# Dumb question about gyroscopic forces to clarify if you'd be so kind

1. Dec 12, 2013

### Fanum

Good evening all.

First off, apologies for the USE of your forum to try and get an answer, but just trying to be honest up front :)

Over on UKGSer (A bike forum, sorry) we're having a bit of a debate raised by the question of simply
(the GS is our type of bike)

From that, the question of gyroscopic forces has arisen.....

I'm no physicist nor mathematician, but it seems to me that, top put it simply, the faster our bikes go in a given crosswind, the more stable they become because of the gyroscopic forces of the wheels (and they're not insubstantial)

The thread is HERE and there are a few other associated questions (I like my theory that the precessive force will possibly explain why our bikes (which have a single sided swing arm) tend to be forced to one side when/if you take your hands off the bars)

Again, apologies for a bit of a 'hit and run' registration and post, but it's sod's law....there's never an expert around when you need one, so I thought I'd come out and try and find one :D

Thanks

Bill (fanum)

PS Very first visit, so if there's a more suitable section for this, mods please feel free to dump it anywhere :)

2. Dec 12, 2013

### rcgldr

Most of the stability for a typical bicycle or motorcycle is due to the steering geometry: the front tire's contact patch with the pavement is "behind" the extended axis of steering. The distance from where the extended axis intersects with the pavement back to the contact patch is called "trail". This causes the front tire to steer in the direction of lean and is responsible for the self stability of a bike.

Gyroscopic forces might possibly work on some bikes, but only within a vary narrow range of speeds. At higher speeds, the net effect of gyroscopic forces is dampen steering response and the rate of change in lean angle, and as speeds increase the gyroscopic forces cause a bike to tend to hold a lean angle as opposed to straightening back to a vertical position.

In a crosswind situation, the net effect of the wind and steering geometry is to move the tires downwind from under the center of mass, causing the bike to lean into the wind. If the wind is strong enough, additional steering input is required to generate and hold enough lean angle.

3. Dec 12, 2013

### Fanum

Thanks for that :)

The initial query was raised specifically because of the stability aspect......if "the net effect of gyroscopic forces is dampen steering response", then doesn't hat mean it's more stable? EG a minor twitch whether from the wind or from steering input is minimised?

That also seems to confirm my side of the debate.......holding a lean angle means (to me anyway) that it's more stable.....yes, it takes more force to bring upright (fighting against the gyroscopic forces) but as motorcyclists, that is true with or without a side wind......we compensate for it unconsciously with counter steering (I'd love to see the final equation for that!) and subconsciously shifting weight to the 'inside' footpeg etc.

I've looked at page after page of (to me) meaningless equations that I truly can't follow......I feel physics far more intuitively IYKWIM....this diagram makes total sense to me(even if I can't name the symbols :D)

I did spend some time sifting through a Google search on 'Gyroscopic force calculator" and similar, to try and find a simplistic model that I could use to try and work out what sort of force is being exerted by our wheels/tyres given a set of weights, revolutions per time unit and so on.....I think we'd have to assume some values (distance from axle pivot centre to centre of radial mass for example, and seeing the wheel as a solid disc would make things a lot easier even though the vast majority of the mas is way out on the periphery) but a layman's gross figure of force v speed would be great.

I do know that gyroscopic forces are much stronger than I had imagined.....yachts for example, and ocean liners, they all use/used physical gyroscopes as stabilisers.

A 120 metre yacht's gyroscope is only the size of a large beachball, yet it exerts enough force at 10K rpm (no idea of the mass) to keep that boat dead stable!

Relating a 200KG motorcycle to that, with a 19 inch wheel on the front and a 17 inch on the back, perhaps weighing in at 8 and 10 or so KG, and spinning at (dodges the maths again) the equivalent of 80KM/H, there has to be a SIGNIFICANT force acting on that bike's forces.

I don't know, I'm a 'gut feeling' engineer *shrugs*

EDIT....there's a follow up question concerning the 'precession force' as it acts on a single sided swing arm as on my bike, in that I suspect that it might be the cause all of them tend to steer to one side if 'left', but that's very much a secondary query :)

4. Dec 12, 2013

### rcgldr

For a bike, "stability" refers to the tendency to return to a vertical orienation if disturbed. The direction of travel may change, but a "stable" bike tends to remain or return to a vertical orientation. This could be called "positive stability". The tendency to hold a lean angle could be called "neutral stability" (no tendency to return to vertical orientation (no tendency to change lean angle)), and the tendency to fall inwards or outwards could be called "negative stability" or "unstable".

5. Dec 12, 2013

### AlephZero

I suspect it's simpler than that. When you ride fast into a crosswind, the wind velocity relative to the moving bike turns more into a headwind.

I've never seen an actual G5 bike, but looking at pictures on the web, I would say the reason is because when the bike is perfectly upright, the weight of the bike and rider is pushng down on one side of the wheel axle, not equally on both sides like a conventional bike. So your weight it tending to tip the bike over towards the side with the swing arm.

If you fix that problem by leaning the bike slightly the other way, then the camber angle of the steering (as described by other posts) will tend to make you ride in a large circle instead of straight.

Personally I'm not convinced that gyroscopic forces are big enough to be significant for bike stability, though they do act in the right direction.

FWIW you can make a bike that is self stable without gyroscopic wheels and without steering camber.
http://www.scientificamerican.com/article.cfm?id=self-stable-bike

Last edited: Dec 12, 2013
6. Dec 13, 2013

### Malverin

When you have fast moving body, it is hard to change its direction by pushing it a side (with force perpendicular to its movement direction )
And it is harder when it moves faster (the angle between changed direction, and previous direction is smaller)

The same happens when you have a rotation.
When the wheel rotates fast, it is hard to change the direction of its axle

So it seems more stable (you need a great force(torque) to change the direction of its axle).

http://scienceblogs.com/dotphysics/2009/12/03/angular-momentum-example/

Last edited: Dec 13, 2013
7. Dec 13, 2013

### rcgldr

but the component of side force on the bike remains about the same regardless of the bikes speed. The sideways force on the bike will tend to steer the front tire downwind, causing the bike to lean in to the wind, but the effect is limited. If the cross wind is strong enough, the rider will have apply additional counter-steering input to maintain a lean. During the transition, the center of mass of the bike will move a bit downwind (Newtons second law, force = mass x acceleration, in this case a sideways force) and the tires will move even further downwind as the bike leans, so the rider should ride on the upwind side of a lane if the rider is aware of which way cross wind gusts could be anticipated.