I have a 20" wheel trike I plan to motorize. Can I add a 24" or 26" Hub front wheel motor and can I mix the front and rear tire sizes?
What are the consequences for stability and cornering?
I am new to e-biking and would greatly appreciate any guidance.
This answer addresses "stability and cornering"
If you do motorise your trike with a hub motor, it will gain 2-5 kilograms in the middle of the wheel. This is "unsprung weight" Depending on the battery number, size and chemistry you will also add 0.5-10 kilograms of storage somewhere on the frame.
So your bike will get heavier by ~3-15 kilos. This is balanced out by the assistance provided by the motor as long as you have power, for regular straight-ahead riding.
A hub motor in a regular bike's front wheel aggravates any conditions. If you ride in the wet or on damp leaves, its more likely to slide out under you. I've never ridden a rear-hub motor, but I imagine its similar, and powering through any corners is a bad idea.
The motor is also unsprung weight, in vehicle terms that's the weight that is always pressing on the road with no suspension. Increased weight makes jumps much harder, and increases the impact of anything like potholes and road debris.
A test you can do is to get some weights that are about as heavy as the kit you want to fit, and got for a ride with these weights on your bike. Bags or plastic bottles of water might be safest. This will show you about how it will feel if the battery is flat.
Personal story - I had an electrified MTB for a while, and it was capable of 40+ km/h with pedalling. I had multiple instances of cars seeing me in the distance and pulling out without anticipating my speed.
Now I have a road bike and can get to similar speeds without the boost, and because I look faster then cars have been less likely to pull out.
I also found it was too easy to take corners at speed. Several times I turned 90 degrees and found myself halfway across the car lane, because I was riding to preserve my momentum. Once I was clear on the wrong side of the road, and fortunately only good luck meant no cars were oncoming.
Summary - ebikes are a gateway drug into riding your bike more. They're a sop to the conscience for those who want to but can't quite.... as such they do have a place in cycling.
Changing the wheel size is well covered in other questions, like:
Short summery is "maybe" if it fits without rubbing, but brakes may be an issue. Geometry of the bike is also changed in various ways. I'd expect your forks to be too short for a larger wheel, so you're up for replacing them too, if the trike has forks.
No, you can't just plop a larger front-wheel hub motor into a bike or trike willy-nilly. There are various considerations including wheelsize and drop-out width. For example, although it might be deadly obvious, you can't put a 100mm front ehub wheel in the rear of a tadpole because: 1) it will likely be the wrong dropout width; 2) it won't have a freehub for your chain (if applicable).
Once you got that sorted out, changing the size of the front wheel in a delta or rear wheel in a tadpole depends on the frame. In most cases, you won't have enough frame or fork clearance to put in a larger wheel. Furthermore, you'll also run into problems with your brakes (unless you have disc brakes) as the brake bosses will be in the wrong place. Your rim and associated tires might be too thick for the frame. Finally, you'll also change the angle at which the seat sits in relation to the handlebars. So in general, thinking about changing wheel size is a Bad Idea® because it won't fit, let alone stability issues.
Rather than using whatever wheel you found, it's so much easier to just build-up the right 20" ebike wheel. That way you can get the right drop-out width, rim width, and add options for disc brakes or a freehub, if applicable. You'd start with the appropriate sized e-hub -- so a rear wheel (135mm dropout) ehub for the rear of a tadpole -- or 100mm for front wheel of a delta. This assumes your bike's dropouts or forks are designed for those widths.
There are many places will build you the exact type of 20" wheel that you need. We're not a shopping recommendation site but Lunacycle.com is a good first place to browse to get a sense of what options you might have.
If I've convinced you changing wheel sizes is a bad idea but you already have the wheel, as @Criggie notes, you could have the hub motor of the wheel rebuilt using your existing rims. However, generally ehubs for smaller wheels are wound different (have a different rpm/V) than those for bigger wheels, so you may have more starting torque but a lower top speed. And many hub motors will not physically fit into a 20" wheels -- really only the smaller geared ones work.
Note that because of the complexities of tricycles (and recumbents), many people use mid-drive motors (like the Bafang mid-drive) to instead motorize the crank.
As said you could get a 24” wheel and have the hub taken out and laced into your existing front wheel. If going down this route make sure the e-bike controller can be set for a 20” wheel otherwise the speedometer might not display the wrong speed and it MIGHT affect the pedal assist ratios. I’m not getting what issue is about the xtra hub and battery weight as it a bit like saying Fred at 90kg will be OK on it but as soon as Mike gets on it at 95Kg there going to be additional stability issues. There might be but these are manageable. I have fitted as 26 front wheel e-bike kit on my 26” wheeled trike that has a thumb throttle limited to 4mph and I found that when crossing inclines the trike wants to run to the downhill side but with a little throttle the front wheel pulls the trike in the direction it’s steered. Yes it will wheel spin on wet grass but you can change the power ratios from 5 settings to 9, thus lowering the power asist percentage delivered to the front wheel. You’ll find you’ll probably need to mount the battery in front of the rear basket or actually in it is mounting it typically on the down tube may well restrict you stepping through. Key thing to know about tradition trikes is you cannot take bends at high speed unless you employ some serious leaning into the bend. I would take a bend at no more than 5mph and going across inclines is not advised but can’t always be avoided so leaning towards up hill is essential whereas on a bike you automatically remain upright. Shop around and find a supplier that does a 20” wheel as you can get them as small as 16”. Brompton bike have small wheels so there you go.
