How much low speed power difference with hub motor 26x3 vs 27.5x3 wheels/tires?

[leelorr]

Hello,

I have a 27.5x3 bike with a MAC12T that I have been trail riding for a couple of years. I love everything about it except that the large wheel/tire diameter make it somewhat under powered and prone to overheating on slow technical climbs (I have a CAV3 that is set up to show temp and shut things done if it gets hot). I built this bike with a 22 inch frame, but find that I feel much better riding a smaller framed bike. I am thinking of just buying the same bike in a smaller frame and just moving everything over to the new bike (Bikes Direct Bullseye 27.5+, so only $499), then I saw that they sell the same bike in 26x3 configuration. Would it be worth just buying the 26x3 bike to swap things into (and having to pay to move the MAC12T built into the new wheel) for the extra low speed power? They both show to have the 170mm width rear hub with QR axles that I need for the hub motor). I am not worried about the drop of a couple of miles per hour (currently 26 mph without pedaling), or will the approximately 5% difference in wheel diameter not really be much of a difference? I can probably give my current bike to my tall son after converting it back to stock...

Thanks in advance,
Lorrin

 

[Tommm]

This is not an ebike specific question, so you can use a regular bike calculator.

https://www.bikecalc.com/speed_at_cadence

 

[leelorr]

I have done the math. My question is really if anyone has experience to let me know if 5% more low end torque is really enough to notice or make a difference.

Thanks

 

[Tommm]

I have done the math. My question is really if anyone has experience to let me know if 5% more low end torque is really enough to notice or make a difference.

Thanks

I have done that same switch, it is not noticable. I wouldn't worry about it. Even if you notice(placebo probably) it is something you get used to in a day and forget what it was before.

 

[motomech]

Hello,

I have a 27.5x3 bike with a MAC12T that I have been trail riding for a couple of years. I love everything about it except that the large wheel/tire diameter make it somewhat under powered and prone to overheating on slow technical climbs (I have a CAV3 that is set up to show temp and shut things done if it gets hot). I built this bike with a 22 inch frame, but find that I feel much better riding a smaller framed bike. I am thinking of just buying the same bike in a smaller frame and just moving everything over to the new bike (Bikes Direct Bullseye 27.5+, so only $499), then I saw that they sell the same bike in 26x3 configuration. Would it be worth just buying the 26x3 bike to swap things into (and having to pay to move the MAC12T built into the new wheel) for the extra low speed power? They both show to have the 170mm width rear hub with QR axles that I need for the hub motor). I am not worried about the drop of a couple of miles per hour (currently 26 mph without pedaling), or will the approximately 5% difference in wheel diameter not really be much of a difference? I can probably give my current bike to my tall son after converting it back to stock...

Thanks in advance,
Lorrin

In a way you are asking about the hardest thing about hub motors. Hardest to understand and the hardest to deal with.
It all comes about when the First Law of Hub Motors and Hills is violated. It states; When the motor RPM falls below 1/2 the max RPM (no load speed?), more heat is generated than locomotion.
So the idea is to keep the motor spinning and not let it "bog-out". Hard to do on steeper incline, especially ones where you get slowed down at the bottom. In these low RPM situations, the motor is screaming for more Amps and the controller is trying to to feed it.
Really "low speed power" is more like how long you can keep climbing before the motor get so slow that the;
1)Phase wires and/or connectors will melt
2)something in the controller will overheat and fail.
3)wires, phase and hall can melt and Halls go bad.
This is why hubbies are not that great for the trails.
So how much will going from a 27.5" whl. to a 26" whl. help?
Only to the degree that it will help keep you moving up a hill.
Not that much I suspect, especially if you mount a larger tire than the standard 2.0"er.
I guess you could interpolate from the inverse. Going from the 27.5" to the 26" would probably lower the top speed 1 1/2 MPH, 2 MPH Max.

 

[DogDipstick]

Grin Calculator will tell you exactly how fast in Mph/sec. and the tq. with any throttle application between the two.

 

[leelorr]

Thanks for all of your input, which confirms my gut feeling.

Lorrin

 

[2old]

Before you spend any more money, you might try a 27.5 X 2.0 tire on the rear of the bike (unless you're really in love with 3.0). Should give you some idea of what to expect with 26 X 3.0, but think you'll still be dissatisfied.

 

[DogDipstick]

I mean, am I getting it wrong, but isn't it just about 1-2mph differences across the acceleration/speed range? The Tq is almost the same?

 

[Bullfrog]

I have a 12T MAC...use the Grin Tech Motor Simulator to figure out speeds and motor temperatures.

I have tried all kinds of diameter...everything from a 20x2.00 tire to a 29x3.00 and my conclusion is a tire that is about 690 mm OD gives me the fastest top speed, good acceleration, and doesn't overheat my motor.

The two tires that fit the bill for me are the Maxxis Hookworm 26x2.5 and the Schwalbe Super Moto-X 26x2.4.

FYI, I am running a 52v/14s battery, total weight of me and the bike is about 264 lbs, I am running a 40A controller with 112A phase current. I found plugging in 1.4% for the incline in the motor simulator works best to match my actual temperatures but you may need to use something different depending on how hilly it is where you ride. Remember the temps predicted by the motor simulator are after you have been riding for about two hours in the conditions you plugged in so just use them as a general guideline and don't worry too much about the exact number.

 

[Voltron]

During testing I laced one motor up to a 24, a 26, and a 700c, and it got noticably worse in performance with each larger size wheel. I think you will notice a difference if you step down a size, as long as you're not dragging the cranks.