After a couple of years of more or less daily riding, I need to replace the 24" x 2.5" Hookworm on my Nimbus. But when I went to order one, I saw there two types available. One was “Wire beaded 60a,” and the other was “Wire beaded 70a.” What is the difference, and which to get for road riding?
in theory, 70a could probably roll a bit easier on the road. However, the other factors - in particular the thread count tpi - must also be taken into account. 120tpi tires are much smoother than 30tpi. The structure of the carcass etc. are also factors.
It’s about rubber hardness. Harder = less abrasion, less resistance, less grip; softer = more abrasion, more resistance, more grip.
For road riding the harder version would be the obvious choice, thus 70a.
The Hookworm’s major advantage is the bead to bead tread which makes it nearly indestructible. However it is notoriously susceptible to camber which must be dealt with by maximum inflation pressure, when it has almost no traction on smooth wet surfaces. It is a very heavy tyre.
I run a Maxxis DTH 24 x 1.75 on my Torker (32 mm wide rim). It is an amazingly responsive uni because the tyre is so light at 440 grams which is far less then half the weight of the Hookworm. The grip is great.
Another high performance uni road/track tyre worth considering is the Maxxis Torch.
I have one on my 29.
Thank you all. That of course raises more questions.
- It sounds like some people prefer lighter tires. Are there other opinions? I thought that there would be an advantage in a heavier tire. Greater rolling inertia to smooth travel over uneven surfaces, and greater gyroscopic stability. I am not worried about the slower acceleration.
- It never occurred to me that the TPI weave would make a difference. In what way is that noticeable?
I’d call the rotational inertia an underappreciated feature of unicycle wheels.
It helps you to keep rolling over bumps and surface irregularities, it gives you gyroscopic stability on one axis and it buys you time to correct your own imbalance on the other axis.
2 Likes
This is is very apparent on my Nimbus 29” Road uni with 114mm cranks. Not as much on my Oracle 27.5” (same diameter tire as the 29”), but I think this is because I still have the cranks set at 150mm.
The inertial benefits depend on the scale. Small bumps benefit from the inertia but sometimes the wheel needs to be rapidly accelerated for a change in terrain such as when encountering a kerb ramp. A lighter wheel can be put into position underneath you faster. It depends on the rider’s technique. Personally I like a very responsive wheel.
Gyroscopic effects are not that important in a unicycle as they are small compared to the destabilising forces of the dominating off axis pedaling forces.
A finer weave makes a tyre more flexible. A more flexible tyre is less prone to camber effects. The rigidity of the Hookworm due to the bead to bead tread is why it is so horrid in this aspect on large wheels.
The roll over is effect of the heavy Hookworm is good in a 20 inch uni where there is so little radius. On a small wheel, the contact patch is short enough to not cause camber problems so the rigidity of the tyre isn’t a problem.
You say that a more flexible tire is less prone to camber deviation. For some reason I got the idea that you can reduce the influence of camber by increasing tire pressure, thus making it less flexible. So which of the two conjectures is the true one?
I tend to think that the flexibility of the tire casing has nothing to to with the camber effect because:
- When you think of the weight of the rider and pressure in the tire the flexing of the tire is a very small part of what is happening.
- On my 36er uni’s I have one with a very stiff Coker non-skid tire and one with a more flexible Nightrider and the Nightrider is way more sensitive to camber then the Coker. The flexibility of the casing does not seem to affect the camber sensitivity.
- The very same tire can react completely differently to camber only based on the amount of friction between the tire and road. A Nightrider tire can be very sensitive to camber on a dry high friction surface but the very same tire can have little or no effect on the same surface when wet or even icy. The tire flexibility does not change.
- Higher the air pressure in the tire and smaller contact area reduces the camber effect. At the same time the flexibility of the un-inflated tire casing does not change.
I think that one reason it is easier to maintain left-right balance at speed in due to the gyroscopic effect. If you hold a uni off the ground and spin the wheel, then lean the uni to the left or right the whole uni tends to turn in the direction of the lean with quite a but of force. This automatic turning is due to the gyroscopic effect of the rotating wheel. Try it.
1 Like
The increased pressure decreases the length of the contact patch as well as the grip. Both those effects reduce the camber effect.
1 Like
I pretty much agree with JimT regarding camber.
“Camber steer” is fascinating and is the reason I re-spoked my nimbus 32 to a 29er.
If you search google scholar you should find a relevant paper entitled:
“Characterization and Modelling of Various Sized Mountain Bike Tires and the Effects of Tire Tread Knobs and Inflation Pressure”.
That paper is engineering measurement based, rather than pure mathematical and has a few tables where twisting torque is graphed against camber angle (and there is some data on a hookworm tire).
My own experience, combined with reading the paper above, is that it is all about the contact patch area (with a wide patch being worse), combined with the friction between the patch and the road. So knobby tires and/or gravel (or wet) road have less friction (even though knobbies have more traction on dirt) and less camber effect. I believe that flatter profile tires may be worse. And I know from experience (as do most riders, I guess) that higher pressure reduces the camber effect.
So with the hookworm, the harder compound will have less friction (so slightly less camber effect than the softer compound as well as being more long lasting), but due to its smooth tread the hookworm has a larger contact patch (so more camber effect, but at least it won’t get worse as the tire wears down).
But maybe it is an open question what effect a “flexible rubber” (definition needed) will have on camber effect when all the other variables are held constant.
Come to think of it, it is an open question about the physics of how a unicycle can be controlled (and even bicycle riding physics is tricky!).
1 Like
The camber effect is due to the sideways forces of the tread being forced up the camber from the centreline circle as it traverses the contact patch. Lower grip will let it slip at lower forces. Higher inflation reduces both the grip and the length of the contact patch and thus the camber effect.
All else being equal, a tyre that doesn’t easily deform sideways will maintain a higher sideways force on camber. A tyre with flexible carcass can still deform sideways even at high pressures. A thick circular profile like the Hookworm won’t flex much sideways at all. A knobbly tyre will isolate the thrust into each knob, slipping and releasing multiple times as they traverse the contact patch.
There are multiple components affected by the camber thrust. Although a unicycle doesn’t have a mechanical steering pivot like a bike, it does have a dynamic incredibly complex steering axis. Leaning the uni backwards slightly increases stability, partly for exactly the same reasons that a bicycle forks put the contact point behind the steering axis. (In fact bicycles have been built with counter-rotating pairs of wheels wheels to eliminate gyroscopic effects and were found to be stable as long as there was some trail of the front wheel contact patch behind the steering axis.)
Camber thrust in front of the steering axis will steer the wheel up the camber while thrust behind it will steer the wheel down the camber. The combination of these forces can be altered by changing the lean. Leaning the unicycle backwards (by leaning the body forwards slightly) will put more more of the thrust behind the steering axis compared to being more upright.
Other factors involves the camber pushing the tyre up the slope while the tyre slides down the slope under the influence of gravity … The key to dealing with camber is balancing all these forces.
Adjusting the angle of the uni is the main thing that the rider can do. Leaning the uni so it is perpendicular to the camber is a common solution. (You counterlean your body to maintain balance.) The back lean can change the balance of thrust behind and in front of the steering axis. You can;t do much about gravity.
Now add what you can do with sideways forces on your feet.
Like pretty much everything involving unicycling, the analysis of the physics quickly leads to brain overload so we usually just get out and do it. Just keep trying different things. until it works and don’t worry too much about what is happening.
2 Likes
A family friend has more than a lifetime of experience racing motorcycles (his father was also a motorcycle racer). What he doesn’t know about motorcycle geometry doesn’t need to be known. He once built a frame with an adjustable steering axis rake so he could analyse how it affected the bike with everything else the same.
We spent an evening discussing bikes and unicycles. Next morning he said he had laid awake for hours trying to come to terms with the dynamics of a unicycle and it “did his head in”.
1 Like
On a uni I tend to believe the camber effect is a steering or tuning effect and not a sideways force. The camber effect on a unicycle is different then any vehicle with more then one wheel. Here is what I’m thinking:
Jim,
Your logic is impeccable. But if that is how things stand, why do I have to lean toward the crown of the road to correct for the pull into the gutter?
That is because as you lean, you are actually allowing your unicycle to stand up straighter, or more perpendicular to the road surface.
As you lean more, the bases of the truncated cones (using the example provided by @JimT above) will eventually touch the road surface.
If you lean too far, then the reverse happens, and “reverse camber” applies AND the effect can feel like you are suddenly running downhill into the gutter.
1 Like
Because when you lean toward the crown, the wheel leans the opposite way and becomes more perpendicular to the road surface. Just like when standing on the ground, if you lean over to the side with your head and shoulder, your hips and waist moves the opposite way. If the wheel is perpendicular to the surface, there is no turning or steering effect.