Rear wheel out of alignment

[Don in Lodi]

Not having the front and rear tires on the same alignment does cause some "scrubbing" although I agree that it would have to be a major mis-alignment to affect tire life. Any mis-alignment will cause the bike to want to continually turn and to counter this and make the bike go straight, the rider has to feed in some opposite bar pressure to counter the turn. To a lesser degree, its identical to the cross-control inputs you see in a power-slide and does cause the tires to slighlty scrub sideways across the road.



- Mark

Wouldn't the steering just be crooked, because both tires have to be pointed in the same direction. With three or more tires in contact with the ground, all sorts of wear stuff can happen, but with two, everything is self centering. Yes, the ass end could be crabbed side ways visibly, but both tires are pointed in the same direction. Neither tire has any need to push itself into the other tire's track, they're both pointed the same way.

 

[scott123007]

Don, I'm giving YOU the nod on that one. The example Mark is showing is no good. That back tire is "scrubbing" in that picture whether the front wheel is on the ground or not.

 

[markjenn]

Wouldn't the steering just be crooked, because both tires have to be pointed in the same direction. With three or more tires in contact with the ground, all sorts of wear stuff can happen, but with two, everything is self centering. Yes, the ass end could be crabbed side ways visibly, but both tires are pointed in the same direction. Neither tire has any need to push itself into the other tire's track, they're both pointed the same way.

When the two tires are not inline, the bike will tend to turn. To counter the turn, you have to turn the other way. Then the two tires are not rolling in the same direction anymore. The bike is in a constant slight turn and whenever you turn the bike, there is a slight scrubbing action.

My example of a powerslide is illustrative because whether the back tire is out of line due to power application or due to misalignment, the point is that it wants to track out of line. And this causes the bike to want to turn. And since the rider wants the bike to go in a straight line, you have to counter this turning tendency. Whether the out of line condition is rider induced with the throttle or alignment induced, it still requires the rider to turn the bike the other way to counter it.

If you fly airplanes, an analogy would be the way you have to cross-control an airplane in a crosswind. And when you cross-control, you create extra drag. This is analogous to the extra drag caused the the tire scrubbing slightly sideways. Any time a tire is forced to move in a direction that is not directly in its rolling path, it goes through a slip angle and this causes extra wear. And any tire that is turning goes through a slip angle where the rolling path and the actual path are not perfectly aligned.

- Mark

 

[jaeger22]

Wouldn't the steering just be crooked, because both tires have to be pointed in the same direction. With three or more tires in contact with the ground, all sorts of wear stuff can happen, but with two, everything is self centering. Yes, the ass end could be crabbed side ways visibly, but both tires are pointed in the same direction. Neither tire has any need to push itself into the other tire's track, they're both pointed the same way

Actually No. Or at least I sure don't think they do. The front wheel is self centering but not the back wheel. If it is not in alignment with the front wheel it will as you say, crab off to the side but also point to the side and scrub the tire and increase wear. It is only a matter of degree. To see this more clearly, imagine it in extreme. In your minds eye, mount the rear wheel off 45 degrees and take it for a ride. Now does the rear tire follow he same track as the front? How is it wearing? ???

 

[markjenn]

Actually No. Or at least I sure don't think they do. The front wheel is self centering but not the back wheel. If it is not in alignment with the front wheel it will as you say, crab off to the side but also point to the side and scrub the tire and increase wear. It is only a matter of degree. To see this more clearly, imagine it in extreme. In your minds eye, mount the rear wheel off 45 degrees and take it for a ride. Now does the rear tire follow he same track as the front? How is it wearing? ???

Well to play devil's advocate, what if the front tire were also turned the exact same 45-deg? Wouldn't the bike simply go down the road with the center-line of the bike 45-deg cocked from the path of travel?

I think what is throwing everyone is that a two-wheel track vehicle turns by leaning and has to always be in balance vertically or it will fall over. You could conceive of a bike with both wheels identically cocked to one side with enough weight imbalance that exactly matched the mis-alignment such that the bike stays upright and doesn't tend to turn. But the CG of the bike would have to be offset exactly the right amount. This is the reason most riders compensate for mis-alignment by shifting weight to one side - this is easier than applying constant pressure on the bar.

Perhaps if you totally compensate for the turning tendency of mis-alignment with weight shift, there is no scrubbing. But then you'd get the increased wear of the tire being constantly in a slight lean.

- Mark

 

[Don in Lodi]

When the two tires are not inline, the bike will tend to turn. To counter the turn, you have to turn the other way. Then the two tires are not rolling in the same direction anymore. The bike is in a constant slight turn and whenever you turn the bike, there is a slight scrubbing action.


- Mark

We'll just have to disagree I guess. Countering the turn caused by a crooked rear end by steering the same way the rear is pointed means that both tires are then going the same direction, straight down the road. They have to be pointed the same way, not scrubbing, if the bike is going straight. The chassis may point elsewhere, bars may be crooked, but you're going straight.Two degrees pointed out on the right rear, turn bars two degrees right, bike goes straight, tires are parallel, not in the same track. Your photo shows what it would look like, LOL, without the lean, and if the rear tire was pointed in the direction of travel instead of sliding, the steering gets turned in the direction of rear wheel travel, and everything is parallel again. They have to be straight or parallel to go in a straight line, chassis won't point centered in the lane, but the wheels will. No extra wear.
Turning with a toe issue on the rear is a whole new game, kind of like stuck two wheel steering.
Back to something in the front end. ;D

 

[jaeger22]

Well to play devil's advocate, what if the front tire were also turned the exact same 45-deg? Wouldn't the bike simply go down the road with the center-line of the bike 45-deg cocked from the path of travel?

Interesting thought Mark. But now add acceleration or drag. If you decelerate, would not the rear wheel tend to drag behind? And would it not be at an angle to the line of travel at that point? Also the front wheel has castor so is always self centering unless you hold it off. So unless you go down the road pushing and or pulling the bars, you are back to the front straight and the rear at an angle.
I once followed a friend that had just changed a rear tire and I noticed his rear wheel tracking out of align. I could see it. I stopped him because something had to be WAY out. Sure enough, the chain adjuster marks were way way off. We adjusted it straight by the sight method, and it was back to normal. He didn't even notice when he was riding it but I bet he would have he he had taken his hands off the bars. And now that I think about it, his first tire wore out very quickly. Like 4K miles on his KLR. His second tire is still going strong with a lot more miles.
Turing is a more complicated case for sure but for these examples I think is clearer to imagine the simpler straight down the road case. That is messy enough! : I have to go rest now, my brain hurts.

 

[markjenn]

We'll just have to disagree I guess. Countering the turn caused by a crooked rear end by steering the same way the rear is pointed means that both tires are then going the same direction, straight down the road. They have to be pointed the same way, not scrubbing, if the bike is going straight. The chassis may point elsewhere, bars may be crooked, but you're going straight.Two degrees pointed out on the right rear, turn bars two degrees right, bike goes straight, tires are parallel, not in the same track.

As I said in another post, it may be theoretically possible to build a bike that would crab down the road like this, but I think if the bike is laterally balanced with the wheels aligned, it will have a turning tendency in this crabbed attitude.

I just don't think that with a single-track vehicle that turns by leaning you can just assume that the front wheel can be turned to match the misalignment in the rear and the bike will go straight. There are other forces at work.

Interesting discussion.

- Mark

 

[markjenn]

But now add acceleration or drag.

I think if you start including transient behavior, everything goes out the window and all sorts of new problems arise that could lead to different aspects of tire scrubbing. The problem is hard enough to get wrapped around in steady-state, I'm not even going down this road.

- Mark

 

[Don in Lodi]

As I said in another post, it may be theoretically possible to build a bike that would crab down the road like this, but I think if the bike is laterally balanced with the wheels aligned, it will have a turning tendency in this crabbed attitude.

I just don't think that with a single-track vehicle that turns by leaning you can just assume that the front wheel can be turned to match the misalignment in the rear and the bike will go straight. There are other forces at work.

Interesting discussion.

- Mark

Torque; acceleration, decelleration would make the rear tire either want to squirt out from under the bike, or tuck under the bike. A pull like effect left or right depending. A couple tenths of a degree though, dunno. A few tenths split of caster/trail-rake now...
How does one confirm trail and rake numbers without some sort of jig?