Roller stability?

9606 said:

Which is more (or less) stable, the front axle (skewer) slightly forward of, in front of the front roller axle; or the front wheel axle slightly behind the front roller axle.

Stable from a steering (lateral) perspective or stable from a not bouncing off the front (fore-aft) perspective?

9606 said:

We all have hunches and guesses, does someone have the correct answer?

The *correct* answer probably depends upon the fore-aft weight distribution of the particular rider. Why not just give it a try?? 😎

9606 said:

A misc question on physics and dynamics, just downtime chatter at our LBS:

Which is more (or less) stable, the front axle (skewer) slightly forward of, in front of the front roller axle; or the front wheel axle slightly behind the front roller axle.

We all have hunches and guesses, does someone have the correct answer?

Thanks

Would you really want to be on a set of rollers where the front roller is behind the contact point of the front wheel? Personally, I'd take solice in having two rollers (one front and back) arresting my forward motion...

The correct answer would be - don't place the roller directly underneath the center of the wheel.

frenchyge said:

Stable from a steering (lateral) perspective or stable from a not bouncing off the front (fore-aft) perspective? - Either, both. How would the answer change?

The *correct* answer probably depends upon the fore-aft weight distribution of the particular rider. Why not just give it a try?? 😎

Becasue it is a pain in the ass to change the roller wheel position. Assume neutral weight distribution.

swampy1970 said:

Would you really want to be on a set of rollers where the front roller is behind the contact point of the front wheel? Personally, I'd take solice in having two rollers (one front and back) arresting my forward motion... Most rollers aren't made that way.

The correct answer would be - don't place the roller directly underneath the center of the wheel.

Why not? Most rollers are set up this way.

The original question is more a thought experiment. Just idle chatter at the LBS.

9606 said:

Assume neutral weight distribution.

Roller in front of the fromt wheel is more stable. With 50/50 weight distribution if you put the roller behind the front wheel then the rider's weight will cause the rear wheel to climb the roller and they will ride off the front of the set.

For today's brain teaser: if the rider is doing 60kph on the rollers when they ride off the front, how fast will they be travelling when they hit the wall 10 feet in front of the rollers?

Bonus: what is the length of the tire burnout streak left on the cement floor (assume Conti 4000's)?

9606 said:

Why not? Most rollers are set up this way.

I don't think so. Kreitler has always recommended that the front axle be ~1/2" behind that front roller axle.

Dave

as others have said you don't want to put the front wheel's axle in front of the axle of the front roller or you are likely to ride off the front of the rollers.

this can also happen if you don't ensure that the height of both axles on your bike are the same height off the ground.. if the front wheel is significantly higher you can bound off the back..

anyway, i say that having the wheel in front of the roller is less stable because i find it hard to control my bike as i'm riding off the front of my rollers and skidding all over the ground...

frenchyge said:

Roller in front of the fromt wheel is more stable. With 50/50 weight distribution if you put the roller behind the front wheel then the rider's weight will cause the rear wheel to climb the roller and they will ride off the front of the set.

For today's brain teaser: if the rider is doing 60kph on the rollers when they ride off the front, how fast will they be travelling when they hit the wall 10 feet in front of the rollers?

Bonus: what is the length of the tire burnout streak left on the cement floor (assume Conti 4000's)?

ANSWER - you will never hit a wall 10' away.. 10 inches maybe..

i've had a roller pulley bounce off and jam and ridden off the front of my rollers before.. and can confirm that because of the small amount of inertia provided by the rollers and bicycle wheels you only travel a foot maybe two feet travelling maybe 2-3 km/hr at the most..

doctorSpoc said:

ANSWER - you will never hit a wall 10' away.. 10 inches maybe..

i've had a roller pulley bounce off and jam and ridden off the front of my rollers before.. and can confirm that because of the small amount of inertia provided by the rollers and bicycle wheels you only travel a foot maybe two feet travelling maybe 2-3 km/hr at the most..

Alright...... since you got this one, you have to pose the next question. :p

doctorSpoc said:

anyway, i say that having the wheel in front of the roller is less stable because i find it hard to control my bike as i'm riding off the front of my rollers and skidding all over the ground...

Maybe you should look into a trainer 😉

Dave

With the wheel axle directly over the roller axle, the system is more unstable than with axle before or after the roller axle. In that position, the system has more potential energy, while with the wheel axle not over the roller axle, the system has lower potential energy. There is no absolutely stable position, but the positions with lower potential energy have greater relative stability.

You can also see that with the wheel axle before or after the roller axle, the front wheel will have more trail, meaning greater stability via greater trail. With this in mind, the position with the wheel axle in front of the roller axle should be a bit more stable because it will have greater trail than when the wheel axle is behind the roller axle. Note stability here is in terms of steering stability.

There are other factors that impact overall stability, but I think the two mentioned are dominant......or so I'm assuming. Of the two, I'm going to guess that steering stability is dominant, but of that'd depend on the forces and energy required for the rear wheel to climb "out of its well". I didn't draw any force diagrams or solve any equations, so I reserve the right to be wrong.

I don't think trail applies when there's no forward motion of the forks relative to the contact patch to pull the wheel back in line. I'd guess that's why tighter input is required to keep the bike on rollers upright.

frenchyge said:

I don't think trail applies when there's no forward motion of the forks relative to the contact patch to pull the wheel back in line. I'd guess that's why tighter input is required to keep the bike on rollers upright.

Ah, but the wheel and bike don't know they're not going anywhere. The wheels still turn and are tracking the surface beneath them. The only thing missing is relative wind. You can also see this by choosing different frames of reference. If the bike is the frame of reference, there is a surface passing under the wheels as they rotate. If a point on a roller is the frame of reference--just like having an observer by the road as you ride by--then the the wheels are riding past that point of reference. You can exchange any of them, and the laws of motion still apply the same.

Now there is a difference in friction between the tire and the roller and the tire and a street, with the roller providing less friction, generally, than the street. As a result, you can turn the wheel, while on the rollers, and have the wheel pivot/slide as opposed to tracking in an arc as on the road. That, I think, is a big contributor to instability on rollers. I also wonder what the differences in shape of the contact patch between road and roller does. The contact patches are definitely smaller since the load (weight) at each wheel axle isn't perpendicular to the contact patch.

Lastly, perhaps another huge issue is the bike's balance. On the ground, with a rider balanced such that the bike is perfectly vertical, there's no moment on the wheels/tires (in a perfect model). On the rollers, though, weight of the rider/bike combo introduces a moment about each wheel axle. This is just hypothesis. Do people do trackstands on rollers? I don't know. If so, are they harder than when they're done on the ground?

alienator said:

Ah, but the wheel and bike don't know they're not going anywhere. The wheels still turn and are tracking the surface beneath them.

Yeah, after some sleep and a clear head I realized you're right about that. :o

I don't think wheel in front of roller is really an option because the contact patch moves forward as you turn the bars, which means the front wheel would want to fall further down the roller with the rider's weight behind it. It would be harder to get the wheel back underneath the rider once this occurred as they'd have to lift their weight by straightening the bars, or pull the bike forward.

With the wheel behind the front roller, as you turn the bars the wheel climbs the front roller slightly and the rider's weight is trying to pull it back down to where it was, helping straighten the wheel/bars in the process. The steering is still somewhat self-righting.

Handling aside, I think you need that little lip to help offset the forward drive of the rider against the resistance of the rollers and pulleys. Are there any fans or resistance units which drive off the front pulley? If so, it seems like those would generate a lot more resistance against which the bike could climb over the rollers.

frenchyge said:

Yeah, after some sleep and a clear head I realized you're right about that. :o

I don't think wheel in front of roller is really an option because the contact patch moves forward as you turn the bars, which means the front wheel would want to fall further down the roller with the rider's weight behind it. It would be harder to get the wheel back underneath the rider once this occurred as they'd have to lift their weight by straightening the bars, or pull the bike forward.

With the wheel behind the front roller, as you turn the bars the wheel climbs the front roller slightly and the rider's weight is trying to pull it back down to where it was, helping straighten the wheel/bars in the process. The steering is still somewhat self-righting.

Handling aside, I think you need that little lip to help offset the forward drive of the rider against the resistance of the rollers and pulleys. Are there any fans or resistance units which drive off the front pulley? If so, it seems like those would generate a lot more resistance against which the bike could climb over the rollers.

Kreitler has--or used to make--a Killer Headwind fan for their rollers. I dunno about anyone else.

After I thought a bit more, I realized that the tire in front of the roller would have less trail than it would on level ground, so essentially everything points to the bike being more stable with the tire behind the roller.

That was an interesting question, OP.

alienator said:

After I thought a bit more, I realized that the tire in front of the roller would have less trail than it would on level ground,

I'm using this as a reference to my understanding, and if it's correct then I don't see how your statement could be so.

frenchyge said:

I'm using this as a reference to my understanding, and if it's correct then I don't see how your statement could be so.

That's because it isn't so. I said it backwards. I had it right last night, then edited it, apparently to make it wrong. [censored] and engineering don't always mix.

alienator said:

[censored] and engineering don't always mix.

Let's just hope the folks designing safe and effective marital aids are better at multi-tasking. 😄

It seems to me the trail is the same in all three scenarios - axles aligned, wheel in front, wheel in back.
As the axles(plumb line) moves, so too does the projected line from the fork blade.