40mph commuter: slack hardtail on 29" tire or steep full-suspension on 26"?

For a 40mph commuter over rough cracked roads, would you choose a slack hardtail on a 29" tire, or a steep full-suspension on a 26" tire?

Slack hardtail:
Chainstay: 430-440mm
BB drop: 55-65mm
Headtube angle: 66-67deg
Seattube angle: 72-74deg
Tire: 29x2.35 Schwalbe G-ONE Speed tubeless

Steep full-suspension:
Chainstay: 397mm
BB drop: 20mm
Headtube angle: 69deg
Seattube angle: 72.5deg
Tire: 26x2.35 Kenda Kwick Drumlin Cargo or Schwalbe Super Moto-X HS 439 tubed
Travel: 100mm

Going from 29x2.35 down to 26x2.35 is about 9% less volume, or conversely, going from 26 up to 29 is about 10% larger volume.
Does the difference in tire volume compare to suspension travel?

At speed, does suspension matter more than stable geometry?

Many thanks

Nothing compares to actual suspension.

I have 3 semi recumbent bikes that have 20" wheels and nice rear suspensions. They glide along my absolutely broken roads better than my lady's Cadillac. And on these bikes, you have more of the rider weight sitting on the rear tire than an upright. Even still, the suspension makes up for the small wheel and weight balance disadvantage.



If you want to use a hub motor, then you should be aware that the larger the tire diameter, the less continuous power you can make from the hub motor due to it being at a lower RPM per MPH.

In other words, go for the 26er and if you need to do things like change the fork height angle so that the geometry isn't messed up, then do it.

I previously read your impressive builds. Thanks for your feedback.

However, a typical 20x1.5 tire has just 29% of the volume of a 29x2.35, and 32% of a 26x2.35. So full suspension would seem quite necessary on that tire, especially on a 'bent that you can't post up on the pedals for a big bump.

These larger tires would offer substantially more pneumatic suspension.

fatty said:

These larger tires would offer substantially more pneumatic suspension.

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I think you may misunderstand how tires provide suspension. They don't compress their entire volume; they have almost invariable constant pressure. It's the inward deflection at the contact patch that does the suspension. So it isn't the tire's total air volume that matters so much, but the height of the rim off the ground.

Wheel diameter matters more, because it determines the rate at which the axle height will change when encountering bumps.

40 mph is probably too fast for comfort on any hardtail (and most full suspension pedal bikes too). But between the two bikes you're comparing, the full suspension one is clearly a better bet for that speed. It's the speed which is a bad idea, in my opinion. If you want to go fast like a motorcycle, use a motorcycle.

The Kenda Kwick Drumlin is a much better choice of tire for daily commuting-- very sturdy and puncture resistant. Tubeless is fine for play bikes that you don't mind maintaining frequently, but it's a liability for regular transportation.

Chalo said:

I think you may misunderstand how tires provide suspension. They don't compress their entire volume; they have almost invariable constant pressure. It's the inward deflection at the contact patch that does the suspension. So it isn't the tire's total air volume that matters so much, but the height of the rim off the ground.

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Thanks for your input. My experience is automotive, with much greater variation in volume to aspect ratio compared to round bicycle tires, but I still think your explanation is incomplete. Skinny trailer tires offer remarkably poor pneumatic suspension, even though they are quite tall (aspect ratio 80-85).

A larger volume at commensurately lower pressure deflects more (pneumatic suspension) up to the limit of the rim striking the ground. But since bicycle tires are round, height is directly proportional to volume.

Chalo said:

Wheel diameter matters more, because it determines the rate at which the axle height will change when encountering bumps.

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Wheel diameter independently matters only for obstacle rollover (cracks). Again, 26" donk car wheels have huge diameters, but ride remarkably poorly.

As far as pneumatic tire suspension is concerned, total air volume is all that matters.

Agreed on the general points on suspension and tires.

fatty said:

However, a typical 20x1.5 tire has just 29% of the volume of a 29x2.35, and 32% of a 26x2.35. So full suspension would seem quite necessary on that tire, especially on a 'bent that you can't post up on the pedals for a big bump.

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I run 20 x 2.0-2.5" and don't expect the tire to do much of the work. The suspension does 90% of the work of dealing with bumps.

Honestly, the air volume in a tire will do some work in suspending things. It works better the less there is. So you want 30psi or lower to get it to work. But then you incur a huge rolling friction penalty. In my opinion it is not worth it.

Rubber also has innate suspension qualities. I ran a 20" on a hardtail upright with a motorcycle tire on it and it was surprisingly good at soaking up irregularities however again.. the rubber creates rolling friction.

I've also experimented with a shit ton of different tires on 26-er/29-er hardtail bikes. There isn't much of a difference in them. I hated how 29ers with front suspension ride when you get beyond 30mph. The back wheel can jump in the air at times and land in a place you don't expect when hitting an irregular bump.. NOT SAFE!

If you are going 40mph then you are best to design your bike like an offroad motorcycle. You'll notice they tend to have very expensive suspension but rarely come with anything resembling a baloon tire. The suspension has damping, rebound, and a much longer distance it can travel to soak up bumps than the tire.

Only in the bicycle world do you see a lot of suspensionless vehicles produced. The reason is because suspension can eat into pedal power, and when your 'engine' only puts out 100-150watts, every watt is sacred. This kind of thinking does not translate into the design of a 40mph ebike.

neptronix said:

If you are going 40mph then you are best to design your bike like an offroad motorcycle. You'll notice they tend to have very expensive suspension but rarely come with anything resembling a baloon tire. The suspension has damping, rebound, and a much longer distance it can travel to soak up bumps than the tire.

Only in the bicycle world do you see a lot of suspensionless vehicles produced. The reason is because suspension can eat into pedal power, and when your 'engine' only puts out 100-150watts, every watt is sacred. This kind of thinking does not translate into the design of a 40mph ebike.

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This was my rationale as well. Even 50cc MiniGP bikes raced on glass-smooth tracks have full suspension.

I can say with confidence that a hardtail with 4" wide tires and front suspension going 30 mph is fine on broken pavement and gravel, but I wouldn't want to go any skinnier at my speed, or much faster with my setup (on rough roads). I definitely wouldn't want to use skinnier tires going faster on a hardtail if the road is at all rough.

I know it's risky messing with geometry but if you already have the bikes have you tried putting the 29" fork and wheel on the 26 FS and see how it feels? It should slacken it out a bit.

Or maybe even a 24" wheel in the back.

Tires are not making suspension, because they have no negative travel. A suspension does move up with a bump, and down with a hole, while the frame tends to maintain a constant trajectory. Also, a suspension does have a damper, that slows down the rebound effect.

Tires can make a smoother ride by limiting the transfer of small imperfections of the surface to the frame, avoiding vibrations. A suspension does very little to reduce high frequency vibrations. It is not its purpose. The purpose of a suspension is to keep the wheel in contact with the ground. The faster you ride and/or the higher the amplitude of hits, the more a wheel want to bounce off the ground. There are 2 effects produced by that bouncing: loss of traction/grip, and loss of control. No tire can help keeping a bike from bouncing off the ground on a hit, or turning into a bucking horse on a series of bumps. Tires only help with vibrations. We need both, proper suspension and tires for handling and comfort, according to the speed and surface that we ride.

Yep. Agreed with most people here. Go with a 26 inch wheel and full suspension. It's a lot easier to increase speed than torque in a hubmotor, so the smaller the wheel the better.

If you're going with a mid drive size doesn't matter as much. But gearing becomes a bigger issue; hard to get a range you can pedal on reasonably at lower speeds and still hit ~40mph.

fatty said:

For a 40mph commuter over rough cracked roads, would you choose a slack hardtail on a 29" tire, or a steep full-suspension on a 26" tire?

Slack hardtail:
Chainstay: 430-440mm
BB drop: 55-65mm
Headtube angle: 66-67deg
Seattube angle: 72-74deg
Tire: 29x2.35 Schwalbe G-ONE Speed tubeless

Steep full-suspension:
Chainstay: 397mm
BB drop: 20mm
Headtube angle: 69deg
Seattube angle: 72.5deg
Tire: 26x2.35 Kenda Kwick Drumlin Cargo or Schwalbe Super Moto-X HS 439 tubed
Travel: 100mm

Going from 29x2.35 down to 26x2.35 is about 9% less volume, or conversely, going from 26 up to 29 is about 10% larger volume.
Does the difference in tire volume compare to suspension travel?

At speed, does suspension matter more than stable geometry?

Many thanks

Click to expand...

Get an agled headset to slacken geometry by 2'. That's what I did with an old DH bike.

Also, if you got free clearance to fit a 27.5 wheel front, you get a slacker head angle and better monster trucking ability.

fatty said:

As far as pneumatic tire suspension is concerned, total air volume is all that matters.

Click to expand...

Sorry, you're wrong. Cross-sectional area of the tire gives suspension, sure-- and that's partly proportional to volume. A large height of tire can help a lot, and you have to have enough air volume for that. But it isn't the total volume of air that does the work, because from unloaded to bottomed out makes almost no difference in the tire's total volume and therefore pressure.

In the bicycle world, the wider but shorter you spread out a given tire on a wide rim, the worse it rides-- even though its volume increased. You have to lower the tire pressure to compensate. Bicycle tires are not the same as murder vehicle tires. They're much more flexible, and they're always bias ply.

Anything you can feel from the surface as you ride has a height (which is why you can feel it), and for that reason the wheel outside diameter makes a disproportionate difference in ride quality. That's why rollerblades ruin your feet quickly, but high wheeler bikes from the 19th century ride nicely enough on dirt roads, with solid rubber tires.

MadRhino said:

Tires are not making suspension, because they have no negative travel.

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Tires have static sag just like any other suspension. The late Jobst Brandt used to recommend using sag to decide what tire pressure to use. I think he recommended 20% of the unloaded casing height, which is not too far off what you'd use for a mechanical suspension.

saskatchewanian said:

I can say with confidence that a hardtail with 4" wide tires and front suspension going 30 mph is fine on broken pavement and gravel, but I wouldn't want to go any skinnier at my speed, or much faster with my setup (on rough roads). I definitely wouldn't want to use skinnier tires going faster on a hardtail if the road is at all rough.

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Thanks. I initially thought about a full-fat 26 x 3.5 or 4.0, but couldn't plan a complete system that I liked.

saskatchewanian said:

I know it's risky messing with geometry but if you already have the bikes have you tried putting the 29" fork and wheel on the 26 FS and see how it feels? It should slacken it out a bit.

Click to expand...

Yeah, I figured I could pick up about 4.5deg with a longer fork, taking it from 69 to 64.5deg.

24x3.0 is about 26od, i like that size on the 26 rim bikes
how long is the commute

MadRhino said:

Tires are not making suspension, because they have no negative travel. A suspension does move up with a bump, and down with a hole, while the frame tends to maintain a constant trajectory. Also, a suspension does have a damper, that slows down the rebound effect.

Click to expand...

This runs counter to every vehicle dynamics course and textbook: tires are universally modeled in series with the main vehicle suspension. Tires do have "negative travel" -- your tire contact patch deflects, and if ground contact is lost, the tire "travels" to the extent that the tire is not deflected. That "travel" is the amount your tire sags, say 10-20mm.

Rubber tires also provide (limited) damping, and indeed, rubber has been used historically and continues to be used in low-cost racing as the sole suspension element.

MadRhino said:

Tires can make a smoother ride by limiting the transfer of small imperfections of the surface to the frame, avoiding vibrations. A suspension does very little to reduce high frequency vibrations. It is not its purpose. The purpose of a suspension is to keep the wheel in contact with the ground. The faster you ride and/or the higher the amplitude of hits, the more a wheel want to bounce off the ground. There are 2 effects produced by that bouncing: loss of traction/grip, and loss of control. No tire can help keeping a bike from bouncing off the ground on a hit, or turning into a bucking horse on a series of bumps. Tires only help with vibrations. We need both, proper suspension and tires for handling and comfort, according to the speed and surface that we ride.

Click to expand...

No, there is only a clear distinction under the "cracking" force of the damper, such that only tires are providing suspension. Past the cracking force for the damper, both tire and suspension operate in series (simultaneously).
Both tire deflection and suspension keep the wheel in contact with the ground, and both help with vibrations.

JackFlorey said:

Yep. Agreed with most people here. Go with a 26 inch wheel and full suspension. It's a lot easier to increase speed than torque in a hubmotor, so the smaller the wheel the better.

If you're going with a mid drive size doesn't matter as much. But gearing becomes a bigger issue; hard to get a range you can pedal on reasonably at lower speeds and still hit ~40mph.

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Thanks Jack. The difference in this case is 9% thrust up to 12.5mph, and 0.5% top speed. I didn't think that would be worth designing around, but I'm curious what you think based on hard numbers.
Full suspension also begs for mid-drive, which would be more flexible with gearing.

Tommm said:

Get an agled headset to slacken geometry by 2'. That's what I did with an old DH bike.

Also, if you got free clearance to fit a 27.5 wheel front, you get a slacker head angle and better monster trucking ability.

Click to expand...

Thanks Tom. Thought about it, but I'd prefer to pick up that 1.5-2deg with a longer fork instead of an Angleset.

Chalo said:

Sorry, you're wrong. Cross-sectional area of the tire gives suspension, sure-- and that's partly proportional to volume. A large height of tire can help a lot, and you have to have enough air volume for that. But it isn't the total volume of air that does the work, because from unloaded to bottomed out makes almost no difference in the tire's total volume and therefore pressure.

Click to expand...

This is incorrect -- it is entirely the total volume and pressure that "does the work", and "unloaded to bottomed out" makes all the difference in the tire's total volume and therefore pressure. When you hit a bump, the tire begins deflecting, therefore the volume begins to decrease, therefore the pressure begins to increase, until the increasing pressure prevents further deflection. If tires didn't work this way, every bump would result in the wheel bottoming out and a pinch flat, and this is why larger volume tires can be run at lower pressures still sufficient to prevent such flats, as well as provide cushy ride.

What do you think prevents bottoming out? Sidewall stiffness? Surely you've seen a deflated tire to know this isn't the case...

Chalo said:

In the bicycle world, the wider but shorter you spread out a given tire on a wide rim, the worse it rides-- even though its volume increased. You have to lower the tire pressure to compensate. Bicycle tires are not the same as murder vehicle tires. They're much more flexible, and they're always bias ply.

Click to expand...

I don't think this is the consensus view. A wider rim certainly handles and corners better.

Chalo said:

Anything you can feel from the surface as you ride has a height (which is why you can feel it), and for that reason the wheel outside diameter makes a disproportionate difference in ride quality. That's why rollerblades ruin your feet quickly, but high wheeler bikes from the 19th century ride nicely enough on dirt roads, with solid rubber tires.

Click to expand...

Again, a larger diameter tire does independently improve obstacle crossover, nothing else (not that crossover isn't important -- it is) -- but the rest of this is incorrect. It isn't relevant to compare rollerblades to penny-farthings. A better comparison would be small diameter scooter tires, which do ride infinitely more nicely than 700c18 track tires.

We'll have to agree to disagree. You're applying (faulty) car thinking to bicycles, and you're wrong.

Good luck with that.

Chalo said:

We'll have to agree to disagree. You're applying (faulty) car thinking to bicycles, and you're wrong.

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There isn't anything specific to cars in my explanation of how tires work. I'm certainly curious to hear your alternative explanation, but rollerblades and high-wheel bikes don't have much explanatory power here.

fatty said:

Tommm said:

Get an agled headset to slacken geometry by 2'. That's what I did with an old DH bike.

Also, if you got free clearance to fit a 27.5 wheel front, you get a slacker head angle and better monster trucking ability.

Click to expand...

Thanks Tom. Thought about it, but I'd prefer to pick up that 1.5-2deg with a longer fork instead of an Angleset.

Click to expand...

That's even better, if you were planning on a bigger fork anyway. Make it 27.5 compatible.

fatty said:

Chalo said:

We'll have to agree to disagree. You're applying (faulty) car thinking to bicycles, and you're wrong.

Click to expand...

There isn't anything specific to cars in my explanation of how tires work.

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"A wider rim certainly handles and corners better". That's car nonsense. Cars don't lean to turn. Bikes do. That makes all the difference, and is why tall section tires are better for bikes than for murder vehicles.

It's not too complicated to calculate the volume difference between a torus with a flat spot and a torus without. That isn't much volume (pressure) difference, but it ignores the fact that the remainder of the tire expands slightly to attenuate any change in pressure. In the most extreme case (wide tire, small wheel diameter, fully bottomed out), it's still only a few percent change in pressure at most. In a normal full sized bike tire, it's negligible.

I'm not suggesting to disregard ETRTO width guidelines.
But even Schwalbe recommends the wider end of the guidelines:

Chalo said:

It's not too complicated to calculate the volume difference between a torus with a flat spot and a torus without. That isn't much volume (pressure) difference, but it ignores the fact that the remainder of the tire expands slightly to attenuate any change in pressure. In the most extreme case (wide tire, small wheel diameter, fully bottomed out), it's still only a few percent change in pressure at most. In a normal full sized bike tire, it's negligible.

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I'm not clear on what our argument is here. The decrease in volume and concomitant increase in pressure is enough to resist further deflection and prevent rim strike and pinch flat.

If you disagree and believe that a mechanism other than pressure is supporting the tire from bottoming out, I'd love to read it clearly explained. We've already ruled out sidewall stiffness, so I'm at a loss as to what you believe occurs.

"Wider tires help protect against pinch flats because they contain more protective air volume."