Tyre slip angle question

[raymondu999]

From what I've understood based on stuff I've read from this forum, slip angle is generally basically the slides a tyre does across the asphalt, at an almost-microscopic level. In laymen terms, at least.

I read something earlier on about tyre slip angles, which really confused the hell out of me as I know contend with two different definitions. Perhaps someone here can enlighten me more?

Here is what I read:

The tyre slip angle results from the tyre being elastic. The centripetal acceleration of the car distorts the tyre around the contact patch. The contact patch is pointing at different angle to the wheel. If a tyre is sliding then this angle loses relevance. The tyre isn’t always sliding. The tread surface is able to work in a regime of static friction. An example of this is driving a car on sand. At low speeds the tyres will leave their tread pattern behind. With sliding the tyres will pump much more sand out and you won’t get a clear pattern.

Under braking a similar thing happens. Optimum braking performance on dry roads occurs when the tyres are rotating at a bit less than road speed, but it is not necessary for braking. The tyres will distort under any braking load, so there will be a brief lag between the braking force being translated to the tyre surface, but the tyre contact patch will not slip until the braking force exceeds the static friction coefficient multiplied by the dynamic weight on the tyre.

The tyre’s ability to key itself into the surface means that it is quite able to exceed a coefficient of friction of 1. Greater forces can be transmitted through sliding, but with additional wasted energy.

For a given tyre greater downforce will allow a greater slip angle before the tyre starts sliding. There isn’t a binary transition from not sliding to sliding, but pretty much by definition the maximum slip angle is the point just before the tyre starts sliding.

[bhall]

I'm sure JT will smack this down for being a gross oversimplification, but I'll nonetheless press on and disseminate my preschool knowledge.

This is what my brain says to me when I'm reading something in which the term is used. The picture is why my brain says it.

Slip angle is the difference between a tire's orientation and the direction it is actually traveling while still adhering to the road surface. When it is no longer adhering to the road surface, the tire is then sliding. As I've termed it here, adherence is the tire gripping the road surface through deformation of the contact patch.

In other words, maximum slip angle occurs just before the tire starts sliding.

[Jersey Tom]

I'm curious where you read that, Raymond.

First off, there is no such thing as maximum slip angle. Or you might say there is- it is 90 degrees. We'll get to that in a second. As for definition, you can think of slip angle in one of two ways - either the difference between the direction the wheel / beads of the tire are pointed, and the direction it is traveling (don't get caught up thinking of tread twist or crap like that)... or a ratio of lateral (sideslip) velocity to longitudinal (rolling) velocity.

Here's an example. Next time you're changing the tires on your car... take one, put it on the ground, and drag it sideways. Congratulations, you have achieved 90 degrees slip angle.

Or another example, let's say you're driving on the highway at 100 kph with the steering straight ahead. A strong gust of wind hits your car from the side and for a moment has you drifting sideways at 1 kph. Since you have a ratio of lateral to longitudinal velocity you now have slip angle on your tires even though they haven't been steered at all. How much? Using small angle approximation, 1 kph / 100 kph * 180 / pi = ~0.6 degrees of slip. The tires generate forces to oppose that slip and wind force... which is why a gust of wind just makes you drift a bit on the road rather than completely blow you off of it!

Was that helpful?

[Tommy Cookers]

Don't we all agree that the so-called slip angle is the just the (angular) difference between where the wheel is pointing and where it's going ?

The side/cornering force cannot be generated/transmitted without developing at least a proporionate distortion in the rotating tyre, that causes the visible angular difference as above. For most of us that's the hardest we can drive.

Those who can drive like Lewis Hamilton can (importantly) generate a bit more sideforce, this produces an extra (and disproportionate) angular difference as the tyre is starting in places to lose adhesion (not every part of the contacting tyre works equally well).
If LH overdoes it there will be more angular difference without more sideforce. Visibly disproportionate, spectators like it.

How the tyre generates the sideforce is another matter, but should not impede agreement on the above definition.

The traditional term 'slip' is confusing, and seems founded on obsolete ideas concerning how the tyre works in cornering.

Most of the sideforce is available from modern tyres with little difficulty and with a distortion close to (linearly) proportionate to the sideforce. Why wouldn't this be so ?
Tyre design essentially went this way starting 70 years ago (Michelin X radial ply).

So for all normal purposes what we call slip is related to distortion (like torque and power are always related), and we don't need to think otherwise.

[Jersey Tom]

For most of us that's the hardest we can drive.

I don't follow. I'm not Lewis Hamilton, but I can generate as much slip angle in a tire as I want by just adding more steering (until I hit steering lock anyway!)

My point earlier though, is still to focus on the definition and what it means. Clearly we all AREN'T on the same page there, as there was the belief that there's a maximum slip angle you can generate, somehow limited by the amount of deformation or force the tire could supply. That's just not true.

For that matter I've also read.. maybe even on this forum.. someone describing driving "within the slip angle of the tires." As if the tire had a single slip angle that was defined as the limit of adhesion. That's clearly BS as well.

I just suspect that if you talk too much about the internal mechanisms of carcass, belt, and tread deformation etc it's easy to focus on that rather than the big picture of what slip angle is. For that matter, it's not unique to tires. An entire car has a "slip angle." Planes have "slip angles."

[Tommy Cookers]

99.99% of those referring to slip angle have in mind useful slip angle or meaningful slip angle (they just don't say it)
that is the angles in progress to maximum cornering (and the related force regime)

angles beyond these are useful only to the LHs of this world, and we should not imply otherwise

angles beyond these have (limited) value in aircraft, but their use is not a trivial matter

IMO

[Jersey Tom]

99.99% of those referring to slip angle have in mind useful slip angle or meaningful slip angle (they just don't say it)
that is the angles in progress to maximum cornering (and the related force regime)

angles beyond these are useful only to the LHs of this world, and we should not imply otherwise

angles beyond these have (limited) value in aircraft, but their use is not a trivial matter

IMO

Whether or not you're on the way to maximum cornering or not.. is irrelevant to me. Slip angle is slip angle. I want to keep hammering that point... and to avoid any implication or insinuation that it's something special to a specific range of twist or deformation or performance or whatever.

Focus on the basic definition - which many struggle understanding to begin with.

[olefud]

Some of us have a problem with definition –or semantics. That is, are slip and slid the same? With regard to tires, slip is sometimes thought of as static friction while slid is thought of as, well, sliding friction. When working with (lower level) NASCAR people the latter was referred to as “washing”. This was considered distinct from slip, i.e. difference in tire travel and pointing as a result of contact patch deformation while maintaining a static, nonsliding –though perhaps squiggling- contact between the tire and the road.

[Jersey Tom]

Some of us have a problem with definition –or semantics. That is, are slip and slid the same? With regard to tires, slip is sometimes thought of as static friction while slid is thought of as, well, sliding friction. When working with (lower level) NASCAR people the latter was referred to as “washing”. This was considered distinct from slip, i.e. difference in tire travel and pointing as a result of contact patch deformation while maintaining a static, nonsliding –though perhaps squiggling- contact between the tire and the road.

Sideslip is sideslip, slip angle is slip angle. Makes no difference whether it's linear range, saturation, or post saturation.

Again, this is the problem with getting twisted up in thinking of "static" or "sliding" friction or tire deformation or any crap like that. Just forget all that malarkey.

[captainmorgan]

avoid any implication or insinuation that it's something special to a specific range of twist or deformation or performance or whatever.

I dont understand where you're coming from, specifically why you insist on leaving twist/deformation/sliding/static out of any discussion of slip angle. Is there not going to be some slip angle at a given speed, beyond which a contact patch can not deform while continuing to provide at least some static friction?

[Jersey Tom]

avoid any implication or insinuation that it's something special to a specific range of twist or deformation or performance or whatever.

I dont understand where you're coming from, specifically why you insist on leaving twist/deformation/sliding/static out of any discussion of slip angle. Is there not going to be some slip angle at a given speed, beyond which a contact patch can not deform while continuing to provide at least some static friction?

Because as far as the definition is concerned, it just doesn't come into play. There's no limit to slip angle, and that was one of the key misconceptions in the quote in the original post here.

There is a slip angle at which for a given state (load, camber, inflation pressure, speed) the tire produces a maximum amount of lateral force...



...but there's nothing saying you can't go past that. On a front tire just add more steering and your SA goes up even if the tire twist does not.

And again, the concept of a sideslip angle isn't common to tires. A plane flying through the air can have a sideslip angle... as in landing in a crosswind (game screenshot here, but still...)


I could have wooden wagon wheels on the front of a car and give them slip angles while driving around (and looking rather like an ass in the process).

I think of it as, you get tire deformation and forces because of sideslip angle... rather than you get slip angle because of tire deformation.

[Tommy Cookers]

You are entirely wrong in your aircraft example

What you are showing is exactly not a sideslip, for a sideslip the wings would have to be out of level and the plane aligned with the runway.

The B-52 has little scope for landing with wings out of level (having a 200 ft span downward sloping wing)

More or less uniquely, it has landing gear that is set by the pilot angle-off for a landing wings-level angle-off such as shown

If the pilot had to straighten up just before touchdown (eg if he couldn't take out the angle with the gear as above), there is a strong tendency for the plane to roll (either favourably or unfavourably or worse) , due to the swept back design.

Because of this, in airliners the requirement is for the (conventional) gear to be strong enough to take touchdown with the plane still having the angle on, on touchdown there is (mechanical) directional stability, ie the plane straightens itself in a ball of tyre smoke. This is to cater for landings with hydraulic pressure depleted to the point where the spoilers (for roll control at landing speeds) are not working.

Nice image ot the B-52, though !!

BTW there's nothing wrong with a wing-down landing in the right kind of plane (or even takeoff !)



IMO ......... a better term for our slip angle would be Angle of Attack (this doesn't imply a particular cause for the angle)

Usually I defend established terminology (99% of spoilers aren't spoilers, 99% of intercoolers are aftercoolers, and uprights are wheelposts), but here it is bad (we Brits are responsible?).

[Jersey Tom]

You are entirely wrong in your aircraft example

Oh?

http://en.wikipedia.org/wiki/Sideslip_angle



Looks pretty spot on as an analogy to me. Top view difference in orientation to travel (or alternatively an introduction of a lateral velocity relative to the orientation of the body).

Or do you mean the specific case of landing in cross wind? Whether or not that's the case I couldn't care less about. Point is the concept of sideslip angle as a relevant state variable applies to a variety of things. It isn't JUST tires. Purely a measure of relative velocities.

[Tommy Cookers]

I have some experience in these matters. What I said was your B-52 isn't an example of sideslip

2 diagrams taken out of context from Wiki won't change my view on that. I think you need to read more about landing aircraft so that you can pick a appropriate image (if there is one)

perhaps we can keep this thread on the rails

PM me if you like

[hardingfv32]

What is the relationship of side slip to slip angles? Is side slip a subset of slip angles?

Brian