Electric unicycle and bicycle dynamics - gyro effects on steering

[rcgldr]

59 minutes ago, techyiam said:

Should you typically make turns by nudging your inner outside leg against the side of your V8F, you will be in a rude surprise when you attempt to make a turn for the first time on those big and heavy euc's, even at speeds as low as 20 or so mph. Basically, you will find that the euc won't turn as you thought it would. Your body won't instinctively know how to effectively get enough leverage to tilt the wheel.

The V8F is only 35 lbs, and I can tilt steer it without touching either pad (pedal pressure only), touching both pads, or only touching outer pad. If I ever got another wheel, I would repeat the process for how I learned to tilt steer in the first place: on a long straight at around 10 mph (stable speed), do small tilts to get a sense of the response, then mild weaves, then large radius turns, before I would go out on any paved bike trails (I don't do off-road trails). This is a hobby for me, I mostly ride at 10 to 15 mph, 30 minutes max, and weight is an issue for transport with my car, so I don't have any plans to get another wheel. 

Edited August 1, 2022 by rcgldr

[techyiam]

2 hours ago, rcgldr said:

If I ever got another wheel, I would repeat the process for how I learned to tilt steer in the first place: on a long straight at around 10 mph (stable speed), do small tilts to get a sense of the response, then mild weaves, then large radius turns, before I would go out on any paved bike trails (I don't do off-road trails). This is a hobby for me, I mostly ride at 10 to 15 mph, 30 minutes max, and weight is an issue for transport with my car, so I don't have any plans to get another wheel. 

My main point was just that gyroscopic effects exist for some of the heavier wheels. For your use case, I can see why those light wheels are great. They would feel like an extension of your body. Great fun. And the most astonishing thing is that the only moving parts in an euc are the outer parts of the two wheel bearings, and the motor/wheel unit.

Edited August 1, 2022 by techyiam

[mrelwood]

22 hours ago, Eucner said:

The Finnish translation for yaw is "kääntyminen" and for roll "kallistuminen". Pitch would be translated as "nyökkääminen".

Those do make sense, thanks. Just that "kääntyminen" also translates back to "turning", which is a pretty generic word no matter which way it turns.

22 hours ago, Eucner said:

Thank you for the video. Gyroscopic precession has many times been considered to be too weak to be meaningful for EUC riding. This video showed that it might have more meaning than thought before.

Only if the wheel is yanked to a steep tilt very quickly at speeds like 50 km/h and above. No rider can turn as fast at speed as I tilted the wheel in the video. Then again, with speeds nearing 100 km/h there can be some effect, but I still don't think that it can do much next to the camber effect that gets very strong already much much sooner.

22 hours ago, Eucner said:

I'm certain camber effect alone is not enough explain autocorrection of the wheel we have seen on the trick ride videos and experienced ourselves.

Gyroscopic forces don't auto-correct though. They resist a change in yaw or roll, and they don't for example bring the wheel back upright.

22 hours ago, Eucner said:

The gyroscopic precession is speed related.

Yes, both riding speed as well as the speed of changing the roll angle.

22 hours ago, Eucner said:

We need to have a certain speed to be stable and at the higher speeds we get speed wobbles. Based on this thinking, the key against speed wobbles is to learn to tame wheels overcorrection.

The camber effect could explain both though. At slower speeds the effect is not strong enough (unless you tilt the wheel more), and at high speeds even a small tilt causes oversteering which can find a resonance frequency that depends on the rider's posture and technique (muscle tension). I can't see how gyroscopic forces would enter into play other than actually dampening the tendencies to wobble.

Maybe gyroscopic forces are why carving actually works so well in killing the wobbles. When quickly tilting the wheel, the gyroscopic precession causes yaw which fights the wobble resonance more on the other side of the resonance wave. Which we already know is an effective way of killing any resonance.

[techyiam]

11 hours ago, mrelwood said:

Only if the wheel is yanked to a steep tilt very quickly at speeds like 50 km/h and above. No rider can turn as fast at speed as I tilted the wheel in the video. Then again, with speeds nearing 100 km/h there can be some effect, but I still don't think that it can do much next to the camber effect that gets very strong already much much sooner.

That is what I have observed too so far. I could initiate a turn at roughly 45 km/h, but I don't observed a yaw torque that would generate a yaw angular velocity. I just have to exert a high enough roll-torque to overcome gyro effects in order to tilt the wheel so the round profile of the tire enables change of direction (camber effect).

 

11 hours ago, mrelwood said:

Yes, both riding speed as well as the speed of changing the roll angle.

Riding speed because you need the angular momentum.  You observed angular acceleration in roll because you applied a roll-torque. The two main components you need to have precession. 

 

 

Edited August 1, 2022 by techyiam

[Mono]

@rcgldr thanks for the awesome insights and thread (and patience)!

The original learning clips from Solowheel emphasized another aspect of the two types of steering: they recommended to balance the wheel by twisting (swivel, yaw steering) and to ride curves by tilting (leaning sideways, roll steering). I found this distinction pretty useful and it fits to the mechanisms as outlined in this thread. Beginners often overuse tilt-steering to stay balanced. Even with 7+ years of riding experience, when I try to learn a new skill I believe I often fail just because I forget/cease twist steering to balance the wheel while I am focussing on the "new" part.

Edited August 2, 2022 by Mono

[Eucner]

On 8/1/2022 at 2:01 PM, mrelwood said:

Only if the wheel is yanked to a steep tilt very quickly at speeds like 50 km/h and above. No rider can turn as fast at speed as I tilted the wheel in the video. Then again, with speeds nearing 100 km/h there can be some effect, but I still don't think that it can do much next to the camber effect that gets very strong already much much sooner.

Gyroscopic precession works at every riding and steering speeds. At low speeds it just is more subtle and hard to feel. It is like with counter steering. It is always needed, but at low speed not recognized at all by the rider.

If you think forces are not big enough, roll a coin. It is much easier to a keep rolling coin upright than a stationary one. Due to coins edge profile, there is no chamber effect. Moment from coins geometry is same in both cases and don't explain the difference. Gyroscopic precession is enough to keep it upright and autocorrect. If it works with small and slow coin, it will work with the EUC.

On 8/1/2022 at 2:01 PM, mrelwood said:

Gyroscopic forces don't auto-correct though. They resist a change in yaw or roll, and they don't for example bring the wheel back upright.

For simplicity, lets think about a loose wheel moving without a rider. Sooner or later it will start to roll and turn to one side. Chamber effect alone would keep wheel turning in the same direction. At certain speed range the gyroscopic precession will yaw the wheel to same direction and wheel will get its COG back over tires contact patch. It will be autocorrected as we have seen it happening.

On 8/1/2022 at 2:01 PM, mrelwood said:

The camber effect could explain both though. At slower speeds the effect is not strong enough (unless you tilt the wheel more), and at high speeds even a small tilt causes oversteering which can find a resonance frequency that depends on the rider's posture and technique (muscle tension). I can't see how gyroscopic forces would enter into play other than actually dampening the tendencies to wobble.

The chamber effect is not much speed related. It is defined by tires roll angle, pressure and geometry.

On 8/1/2022 at 2:01 PM, mrelwood said:

Maybe gyroscopic forces are why carving actually works so well in killing the wobbles. When quickly tilting the wheel, the gyroscopic precession causes yaw which fights the wobble resonance more on the other side of the resonance wave. Which we already know is an effective way of killing any resonance.

Yes, we agree. The speed wobbling tire makes excessive autocorrection due to gyroscopic precession. Carving puts the gyroscopic precession into the control of rider. It is a battle of command.

[mrelwood]

 

2 hours ago, Eucner said:

If you think forces are not big enough, roll a coin. It is much easier to a keep rolling coin upright than a stationary one. Due to coins edge profile, there is no chamber effect. Moment from coins geometry is same in both cases and don't explain the difference. Gyroscopic precession is enough to keep it upright and autocorrect. If it works with small and slow coin, it will work with the EUC.

Unfortunately I wasn't able to follow your coin example, though I would've sure liked to.

2 hours ago, Eucner said:

For simplicity, lets think about a loose wheel moving without a rider. Sooner or later it will start to roll and turn to one side. Chamber effect alone would keep wheel turning in the same direction.

The problem here is that the test can't separate the gyroscopic forces from a camber effect. So the results don't tell us which force did what.

2 hours ago, Eucner said:

At certain speed range the gyroscopic precession will yaw the wheel to same direction and wheel will get its COG back over tires contact patch. It will be autocorrected as we have seen it happening.

If you roll only a wide outer tire in a certain way, it can have those same auto-correcting tendencies even at walking speeds.

2 hours ago, Eucner said:

The chamber effect is not much speed related. It is defined by tires roll angle, pressure and geometry.

Absolutely. But the speed at which it changes the wheel's trajectory increase with the forward momentum. At higher speeds the wheel doesn't have to tilt (roll) much at all to achieve the intended turn. The rider already has to counter the angular acceleration by leaning more than the wheel. Tilting the wheel further would make it turn tighter, and I'm pretty sure that the wheel's mass alone would be enough to pull the wheel upright and tilting in the opposite direction.

I don't have the tools to prove this though.

2 hours ago, Eucner said:

Yes, we agree. The speed wobbling tire makes excessive autocorrection due to gyroscopic precession.

I don't think it does though. When the wheel starts to wobble, it doesn't seem to do so by it's roll axis as much as it does on it's yaw axis. For the precession to yaw a wobbling wheel the wobble would have to vary the roll angle. To me it has seemed, looked and felt as if that would not be the case.

Although, as the wobble gets (way) worse, the tire does start follow a clear S pattern which does introduce a roll tilt as well. If the wobble is fast enough, the precession can make a difference at this point. But a wobble can start quite slow. And a small and light EUC can be just as susceptible to wobbles despite having a fraction of the rotating weight of a large wheel.

The 18XL doesn't seem to wobble very easily though, and I believe the narrower tire is the main contributor. It's camber effect is so much smaller.

I recommend for you to also make the test I did in the video, and feel the strength of the camber effect at various yanking as well as rotational speeds. I'd be interested in hearing how you'd assess your findings.

[rcgldr]

7 hours ago, Eucner said:

At certain speed range the gyroscopic precession will yaw the wheel to same direction and wheel will get its COG back over tires contact patch. It will be auto-corrected as we have seen it happening.

Precession is a reaction to an imbalanced inwards torque that once stabilized, will hold a lean angle, but will not correct the lean back to vertical. 

7 hours ago, Eucner said:

For simplicity, lets think about a loose wheel moving without a rider. Sooner or later it will start to roll and turn to one side. The camber effect is not much speed related. It is defined by tires roll angle, pressure and geometry.

Since camber effect is not affected much by speed, and is a response to lean angle, at sufficient speed, it will auto-correct for lean and return to vertical. If there is too much speed, there may be excessive over-correction and wobble if there are no dampening factors. 

A riderless EUC at sufficient speed will auto-correct. Example of a dog on a EUC that auto-corrects a few times, then goes straight once the dog stops moving.

 

Edited August 4, 2022 by rcgldr

[Eucner]

14 hours ago, mrelwood said:

Unfortunately I wasn't able to follow your coin example, though I would've sure liked to.

Don't mix the rolling coin to a spinning coin. It is an other interesting, and more commonly studied, phenomenon. We can divide rolling into three phases. At the first, the coin stays upright and autocorrects itself. At the second, friction slows it down and it start gradually turn more and more into one direction, following a spiral path. At the third, it spins faster and faster, almost in one spot, until stops. For EUC analogy the first phase is most interesting. Coins geometry doesn't introduce a chamber effect. We will see gyroscopic precession stabilizing the coin. You can surely still find coins from somewhere. Try it yourself and let me know what you think helps rolling coin stay upright and autocorrect itself.

Here is link to an other discussion about this problem Balance of a rolling coin.

This video has been posted on the forum before. It is about bicycles. At the timestamp he shows how gyroscopic precession will autosteer the spinning front wheel.

This same thing happens to coins and EUC's.

14 hours ago, mrelwood said:

The problem here is that the test can't separate the gyroscopic forces from a camber effect. So the results don't tell us which force did what.

The coin test makes it easier. There is no chamber effect.

14 hours ago, mrelwood said:

If you roll only a wide outer tire in a certain way, it can have those same auto-correcting tendencies even at walking speeds.

Yes, we can see gyroscopic precession also here.

14 hours ago, mrelwood said:

I'm pretty sure that the wheel's mass alone would be enough to pull the wheel upright and tilting in the opposite direction.

Mass don't pull anything up. A force is needed. In gyroscopic precession this force is generated from rolling objects kinetic and rolling energy.

14 hours ago, mrelwood said:

Although, as the wobble gets (way) worse, the tire does start follow a clear S pattern which does introduce a roll tilt as well. If the wobble is fast enough, the precession can make a difference at this point. But a wobble can start quite slow.

Yes, it is difficult to recognize when it is starting small. Still it is exactly the same phenomenon when it is way worse.

Edited August 4, 2022 by Eucner
grammar

[rcgldr]

13 minutes ago, Eucner said:

rolling coin

Start the rolling coin with an initial tilt angle, it will follow a curve and never transition to rolling vertically.

 

13 minutes ago, Eucner said:

This video has been posted on the forum before. It is about bicycles.

Gyroscopic related properties, angular momentum of the front wheel and precession effect interfere with the self-correcting geometry of a typical bike that mostly relies on trail (contact patch behind the extension of the steering axis). At moderate speed, this interference acts as a damper. On a theoretical bike with infinitely thin tires, there is a maximum speed, beyond which the bike goes into capsize mode, falling inwards at a very slow speed. On real motorcycle, at 100+ mph, the bike ceases to tend to return to vertical if the rider relaxes on the handlebars, and instead tends to hold the current lean angle (or the rate of change of lean angle is so slow that it's imperceptible).

Scroll down to eigenvalues and capsize mode sections:

https://en.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics#Lateral_motion_theory

Precession can help a little bit during an initial counter-steer, as the torque to steer outwards will produce an inwards roll torque, about 1/8th that of the roll torque due to the tires being steered outwards.

https://en.wikipedia.org/wiki/Countersteering#Gyroscopic_effects

 

[Eucner]

12 hours ago, rcgldr said:

Precession is a reaction to an imbalanced inwards torque that once stabilized, will hold a lean angle, but will not correct the lean back to vertical.

When the wheel starts to roll (fall) to one side, the gyroscopic precession will also yaw (turn) it into the same direction. Then the wheel will get its COG back over to the contact patch. The wheel has been self-stabilized. It is true that after a movement the wheel will hold its angle. When the speed of wheel is at a correct range, this happens when the wheel is already corrected to upright. If there is not enough speed the wheel stays tilted. And if there is too much speed, the wheel starts to oscillate in increasing amplitude.

The only things working against gyroscopic precession are friction forces at the contact patch and yaw axis inertia. The contact patch is quite small and friction is even easier to overcome when wheel is moving. The combined inertia of wheel and rider is by far the lowest around the yaw axis. Autocorrection is happening at very small roll angles. Only a very subtle yaw movement is enough to make a correction.

12 hours ago, rcgldr said:

Since camber effect is not affected much by speed, and is a response to lean angle, at sufficient speed, it will auto-correct for lean and return to vertical. If there is too much speed, there may be excessive over-correction and wobble if there are no dampening factors.

The chamber effect happens when the wheel is tilted. There isn't any forces which would move the COG back to over the contact patch.

[Eucner]

4 minutes ago, rcgldr said:

Start the rolling coin with an initial tilt angle, it will follow a curve and never transition to rolling vertically.

Yes, when angle is not chancing, there no gyroscopic precession.

6 minutes ago, rcgldr said:

Gyroscopic related properties, angular momentum of the front wheel and precession effect interfere with the self-correcting geometry of a typical bike that mostly relies on trail (contact patch behind the extension of the steering axis). At moderate speed, this interference acts as a damper. On a theoretical bike with infinitely thin tires, there is a maximum speed, beyond which the bike goes into capsize mode, falling inwards at a very slow speed. On real motorcycle, at 100+ mph, the bike ceases to tend to return to vertical if the rider relaxes on the handlebars, and instead tends to hold the current lean angle (or the rate of change of lean angle is so slow that it's imperceptible).

Scroll down to eigenvalues and capsize mode sections:

https://en.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics#Lateral_motion_theory

Precession can help a little bit during an initial counter-steer, as the torque to steer outwards will produce an inwards roll torque, about 1/8th that of the roll torque due to the tires being steered outwards.

https://en.wikipedia.org/wiki/Countersteering#Gyroscopic_effects

Thanks, for links. They had very interesting information. The two wheelers are more complicated then EUC's. They have more interacting components and DOF's, and are self-balanced in many different ways. We got to be careful when making analogies between them and EUC's. Same things don't always apply. Somebody should make a similar scientific study of EUC's.

[rcgldr]

7 minutes ago, Eucner said:

gyroscopic precession will also yaw

Precession requires an unbalanced inwards torque to yaw inwards. If that yaw response is enough to result in a coordinated and leaned turn, the result is no net roll torque and no precession. 

7 minutes ago, Eucner said:

camber effect happens when the wheel is tilted.

Camber effect causes a wheel to follow a radius of curvature based on tilt angle, mostly independent of speed. At sufficient speed, this results in an over-correction (wheel steers inwards beyond what is needed for a coordinated turn) reducing lean angle. The result is a wheel turned inwards of the direction of linear momentum, causing it to return to vertical or a bit past if there is no dampening factor. For an EUC, the response is increased due to the linear momentum of the non-rotating parts of the EUC, and if there's a rider, even more so due to the linear momentum of the rider. 

[rcgldr]

8 minutes ago, Eucner said:

Yes, when angle is not changing, there no gyroscopic precession.

It's OK if the lean angle is not changing, as long as there is an imbalanced inwards torque due to the coin being leaned more than the lean angle for a coordinated turn. In effect, the coin will yaw enough to hold an imbalanced lean angle, but not enough to ever catch up, because if it started to catch up, the inwards torque and resulting precession yaw response would get reduced. The end result is precession prevents the coin from falling, but it doesn't yaw the coin enough to return it to vertical.

Edited August 4, 2022 by rcgldr

[Eucner]

19 minutes ago, rcgldr said:

Precession requires an unbalanced inwards torque to yaw inwards. If that yaw response is enough to result in a coordinated and leaned turn, the result is no net roll torque and no precession. 

Yes, at that point the roll is also corrected. 

14 minutes ago, rcgldr said:

It's OK if the lean angle is not changing, as long as there is an imbalanced inwards torque due to the coin being leaned more than the lean angle for a coordinate turn. In effect, the coin will yaw, but not enough to ever catch up, because if it started to catch up, the inwards torque and resulting precession yaw response would get reduced.

There is no point trying to prove why it is not working, when in reality it works.

[mrelwood]

2 hours ago, Eucner said:

Try it yourself

I mean that I didn't understand how to do the test.

Btw, did you do the lift test yet?

[Eucner]

2 hours ago, rcgldr said:

Camber effect causes a wheel to follow a radius of curvature based on tilt angle, mostly independent of speed.

Agreed.

2 hours ago, rcgldr said:

At sufficient speed, this results in an over-correction (wheel steers inwards beyond what is needed for a coordinated turn) reducing lean angle.

Why the wheel would be over-corrected? High speed or tight turn radius leads to high lateral forces at the contact patch. This deforms the tire and makes turn radius slightly larger, not smaller.

2 hours ago, rcgldr said:

The result is a wheel turned inwards of the direction of linear momentum, causing it to return to vertical or a bit past if there is no dampening factor. For an EUC, the response is increased due to the linear momentum of the non-rotating parts of the EUC, and if there's a rider, even more so due to the linear momentum of the rider. 

When the tire would start to move more upright the chamber effect would be reduced. The wheel would stay in tilted equilibrium and not to be fully corrected.

Why would gyroscopic precession work for autocorrection and chamber effect not? The answer lies in steering efficiency. Yaw axis steering is much more effective than roll axis steering. A smaller angular change is needed to get the wanted change in direction.

[Eucner]

1 hour ago, mrelwood said:

I mean that I didn't understand how to do the test.

Ok, my bad. Here is a video how to roll a coin.

There will be some variation how well one succeeds in this. For the sake of science the best throws are the slightly imperfect ones. Then you can see the autocorrelation. 

1 hour ago, mrelwood said:

Btw, did you do the lift test yet?

Yes, I did. Thanks for reminding me to report my findings. At the first I decided to do it slowly. It turned out to be a large yaw angle movement and a strong moment. I was surprised. I was waiting for more subtle response. This assured me that I'm on right track with autocorrection.

[Mono]

It always disheartens me to see that people can so persistingly disagree on some, not trivial yet still comparatively simple physical mechanisms. It even works with math too, though that I find more funny.

[Mono]

4 hours ago, Eucner said:

The chamber effect happens when the wheel is tilted. There isn't any forces which would move the COG back to over the contact patch.

I believe there is, namely the centrifugal force that affects the top of the wheel when it makes a turn. The centrifugal force at the bottom is resisted by tire friction, hence we get a righting moment acting against gravity. As the camber effect is constant for any given lean (and so is gravity) whereas the centrifugal force increases with speed, there must exist a large enough speed where the centrifugal force brings the COG back and over the contact patch.

[rcgldr]

1 hour ago, Eucner said:

how to roll a coin.

I can stand a penny on its edge without it moving, because the edge is flat.

However, the issue is not about the behavior of a rolling disc, but an EUC with or without a rider.

As Wrong Way mentions, when an EUC is released at a slight angle, it might start turning one way, then switch to the other way, an example of over-correction at a relatively slow speed where precession would not be much of a factor. Walk the dog and Davastato demo:

 

Edited August 4, 2022 by rcgldr

[rcgldr]

23 minutes ago, Mono said:

I believe there is, namely the centrifugal force that affects the top of the wheel when it makes a turn. The centrifugal force at the bottom is resisted by tire friction, hence we get a righting moment acting against gravity. As the camber effect is constant for any given lean (and so is gravity) whereas the centrifugal force increases with speed, there must exist a large enough speed where the centrifugal force brings the COG back and over the contact patch.

The reactive centrifugal force increases with speed squared.

[Eucner]

1 hour ago, Mono said:

I believe there is, namely the centrifugal force that affects the top of the wheel when it makes a turn. The centrifugal force at the bottom is resisted by tire friction, hence we get a righting moment acting against gravity. As the camber effect is constant for any given lean (and so is gravity) whereas the centrifugal force increases with speed, there must exist a large enough speed where the centrifugal force brings the COG back and over the contact patch.

For a smooth constant tilt angle turn the centrifugal force and wheels tilt moment are in balance. To get the righting force the turning radius needs to be reduced or speed increased. The cone effect doesn't do either. It is a constant. The gyroscopic precession will reduce the turning radius, so it is quite obvious which is working here.

[rcgldr]

3 hours ago, Eucner said:

Why would gyroscopic precession work for auto-correction and camber effect not? The answer lies in steering efficiency. Yaw axis steering is much more effective than roll axis steering. A smaller angular change is needed to get the wanted change in direction.

Precession requires an inwards roll torque in order to yaw inwards. If there is no roll torque (such as a coordinated turn), there is no precession. If there's an outwards (correcting) roll torque, precession yaws outwards, tilting the wheel back inwards. I still don't know how much yaw torque precession can generate if there is any angular inertia (non-rotating parts of EUC and rider) that resists any precession related yaw response.

Camber effect only needs an inwards lean angle to produce an inwards yaw torque, independent of speed and even if the roll torque is outwards. So while the camber effect diminishes as the lean angle diminishes, with sufficient speed, the camber effect is always over-correcting for the current lean angle, even for a small lean angles (nearly vertical) reducing the lean angle, and momentum will carry the COM back (or past) the contact patch.

Precession is limited by the mass of rotating parts, while camber effect yaw torque is increased by the weight of the non-rotating parts, including the rider.

As for my own experience, when I only knew how to yaw steer via arm flailing and before I had tried tilt steering, I unexpectedly found my V8F became stable and self-balancing at 6 to 8 mph where I no longer had make any balancing movements. From this video my wife took of me on day 10 on the V8F at night, you can see how stable the V8F becomes at around 8 mph by the headlight beam, and how I can just relax my arms and just stand nearly motionless on the V8F.

 

Edited August 4, 2022 by rcgldr

[Eucner]

1 hour ago, rcgldr said:

However, the issue is not about the behavior of a rolling disc, but an EUC with or without a rider.

True, but the coin case is a simpler. The theory should work also here. Otherwise it is wrong or too limited.

1 hour ago, rcgldr said:

As Wrong Way mentions, when an EUC is released at a slight angle, it might start turning one way, then switch to the other way, an example of over-correction at a relatively slow speed where precession would not be much of a factor. Walk the dog and Davastato demo:

There wasn't anything in the video against gyroscopic precession autocorrection. You shouldn't overlook small forces, because only small yaw angle change is needed and there very little resisting forces.