At what point would you draw the line on maximum tire diameter?

At what point would you draw the line on maximum tire diameter?

Remember as the tire gets larger in diameter it can be less wide while still maintaining good ability to roll over obstacles:

54” is probably too big, but lace a hub motor to that 18” rear wheel and you’ve got yourself a real climber.

I don’t think I can ride a 29er, so 27.5.

Somebody please convert a penny farthing. Please. I need to see it done.

It's known that 29" wheels have a lower rolling resistance than 26"



32-36" wheels are said to roll extremely nicely. Here's the words of an engineer @ Trek:
Interview: Trek's Travis Brown On Experimenting With 32

" Brian Park: So, if we're in make believe land. You said you're 6'1"-ish. And if you were building, let's say your perfect cross country hardtail today, what wheel size would it have? With good tires and good components.

Travis Brown: If I had one fully custom ticket to spend, with what I know now and what I've experienced, I would spend it on a 36 inch wheel race bike."


32" wheeled bikes are a thing:
32" Kent Big League


Clydesdale BFG:



Look at the insane battery space and crank-forwardness they're using to make the geometry work. Too bad it has baby disc brakes instead of vbrakes ( this huge rim could make for a great heat reservoir and it would reduce brake fade )

Now the only negative is that DD hubmotors perform poorly at such huge diameters. You'd really want a geared motor or mid drive.

neptronix said:

Too bad it has baby disc brakes instead of vbrakes ( this huge rim could make for a great heat reservoir and it would reduce brake fade )

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A rim brake grabbing a larger diameter rim also has more braking torque for any given amount of cable pull. This because the larger rim offers a longer moment arm and thus greater leverage.

Yeah, but it is spinning at lower RPM, so you really need that extra force to make up for that. I think you only get ahead in rim brake cooling ability, because you have a lot more rim.

neptronix said:

Yeah, but it is spinning at lower RPM, so you really need that extra force to make up for that.

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That is true.

With that noted at least two things come to mind:

1. For any given tire width and sidewall height (e.g. comparing 32" wheel with 2.215" wide tires to 20" wheel with the 2.125" wide tires) the larger diameter wheel will have a greater proportion of rim diameter to tire diameter. That means the larger diameter wheel of the same tire width and sidewall height would have a small advantage.

2. Smaller diameter tires (at X load) need to be wider than larger diameter tires otherwise rolling resistance increases and shock absorption decreases (due to reduced tire air volume). Therefore if comparing a larger diameter wheel with a skinnier tire to a small diameter wheel with a wider tire the ratio of rim diameter to tire diameter further increases for the larger diameter wheel. This because wider tires of the same aspect ratio have taller side walls.

So rim brakes should still work better on larger wheels from a braking torque standpoint.

29 inch for road and 27.5 for off-road.

neptronix said:

32" Kent Big League
View attachment 354049

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Notice the smaller triangle on the 7 speed version:



Compared to the Single speed version:

Here is a hard tail 36" bike the maker says can be sized for riders 5'5" and up:




And here is a rigid 36" that only comes in one size but the makers says fits him perfectly and he is 5'9" tall (but will also work for riders far taller):




So it looks like 36" will probably enter the mainstream.

My own opinion is that the trend for shorter crank arm lengths will help to further improve the adoption of 36" bicycles. This because crank length is a major determinant of bike fit and cockpit layout. (Re: crank arm length determines BB height, toe overlap, seat position, etc.). Another determinant of bike fit is Q factor....and shorter crank length helps wider Q factor work better for the average person.

I find that short crank length hurts my knees by overloading part of them. Not for me.

32" rear with a nice quality 29" fork up front ( adjust tallness to get back to stock geometry ) sounds like my idea of a good time.
A geared motor on the rear and front would do good to make up for the pitfalls of slower wheel RPM.

With a mid drive, you could run oversized gears and get away with it due to the taller rear diameter. That's nice.

neptronix said:

I find that short crank length hurts my knees by overloading part of them. Not for me.

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When you shortened your crank length did you also lower your gears?

Because shorter cranks offer less leverage, but make up for the lower torque by making higher rpms at the same pedal speed as longer cranks. But if your gears are too high (for the crank length you are using) the pedals can be too hard to push and thus hurt knees.

Yes, i tried that, and went slower as a result. the experience wasn't better, it was worse both ways.

My legs like 175-180mm cranks, shorter hurts. I'm 6 ft. tall but have the legspan of a 6 foot 2 person ( smaller torso )

neptronix said:

Yes, i tried that, and went slower as a result. the experience wasn't better, it was worse both ways.

My legs like 175-180mm cranks, shorter hurts. I'm 6 ft. tall but have the legspan of a 6 foot 2 person ( smaller torso )

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You say you went slower, but why did you go slower?

If you shortened your cranks you would have to raise your seat (and likely pushed it back some too). This to compensate for the fact that the pedal at 6 o'clock is now at at higher position that it was before. This or you would not get full leg extension and lose power. However at the same time raising your seat (to avoid losing power) also means that you will have to raise your handlebars which hurts aerodynamics.

So it basically boils down to two situations when shortening crank length:

Lack of full leg extension= Lower power and thus slower.

Vs.

Worse aerodynamics, but proper leg extension= slower at the same amount of pedal power (or slower at an even a greater amount of pedal power as before).

I've made all kinds of adjustments to compensate, it still hurt and i had worse power output.

I prefer no pain and good output to a small aerodynamic advantage. If we want that, we can get on the recumbent or lean forward ( the upper part of the body has a much larger frontal area than the legs )

If you are looking for small aerodynamics gains at the leg then a crank forward bicycle would be a better route to go.

neptronix said:

I prefer no pain and good output to a small aerodynamic advantage.

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But you are not going to get that by taking an existing bike built around 170mm or 175mm cranks and then shortening the cranks.

One nice side benefit of having a bike specifically designed for shorter cranks is that the lower BB height will result in a frame triangle that is larger for any given amount of standover height.

The larger frame triangle would then allow for a larger battery pack.

this is a funny question, because recent mountain bikes are mostly 29er and that's what i would choose most times, then i would go for 27.5 or 27.5 plus diameter, the smallest i had was 20 inch fat bike which was comfy and nice for city riding but it sucked when you wanted to go faster.

I have an approximately 34" inseam. After much experimenting and tuning (gearing, seat position/height, reach, etc.) during my serious road bike years (80s-90s) I ended up riding 172.5mm cranks religiously. I always felt a bit less efficient on 175s (judged over 100-150km rides) and a bit slower on average on 170s. For me the 2.5s were a sweet spot in range and avg speed.

With E bikes I find the trend with the preponderance of cheap 170s annoying. I'm always looking to not spin out at higher speeds and the use of a throttle to help get rolling means there's less reliance on lower gearing/shorter crank arms, so I've retained my road era drivetrains on my road/distance Ebike setups but it's getting harder and harder to do so with each refit/upgrade.

These 29er tires are 31.338" in diameter:


That is actually a greater diameter than what Neptronix measured for his own 32" tire:




.....which he measured at ~31.25".

With such a big front wheel surely toe-overlap must suck unless the geometry were greatly changed to put the wheel way forward no?

A typical bike frame already has a lot of room for a battery. I haven’t even seen a fully filled front triangle

Hummina Shadeeba said:

With such a big front wheel surely toe-overlap must suck unless the geometry were greatly changed to put the wheel way forward no?

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Yes, for any given crank length there must be some type of adjustment or compensation made to prevent a person's toe from hitting the front tire during a turn when the pedal on the opposite side of the turn happens to be in the 3 o'clock position.

Some options from a design standpoint:

1. Slacken head tube angle while simultaneously increasing fork offset. (This moves front tire forward from Bottom Bracket while keeping the trail constant..... just remember that increasing tire diameter increases trail independently. Therefore fork offset should be increased more than the head tube angle slackened if the plan is to keep trail the same as it was with a smaller diameter tire)

2. Increase top tube length while simultaneously decreasing stem length and/or decrease handlebar reach (e g. Handlebars with a greater sweep so distance to grips is decreased). This basically pushes the BB back X distance and therefore reduces toe overlap without making the handlebars harder to grab. The riding position obviously will be more upright when taking this route.

3. Some combination of #1 and #2 together. Remember trail increases with increase in tire diameter so even if route #2 is taken some increase in fork offset will have to also happen to counter the increase in trail from the larger diameter tire. That or a steepening of head tube angle. Obviously if steepening head tube angle is chosen the top tube would have to be increased even more than it would have with the increased fork offset approach.

Personally I like the idea of reducing crank length to also help the problem. This for at least two reasons:

1. Lower BB height (which is possible with a shorter length crank) increases distance of foot to tire when it is at the 3 o'clock position independently of the decrease in crank length which also increases the distance to the tire. This because the lower the BB is the farther down the curve of the tire the pedal will be when it is at 3 o'clock.

2. If using a higher RPM mid drive like the BBSHD (and wanting to pedal along with the motor) less detuning of the motor needs to happen for any given foot pedal speed (Reason: shorter crank length arms making higher RPMs easier because each revolution of the cranks is a smaller circle than it was before).

With larger tire diameter resulting in better aerodynamics for any given tire volume it would be interesting to see either a 32" or 36" tire used in an eVelomobile like the Northern light 428, 557 or 630: