Large front/small back wheel

JackFlorey

100 kW
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Feb 19, 2020
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San Diego
Saw the Yuba Spicy Curry a while back and it got me to thinking. Geared hubs are often used to allow smaller/lower torque motors to be used as hub drives, and are usually lighter (per watt power) than their direct drive equivalents.

The Curry isn't a hub drive (it's a mid-drive) but it uses a smaller wheel in the back to allow more cargo space above the wheel. Could you take that to an extreme and put a 12" wheel in the back and a 29" wheel in the front (for bump handling) and use a much smaller direct drive hub in back, since it would need much less torque to climb a given hill? What are the implications for handling?
 
When it comes to a direct-drive hubmotor (DD), it is a significant performance improvement to increasing the "tangential magnet speed". There are two ways to do this. You can increase the diameter of the hubmotor, or you can decrease the wheel diameter (or both).

I don't see enough cargobikes with this yet. The "go to" suggestion for significant up-hills is a mid rive, but they are limited to the power that the chain and sprockets can survive, and even if they can survive 2500W, they will experience rapid wear.

A 20-inch bicycle rim can usually seat a 16-inch moped tire (motorcycles and bikes measure the rim by different methods). For me, the optimum setup for a small rear tire is a 1500W DD hubmotor using thin laminations (Leafbike, etc), 12-ga spokes, and mount it to a 16-inch moped rim.

Moped13.png


Moped rims have angled spoke-holes, which allow a severely-angled spoke nipple without stress to the spoke.

Moped3.png
 
You can take things to the extreme, but it's not likely to be something you would enjoy. While a small 12" wheel will give more torque, it will also suck at rolling over even the smallest of crack. you and any cargo you have on the back will feel every bottlecap in the road with a tiny 12" wheel. 20" is about the break even point were you get the best power without sacrificing the ride too far. If I was planning a cargo bike upgrade, I'd follow Spinningmagnets idea, except I'd use a more powerful motor and dump crazy amounts of power into it, and also run a large volume tire, like a moped 16"x3.00"

As for the front wheel, there are reasons 26" wheels are still so popular. the larger the wheel, the smoother the ride, but the worse the handling at low speeds. Since a cargo bike is a long wheelbase bike, and since your ride is already ruined by a small rear wheel, you aren't really going to see much improvement in ride comfort by going to a larger front wheel, but you will notice the decrease in handling. There is a good reason so many cargo trikes use 24" wheels. At the speeds they are designed for, similar to your cargo bike, even a 26" wheel is too much trade off in handling vs ride.
If I was looking to increase the ride comfort on the front wheel, I'd run a larger volume tire, replacing the skinny stock 2.15" with a 2.40" or 2.50"
 
Ditto--that small a wheel is going to have a terrible ride.

I'm using 20" rear wheels on SB Cruiser (and previously on CrazyBike2), and the ride is still harsh on some sections of city streets, even with the large 16"x2.25" Shinko SR714 moped tires (that basically turn them into 22" wheels).

I've actually been working on a wya to rebuild the trike to use 26" or larger wheels back there (like the front does), to improve ride quality.



So...you could use small diameter wheels to improve torque output from a small motor, but it will be at a (drastic) cost of ride quality.


BTW, a common term for a bike with different size wheels is a 96er or 69er, if you ever want to search for bikes like this, or articles about them.


Drunkskunk said:
There is a good reason so many cargo trikes use 24" wheels. At the speeds they are designed for, similar to your cargo bike, even a 26" wheel is too much trade off in handling vs ride.

That depends on the design and geometry of the trike (or bike). If it's designed around a larger wheel, it works fine (at least with the SB Cruiser trike I built and use all the time, and the Raine Trike I built for my brother with very similar geometry and scale--and I don't imagine those two are flukes).

If you change the wheel on a trike or bike designed for one size wheel to a different one, larger or smaller, it changes the handling by changing the geometry. Whether that makes it better or worse depends on the bike or trike and it's usage. Similarly, a larger or smaller tire size changes the diameter which is the same kind of change a different wheel makes (though typically to a lesser degree).

Changing both wheels to a larger size (or smaller) by the same proportion would keep the geometry about the same and keep the handling about the same, but that's not usually what people do. ;)
 
spinningmagnets said:
A 20-inch bicycle rim can usually seat a 16-inch moped tire (motorcycles and bikes measure the rim by different methods). For me, the optimum setup for a small rear tire is a 1500W DD hubmotor using thin laminations (Leafbike, etc), 12-ga spokes, and mount it to a 16-inch moped rim.
OK thanks.

One of the reasons I ask is that about three years ago I did a test with a trailer with a 16 inch wheel/Crystalyte DD hub. It worked surprisingly well. I expected all sorts of traction problems but no issues.
 
It works fine to use a smaller wheel in back, but you'd have to find a mag wheel to go smaller than 20" for most dd hubs.

But you have to start with a frame with the angles adjusted so the ride doesn't get too funny.

I built a bike specifically to use the small wheel in back, for a lower cargo deck, and to get better climbing torque, and better low rpm efficiency. Used a low rpm dd motor, so at 52v top speed was only 18 mph. But it climbed a hill towing a trailer much more efficiently, but slower, than similar cargo bike that used a big wheel, and big motor to just climb the hill at 2000w.

This bike climbed 8% at about 800w, easy on the battery, yet could carry big loads with ease. when I say hill, I don't mean it could climb a quarter mile of hill. I mean it could climb the mountain pass.

See the bike in one of my signature thread links. The mixte bike.
 
I think a dual motor cargo bike could be used with a mid drive and for hills use a geared motor for the extra power to climb. But, I have no experience.
 
Drunkskunk said:
by going to a larger front wheel, but you will notice the decrease in handling.

Curious why this happens? I don't really even know for sure what "handling" means here, so it's hard to guess what's going on in mechanical terms. More gyroscopic inertia? Longer contact patch?
 
The smaller the wheel the more twitchey the bike could be. One day I got off my motorcycle to ride my friend's Vespa, and my first thought was....this thing is a death trap! Subsequent rides on small wheeled bikes and motorbikes convinced me of the fact. Granted, the Vespa was an extremely small wheel, but it's a given that the larger the wheel the more stability you'll have.

Wheels have a flywheel sort of inertia, so the larger the wheel, the more pronounced this will be. You'll reach a point of diminishing returns if you get the wheel too big because it won't want to lean readily into a turn, but bikes don't get anywhere near this point.

Then there's the road contact issue others have pointed out. With more road contact, a large wheel will sorta scoot over a rough road's bumps, while a small wheel will want to drop into every imperfection and shake your teeth out. Can't be good for the bike either.
 
Most cargobikes with a small rear wheel don't have a rear suspension, so when they hit a pothole, it can be very jarring. There might be other issues, but that's the first one that comes to my mind...
 
The other way to look at that would be, of course, you need suspension over a small rear wheel, especially if the frame is going to be heavily loaded there. That would have to be taken into account fairly early in the design for something like a cargo bike, so you have enough usable area on the suspended rear frame.

I'm not saying 12" makes sense, thinking of a more reasonable 20". How does wheel size relate to disk brake performance, by the way - make any difference how fast the axle is turning?
 
I am **very** interested in this topic.

Are there cargo bike frame designs, lots of weight racked over the rear wheel

that make use of a strong swing-arm style suspension?
 
john61ct said:
Are there cargo bike frame designs, lots of weight racked over the rear wheel

that make use of a strong swing-arm style suspension?

Surfing for my own curiosity, I found something on this right here - Why is there such a lack of full suspension cargo bikes? - wherein Chalo proposes that it's hard to set shocks up for a widely varying load, as you're bound to have on a cargo bike.
 
on my trike with rear suspension, ive been playing with fatter tires and stiffer springs, the fatter tire absorbs the little bumps and the stiffer coil keeps my fatter tire from bottoming out on the rear rack. the tire actually bottomed UP into the rack but not now and its still a very comfortable ride
 
Could it be designed to swap out the coilspring when you know you'll be fully loaded?

Or using varying PSI of air as with tires and some suspensions?
 
25 lbs more or less difference can be tuned if the shock is selected especially to let you have that much setting range, but 50 lbs does require another shock or at least, a spring weight change or a different model of the same shock. 100 lbs difference does require a different suspension ratio or a different linkage design.
 
My first homemade cargo bike had rear suspension. It worked good, but later on I realized that the real beauty of the longtail cargo bike is that the real cargo, your ass, is centered between the two wheels. the rear wheel goes up and down, but your ass does not. you sit on the center of the teeter totter and it just rocks.

Sure, you cargo might be 100 pounds, so your rear wheel still needs to be strong. But it feels little of the jolt, unlike your ass. And its much lighter than your ass, which might be 250 pounds that the rear wheel lifts up every bump on normal bikes.

The spicy curry example is a near perfect cargo bike. Handling issues minimized by lowering the cargo deck. No, its not an off road bike. I just mean it wobbles less, does low speed turns better, avoiding the handling issues common with normal 26" bikes overloaded with panniers.

That's why I built one to imitate the spicy curry as much as possible. It handled great, yes, its top speed was only 18 mph, but it ran much faster down the other side of a mountain pass. So it took a corner excellent, at speed. Longer wheelbase helps that too, imo. I had a front shock on that bike, the frame was built so it fit, without affecting the angle of the frame. That meant that the crank came up an inch as well, which was nice for pedaling right through a corner at high lean angles. Then the rear stay angle had to change to make it accept a small wheel. Finished cargo mixte..jpg

I DONT suggest you just put a small wheel on a normal frame.
 
Well, my rack is only rated for 350lbs...

partsleft.jpg
ontherack2.jpg
IMG_20151015_183434.jpg

But I'll try to talk her into bringing two of her friends next time for a foursome ride for load testing 🤣
 
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