|
Main Forum Page
|
The Gyroscope Forum |
23 November 2024 20:36
|
|
Welcome to the gyroscope forum. If you have a question about gyroscopes in general,
want to know how they work, or what they can be used for then you can leave your question here for others to answer.
You may also be able to help others by answering some of the questions on the site.
|
Question |
| Asked by: |
Nitro MacMad |
| Subject: |
Gyros and bikes |
| Question: |
Dear Webmaster,
To start to grasp the application of gyro-dynamics to propulsion needs the best understanding. In the interests of improved understanding of gyro-dynamics, would you consider removing the link (or consider asking for a correction of it) under “uses” - motorbikes “how gyroscopic forces affect motor bikes”? This links to an American Virginia educational web site with an explanation by a Professor L A Bloomsfield. He starts correctly enough to compare the need to move the hand at bottom of a balanced broomstick to maintain its centre of gravity with the balancing of a bike. Then, giving up on fact, he produces a page of pure tosh about how gyroscopic forces are responsible for the dynamic stability of a bike in motion. This is quite simply wrong. (Incidentally Glen, Gyro-dynamics have nothing to do with down force on a racing car either. Any (small - because of the slow rate of turn) precessional force pushing down on the front wheels lifts equally on the rear wheels. The useful down force of racing cars is created by the airfoils and the dynamically induced vacuum under the car acting like a reverse hovercraft).
The gyro-dynamic forces involved in a bike ride are tiny compared to the centrifugal (sorry any scientists but I always used that word and I am sure you are clever enough to know what I mean) forces involved in cornering. This is just as well as any precession caused by turning the handlebars, far from helping cornering or balancing, actually tries to tilt the bike the wrong way.
The centrifugal force of cornering is balanced by the cyclist, incredible though it may seem, steering right to turn left. This initial right steer shifts his c of g until the “lean-in fall under gravity” to the left equals the centrifugal force, to the right, of cornering. The steering is then brought back towards the left until cornering equilibrium is achieved. When he has finished cornering, incredibly again, he steers tighter left this time to reduce the effect of gravity while increasing the centrifugal effect to bring the bike upright again before centring the handlebars to steer straight. This same steering process occurs with ski bikes and other inherently unstable motive devices that have no rotating wheel and therefore no gyro-dynamic effect.
Before you scientists all start pushing the B.S. button you might like to try a simple test. You could borrow your nipper’s bike, attach a string to one side of the handlebars, and get enough speed to steer. Then with hands off the bars pull on the string gently to steer (I accept no responsibility for any injury or damage incurred. It will be your own fault for being daft enough to mess about with your kid’s bike!). You will find that if the string is on the right you will fall off on the left. This is because you have carried out only half of the required inputs to achieve the dynamic balance needed to corner to the left. You will have, by steering right, moved the c of g (should really be called the dynamic c of g) to the left and with no left string to pull on to recover the dynamic centre of gravity while you corner, the bike simply (simply?) keeps falling left or crashing as it is scientifically known.
This process has nothing to do with gyro-dynamics but like gyro-dynamics is counter-intuitive. This is why it took us all so long to “learn” and why we all got grazed knees until our subconscious took over this task that the conscious mind simply could not cope with.
This correction to the links may seem like nit picking taken to Olympic standards but I have found that the greatest viscosity to speedy understanding is caused by immersion in the thick unexamined repetition of others misinformation.
I am sure I have been as guilty as sin of this crime myself but then I am not in the education of physics business in Virginia -shame on you “Loo” Bloomsfield.
Kind regards
NM
PS A further example of duff teaching is in bad versions of the Bernoulli theory of lift that are still out there. Although old Bernoulli was spot on with his venturi law, some bloody fool extended it in incorrect form to wings. As a result, I was taught that a wing created lift because the air passing over the top of the wing had further to go and therefore its unit pressure was less than that on the underside. This would extend to a stationary wing (great boys, we have perpetual motion AND free flight!) and would mean that planes could not fly upside down at air shows. It is, of course, mostly to do with the mass of air deflected downwards - remember Newton’s equal and opposite is still (nearly) universally right. Thanks to my science teacher, I distrusted most of what I was taught and to this day, I hate flying. Lift, after all, may very well decide to disagree with Bernoulli’s law and not work today.
|
| Date: |
18 June 2004
|
| report abuse
|
|
Answers (Ordered by Date)
|
| Answer: |
webmaster@gyroscopes.org - 23/06/2004 13:34:03
| | | Your points have been taken on board and I will make some changes the next time I have some time on my hands. Any volunteers to write update versions are welcome.
The influence of gyroscopic forces on motorbikes and racing cars is quite small. In the case of racing cars it has been used but the forces are small compared to airfoils, I have noted instances where teams reversed the rotation of the engine. I guess this meant they could reduce the airfoil downforce a little and hence less drag. I’ll try and get a reference/weblink when I rewrite it.
Glenn
(webmaster)
|
| Report Abuse |
| Answer: |
Joe Bine - 07/10/2004 03:30:12
| | | You have it almost right, but not quite.
Firstly, balance is not about counteracting centrifugal forces when going in a straight line. In that instance, there is no centrifugal force to speak of, yet you can still balance the bike. It's about applying castering torques. The castering torque results from the displacement of the tire contact patch to either side of the c of g line.
Centrifugal (actually, centripetal) forces are involved only when riding on a curve. What you're trying to describe as the method to initiate a turn is called countersteer. That's because you need to turn the handlebars (i.e. steer) in the direction opposite to where you want to go to get the bike to lean in the right direction.
To get out of a turn, you don't "steer tighter" in the same direction. You simply countersteer in the opposite direction. For example, to initiate a left turn, push the handlebars towards the right. To get out of the left-turn lean to the left, push the handlebars towards the left.
In any case, you are right in that gyroscopic effects are small (although they work in the same direction as the castering torque, contrary to what you stated).
See http://www.rider-ed.com/tips/motorcyclestability.htm for a very thorough and understandable summary.
|
| Report Abuse |
| Add an Answer >> |
|
