brokenrobot

Yes, just as God gave us an infinite number of thrift-store Varsities... it's up to Man to do the rest!

Brian

Gyro effect - with only two front wheels, they would cancel each other out and you'd be unable to balance.

phidauex

Actually there have been bikes made as experiments that have two front wheels that spin opposite directions at the same speed, and while they are a bit more 'twitchy' than normal, they are perfectly ridable. A bicycle doesn't really balance by gyroscopic inertia, though that does play a small part. The greater part of bicycle balance is given to it by the trail of the front wheel, and the rake angle. When you turn the bars on a bike, the front of the bike lifts up a little bit. This means that the point of lowest center of gravity is found with the bars straight ahead, and so a moving bike will naturally attempt to realign the wheel forward to maintain course. When the bike leans, the wheel must turn, due to the trail again. That motion kicks the wheel out from under the center of gravity, and as it pulls back under, the bike stabilizes.

Anyway, the physical interactions end up getting pretty complex, but the end result is that gyroscopic forces don't affect balance as much as we used to think. A 'two-penny' farthing would be a tricky thing to ride, but certainly not unridable.

peace,
sam

randya

Just two frames. That would be a tall 20. Two stacked mini frames (12" or 16" wheels) is a tall mini.

Rev.Chuck

With nothing but experience to back it up I am going to have to disagree on the gyro and rake/trail stuff. Most of a bikes or motorcycles tendency to remain upright is beacause of the gyroscopic effect of the wheels. The tendency to go in a straight line(or not) is from rake and trail.
My coworker and I dicussed the opposing wheels canceling each other out, and concluded that, in the tall bike aplication it would not affect stability. We could be wrong.

phidauex

As a science teacher, I must respectfully re-disagree. Gyroscopic inertia exists, but it is a very minor part of bicycle and motorcycle stability. Here is an old, but interesting study about a guy trying to make unridable bicycles by examining each aspect of bicycle stability, negating it, and then seeing if the new bike is ridable, and if so, how its behavior changes. Gyroscopic inertia holds up a rolling hoop, but it can't hope to hold up a singletrack vehicle.

https://ist-socrates.berkeley.edu/~fa...onesBikeBW.pdf

Bicycle and motorcycle stability are related more to steering geometry and the rider's ability to counteract leaning forces by moving the center of gravity back and forth. A unicycle rider maintains forward/backward balance by having a high center of gravity, and moving the base back and forth beneath it. Imagine balancing a broom on your hand, bristles up. A bicycle doesn't balance front to back this way, but it does balance side to side this way. As it leans to the right, the wheel turns to the right. But because turning the wheel to the right causes the center of gravity to rise, there is a reaction force that kicks the wheel straight again, and pulls the base of the vehicle back under the center of gravity.

You might also be interested in these dynamic simulations of bicycle motion. Note that they show a stable geometry (which looks very realistic) without accounting for gyroscopic inertia at all! This simulated bicycle stays stable through geometry alone.

https://www.win.tue.nl/dynamo/bike/bike.html

Up next is a very detailed analysis of the stability of fully faired two wheel HPVs. It is incredibly complex, but again comes to a conclusion that gyroscopic forces are not a big player in stability.

https://www.coe.uncc.edu/~rkeanini/hu...E_Steering.htm

Intuitively, part of you knows that this is the case. Motorcyclists often replace heavy steel wheels with carbon fiber wheels that might weight as little as 1/3rd the original. Yet they will tell you that stability has not been noticably decreased! With 1/3rd the mass, you'd expect a drastic change in stability if gyroscopic inertia were the primary player in stability, since gyroscopic inertia changes proportionally with mass.

Anyway, I get a huge kick out of things like this, and its one of the reasons I love two wheeled vehicles, they are very facinating! We haven't even gotten to cool stuff like countersteering (which does have a bit more to do with gyroscopic inertia than general stability does).

peace,
sam

Brian

Sounds like Starbucks to me.

Rev.Chuck

Look at it this way, if you put whatever rake you want on a fork but a set of rollerblade wheels on the bike and get it rolling and let go, it will flop right over, do the same thing with a bicycle (ghost riding) and it will go til it slows down or hits something. I have come of the back of a motorcyle and had it go another hundred yards before hitting something and crashing.

Rev.Chuck

Continued(Had to open the store ) As for the motorcycle wheels, while A carbon rim is lighter they are still pretty massy, as the part at the outer most of the hoop, the tire, still weighs the same. One of the reasons they reduce weight is to allow turn in, those big rims resist turning the front wheel. I have a lot of moto books and amgazines that talk about this.

Playing around in the shop, we put a wheel in a fork and spin the heck out of it, you can feel it resist movement as you try and move the fork around.

Rev.Chuck

We decided to do the fork and wheel test again. I put a computer on the fork and calibrated it for the wheel, a 26x1,4 on a cheap single wall rim. Then we did a simple empirical test. We held the fork by the steerer, wheel down and spun it then tried to turn the fork. There was a resistance to turning as low as 2 mph, by 5 mph it is enough to require some effort to turn the fork and by 11 mph, very hard.

In addition I have ridden highwheelers that have zero rake and trail, the fork is pretty much perpindicular to the ground. You have to climb up into the saddle while rolling it down the street, using a peg on the frame. If it is moving the bike stays up by its self, it is pretty spooky the first couple of times because your weight is all to one side but the bike just tracks on until you swing into the saddle.

phidauex

With your empirical test, I think you are feeling the precession that allows for countersteering more than you are feeling the forces of general stability. If you hold the fork free in your hands by the steerer, and attempt to turn it to turn it, as if you were turning the handlebars, you'll feel the wheel dive to the side, which helps initiate a lean for countersteering.

Try this, hold the fork so that the wheel is at a 45 degree lean. Get the wheel spinning, and then try to rotate it to vertical. It will resist you in the same way. If that gyroscopic force was the dominant stability force, you wouldn't be able to right yourself from a lean on a bike, since the same force holding you up at first would now be holding you to the side. Incidentally, it is very hard to recover from a lean on a hiwheeler, because they lack the geometry that a modern bike has.

The rollerblade wheel example doesn't work, because the size of the wheel contributes to the geometry, and the ability of the contact patch to change position as the wheel turns.

How do you explain the computer simulations of the gyroscope-less bikes displaying stability?

These discussions pop up regularly on motorcycling forums and biking forums, and are very difficult to resolve. There are many forces at work, and determining which is dominant is very hard (and hence the subject of phD research at times).

peace,
sam

smurfy

Oh, come on, enough of this tech talk, let's see some more freak bikes!

phidauex

Hear Hear!

I present to you, a friend of mine on SS Hotdog Bike!

Rev.Chuck

"How do you explain the computer simulations of the gyroscope-less bikes displaying stability?"

It is wrong.

Dutchy

More like Dali I think.

CHEERS.

Mark

Brian

Consider how Pollock died...

capsicum

Flopping around on the deck of a purse seiner in the bearring sea?

GeezerGeek

Anybody make a tall trike or quad?

phidauex

A tall trike would be neat.. It could be a truck, meant for hauling stuff, but instead of sitting in front or behind the load, you'd sit above it! Maybe have recumbent style seating position about 7 feet in the air, with a big load basket beneath it. I'd ride it!

peace,
sam

Gurgus

That is a damn cool idea! I wanna see it done. maybe instead of hauling stuff, you could haul people, kinda like a tall-trike taxi.

phidauex

Hmm.. Tall taxi, eh? I'm liking it! It'd be like a double decker bus, only in bike form.

peace,
sam

MadMan2k

You'd have to have a pretty low gear to get that mass moving... Maybe you could have some more sets of pedals down there for the people to help out.

Stacey

I agree, the gearing would be an issue. You could also use a double reduction. A normal triple chainwheel up top (primary) running down to the large ring on another triple at the bottom (secondary) , that would drive a 5, 6 or 7 speeed rear hub. Then use the remaining two chainwheels on the sceondary to 'split' the primary ratios. This would effectively give you a transmission similar to the old twin 'twin stick' Mack truck 13 speed transmission with a two speed rear... only the ordering would be rearranged. This, 3 X 2 X 5, 6 or7 speed gearing should give adequate reduction to get the mass moving, yet offer enough top end to keep you from tiring too fast at an elevated cadence.

Gurgus

Fantastic.

Stacey

Thanks, I try.