Why might a material have more traction on a smooth surface?

Thread Starter

strantor

Joined Oct 3, 2010
6,815
I am looking for the name of something so that I might do some research about it. I suspect it is an "effect" or "law" or "principle" that is well known but I am not googling the right words. I will try to describe it.

I tried 3D printing tires for a robot with TPU. The TPU tires were terrible. On my smooth (porous, unsealed) concrete shop floor they had no traction, it was like Robots On Ice. Same on wood, on rough concrete, on tile, on textured linoleum, and most other surfaces. The rougher the surface the less traction. But on smooth painted steel and smooth powder coated steel, they had great traction. Think what is happening is that, with a normal rubber tire, the tread is finding purchase on features of the "road" which are of observable size. For example a rubber tire would have excellent traction on coarse sand paper, and that's why skateboards are covered in it. But the TPU for whatever reason would just slide over the top of that same sandpaper, not finding purchase. However on a smooth surface, the TPU does find purchase, but on a microscopic level. I think it is similar to how machinists' gauge blocks "wring" together (quick video demo if you don't know what I mean) but it doesn't seem like entirely the same thing. If I had to make up a term for it, I would go with "micro stiction" or something. The closest thing I found so far is Van Der Waals forces but I don't think it's really that either.

I feel like I'm missing something, just one elusive word away from the Googling the right thing. Anyone know the right word(s)?
 

panic mode

Joined Oct 10, 2011
2,818
rubber is soft and deforms. as a result contact of cylindrical tire on a smooth surface is not just single thin line but rectangular area. the larger the area, the better grip (more tractions).
TPU is just too hard. it does not deform nearly enough to create contact with sufficient area. you may consider changing shape from round to something that has number of flat faces. then with even small movement (begin of the motion) larger contact area can be possible to give you initial acceleration. as the wheels spin up faster, you will loose that advantage. or try making groves in tires. this reduces tire surface area but... it also allows tire to deform at the surface and hug the pavement....

something like this
1713233069703.png
 
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LowQCab

Joined Nov 6, 2012
4,218
"Roughness" is a widely, hugely, variable thing.
There's no such thing as a truly "smooth-surface",
it just depends on how much magnification You use to look at it.

You can't "print" Tires that are worth messing with,
but You can Print "Wheels" and then wrap them in a piece of very-soft-stretchy-Silicone-Tubing,
( previously known as "Surgical-Rubber-Hose" ),
then You might start generating enough traction to get some real usable action.
.
.
.
 

Thread Starter

strantor

Joined Oct 3, 2010
6,815
rubber is soft and deforms. as a result contact of cylindrical tire on a smooth surface is not just single thin line but rectangular area. the larger the area, the better grip (more tractions).
TPU is just too hard. it does not deform nearly enough to create contact with sufficient area. you may consider changing shape from round to something that has number of flat faces. then with even small movement (begin of the motion) larger contact area can be possible to give you initial acceleration. as the wheels spin up faster, you will loose that advantage. or try making groves in tires. this reduces tire surface area but... it also allows tire to deform at the surface and hug the pavement....

something like this
View attachment 320037
Thanks, yeah the first thing I tried was holes like you drew and it still wasn't great. A few revisions later this is what the tire looks like:

1713234451867.png

It's got a more-than-ample contact patch now and the traction on smooth surfaces is great, which is a good thing since that's the kind of surface it's intended to operate on. But I really think there is something else going on here apart from the size of the contact patch. There is something specific to the material (any material) which seems to matter, beyond just how much of it is contacting the surface.
 

Thread Starter

strantor

Joined Oct 3, 2010
6,815
"Roughness" is a widely, hugely, variable thing.
There's no such thing as a truly "smooth-surface",
it just depends on how much magnification You use to look at it.
Right, that's what I think is at play here.
The painted surface is "smooth" to the touch (but "rough" on a microscopic level) and the TPU tire is also "smooth" to the touch (but "rough" on a microscopic level). Therefore, these two smooth surfaces have a high tractive force between them.

But a rubber tire with a "smooth" surface has great traction on whatever surface, "smooth" to the touch, or "rough" to the touch.

What's the name for whatever is the cause of this?
 

atferrari

Joined Jan 6, 2004
4,784
I feel like I'm missing something, just one elusive word away from the Googling the right thing. Anyone know the right word(s)?
Something combined with "friction"?

Staying inside the hold of a bulk carrier fully loaded with rebars I was called urgently from someone on main deck. Trying to run on the surface of that stow wearing common safety shoes proved to be literally impossible. Not sure if grip is the right word but it was evidently excessive.

Later, while watching on TV an ad from maybe Adidas (?) focusing on the sneakers of Usain Bolt (?) while racing, you could see, in slow motion, that they seemed to slide a little at every step.

One night while driving during the tormenta de Santa Rosa (end of August) at 120 Km/h, I suffered a spectacular accident with my car in perfect aquaplaning where grip was obviously zero.
 

Ya’akov

Joined Jan 27, 2019
9,237
Thanks, yeah the first thing I tried was holes like you drew and it still wasn't great. A few revisions later this is what the tire looks like:

View attachment 320040

It's got a more-than-ample contact patch now and the traction on smooth surfaces is great, which is a good thing since that's the kind of surface it's intended to operate on. But I really think there is something else going on here apart from the size of the contact patch. There is something specific to the material (any material) which seems to matter, beyond just how much of it is contacting the surface.
I know this is extremely counterintuitive, but in fact friction is proportional to the force applied normal to the surfaces and independent of the size of the contact area. If you haven’t looked into “the laws of friction”, it’s worth doing.
 

panic mode

Joined Oct 10, 2011
2,818
from Physics, for static friction for example, force is F=u*N
where u is friction coefficient (material dependent)
and N is the normal force (weight)

if the weight of the vehicle does not change, N is constant.
so the coefficient of friction is the only thing to play with.

but the things are a bit more complex, which is why racing cars use very wide tires.
contact area does matter.
 

BobTPH

Joined Jun 5, 2013
9,174
I know this is extremely counterintuitive, but in fact friction is proportional to the force applied normal to the surfaces and independent of the size of the contact area. If you haven’t looked into “the laws of friction”, it’s worth doing.
Yes, consider spikes on ice. Low surface area, great friction.
 

BobTPH

Joined Jun 5, 2013
9,174
TPU is just too hard
TPU is very soft and deformable.

Thermoplastic polyurethane (TPU) filament is a 3D printing material that has rubber-like elasticity, tear and abrasion resistance, and durability. It's also known for its high tensile strength, which makes it suitable for printing objects that require durability and strength, such as gears or mechanical components.
 

Thread Starter

strantor

Joined Oct 3, 2010
6,815
I know this is extremely counterintuitive, but in fact friction is proportional to the force applied normal to the surfaces and independent of the size of the contact area.
Yes I have heard it and seen the demonstrations of objects with equal mass but unequal surface area sliding down a ramp at the same speed, so I know it is "true" in that it can be demonstrated in a controlled environment but I think that in the real world surface area does matter (see earlier mention of dragsters deflated tires, also offroad racing with deflated tires), and the reason why it does, is one of the things I hope to learn. I think this is a topic with a lot of caveats and this thread is born of my obsession with just one of them.
If you haven’t looked into “the laws of friction”, it’s worth doing.
Thanks, that gave me a lot of reading material.
 

Ya’akov

Joined Jan 27, 2019
9,237
from Physics, for static friction for example, force is F=u*N
where u is friction coefficient (material dependent)
and N is the normal force (weight)

if the weight of the vehicle does not change, N is constant.
so the coefficient of friction is the only thing to play with.

but the things are a bit more complex, which is why racing cars use very wide tires.
contact area does matter.
Wider tires affect traction which while related to friction and dependent on it is not the same thing. Wider tires offer advantages in maintaining traction when undergoing dynamic forces and become less important as those forces are reduced.

Ordinary cars have relatively narrow tires, and race cars relatively wide ones because of the magnitude of the dynamic forces they must operate under.
 

Thread Starter

strantor

Joined Oct 3, 2010
6,815
".....which makes it suitable for printing objects that require durability and strength, such as gears or mechanical components."
Aren't those two statements contradictory?
Yes they are. That is not a great explanation of TPU. I would not use TPU for gears or anything like that (unless maybe for torque limiting via intention gear deformation and tooth skipping). It's like slippery hard rubber.
 

Ya’akov

Joined Jan 27, 2019
9,237
Yes I have heard it and seen the demonstrations of objects with equal mass but unequal surface area sliding down a ramp at the same speed, so I know it is "true" in that it can be demonstrated in a controlled environment but I think that in the real world surface area does matter (see earlier mention of dragsters deflated tires, also offroad racing with deflated tires), and the reason why it does, is one of the things I hope to learn. I think this is a topic with a lot of caveats and this thread is born of my obsession with just one of them.

Thanks, that gave me a lot of reading material.
As I answered above:

Wider tires affect traction which while related to friction and dependent on it is not the same thing. Wider tires offer advantages in maintaining traction when undergoing dynamic forces and become less important as those forces are reduced.​
Ordinary cars have relatively narrow tires, and race cars relatively wide ones because of the magnitude of the dynamic forces they must operate under.​

As the dynamic forces subside the ideas of traction and the friction it depends on converge. However, even at low speeds things like climbing slopes, which may tend to deform the tires leading to slipping, can make a wider tire more likely to keep some contact with the surface. This is dependent on the composition of the tire material as well.
 

Ya’akov

Joined Jan 27, 2019
9,237
Yes they are. That is not a great explanation of TPU. I would not use TPU for gears or anything like that (unless maybe for torque limiting via intention gear deformation and tooth skipping). It's like slippery hard rubber.
TPU is exceptionally tough. But this is not necessarily what people expect it to be. Toughness is generally in opposition to stiffness which is different. TPU is (generally) not stiff and so, for example, impact and abrasion will have less effect on the very stiff PLA, which is far less tough.

I do have some TPU that is high durometer stuff and is pretty impressive. Still quite tough but much stiffer than you‘d imagine TPU can be. [datasheet] This is a test print from the filament, it’s more difficult to print than PLA but not as difficult as some low durometer TPU, and the results of a good print are excellent.

1713277203573.jpeg 1713277070386.jpeg
 

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GetDeviceInfo

Joined Jun 7, 2009
2,198
I used to spend a lot of time at the race track, and was always amazed at how a wide wrinkle wall tire suddenly became a narrow band when the lights turned green. Ever drive a dual wheel in winter, no fun.

I wouldn’t over think it. Frictional forces can be determined through testing. We used to drag objects with a spring gauge. The larger concern in real world applications, is maintaining the intimacy between the materials. The force between materials, which maintains that intimacy, directly affects resultant frictional forces. The plasticity of a material would certainly have an effect in a dynamic environment. In a comparable application, I would look at a simple drag test to gauge friction, then look at the vehicles abilities to keep the tires in constant contact with the surface. Tire balance and roundness likely have a larger influence.
 
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Thread Starter

strantor

Joined Oct 3, 2010
6,815
TPU is exceptionally tough. But this is not necessarily what people expect it to be. Toughness is generally in opposition to stiffness which is different. TPU is (generally) not stiff and so, for example, impact and abrasion will have less effect on the very stiff PLA, which is far less tough.

I do have some TPU that is high durometer stuff and is pretty impressive. Still quite tough but much stiffer than you‘d imagine TPU can be. [datasheet] This is a test print from the filament, it’s more difficult to print than PLA but not as difficult as some low durometer TPU, and the results of a good print are excellent.

It is ironic that you should post this because I have been looking for some >95A TPU. Also ironic that the product which is marketed as high durometer material, has no durometer specified in the datasheet. Have you measured the durometer? Or have an intuitive estimate? Even "acts like Nylon" or "bends like balsa wood" will be good info.
 

Ya’akov

Joined Jan 27, 2019
9,237
It is ironic that you should post this because I have been looking for some >95A TPU. Also ironic that the product which is marketed as high durometer material, has no durometer specified in the datasheet. Have you measured the durometer? Or have an intuitive estimate? Even "acts like Nylon" or "bends like balsa wood" will be good info.
It is reminiscent of PETG but with a more durable feeling surface. When I get a chance, I will compare it to some other polymers and give you some idea. Let’s say, though—it is not rubbery. I really like TPU, even if it is a pain to print it‘s so durable and can conform in various ways. I find a lot of occasions to print TPU in vase mode and make very tough, thin, parts to act as caps and the like.
 
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