idea at all.

All the best, everyone!

Tklop

[paxtana]

That is a good point, and indeed you can upgrade a bbs02 system to bbshd with nothing more than a bare motor. However, most factory built ebikes are not based off kits like bbs02. Luna sells some bikes that are custom converted with kits for customers, as do a few other companies but it is rare.

This knowledge base post deals more with bikes that are tightly integrated, such as ebikes using Bosch for the drivetrain and electrical components.

And while there are a few examples of upgrades such as the display upgrade for sondors that can unlock speed limit, this post is relating to guidance for more substantial upgrades, for example going very fast or far, with greatly improved torque, while also potentially handling hundreds of pounds of extra load. Sondors is one of the few factory ebikes that has the potential to be relatively easily upgradeable to a larger battery, better controller and unlocked display, and indeed we sold special sondors kits to do that for years, but that sort of easy extendability does not apply to the vast majority of factory ebikes.

[Retrorockit]

You didn't go into what makes a good donor bike. My vote goes for a mid range 8/9 speed disc brake Mountain bike, or the few comfort bikes with actual off road capable forks ( Gary Fisher, and Cannondale made a few). 26" or 29". 27.5 wasn't used yet back in the 8 speed days but maybe they exist. Check to see if you can get the gearing you need for the wheel size your buying. 26" can be hard to gear up for a fast street bike, and 29" can be hard to gear down for offroad with just one chainring Keep in mind that the choice of chainrings that give a good chainline is limited 42-48T. Mid level bikes that are intended for actual offroad riding will have strong wheels, strong forks, and decent brakes. 8 speed has a stronger chain than higher speeds, and E bikes don't need a lot of close gears. Most high end bikes cost more because they have lighter parts. Not necessarily what you want for a faster heavier setup. Downhill bikes, and dirt jump bikes would be the exception. There you are paying for stength. Things I look for are Threadless headset, at least 28mm fork tubes but 30mm+ is better, eyelets around the spokes on the rims ( this shows that corners weren't being cut). I much prefer to fin Rockshox, Manitou,or Marzocchi forks. A Fox fork is high end and it's up to you whether you want to pay extra for that. You can run an 8 speed chain and casette with a 9 speed derailer. Just a shifter swap is needed. So 9 speeds count also. Mid range bikes may tend to be 9 speed, and 29ers were not low end bikes back then (10-15 years or so). A lot of MTB got bought and not used much. Too hard, too dangerous,too dirty. DH and DJ bikes tend to live hard lives. The finish on the fork tubes will give you some idea if it was ever ridden hard. I would prefer a hardatil over a suspension bike and add a decent suspension seat post to the build. Adding a fork, brakes and wheelset to your build can easily cost more than buying a new bike. But the right donor bike can save you a lot of money and produce an excellent result.
The other popular route is a disc brake fatbike.These are newer but their simplicity can keep the cost down. The front supension fork becomes optional on these for street use. The tires are the suspension. But speed, range, and handling won't be as good. But if sand or snow are on the menu an MTB with a mid drive conversion will lose much of it's offroad capabilty due to wekght and balance issues.
The worst case would be to buy a bike that doesn't already have disc brakes. Buying a disc brake fork, brakes, and wheelset will cost you plenty. I did this back in the day when disc brakes were just coming out. Basically you're throwing away 1/2 of the bike.