The Bum Steer

[pete376403]

All this talk of the wheels gyroscopic effect but I dont think anyones mentioned the other gyro -the crankshaft - yet.

[Grub]

Are you suggesting I should move my body to the inside?...

Well yes ... and no. It's a very individual thing and it's raised as a talking and a trial thing to see if anyone finds it interesting enough to have a play with it.

I personally do lean inside, shoulders, head and bum. The degree of that movement is very much dependant on the speed and agressiveness of the ride. At a casual cruising pace the shoulders and head go in which just weights up the inside bum cheek and for me it really really works. Carving through Paekak Hill Rd I can pull the bike down to me or push it away from me to adjust to a tightening or opening of the corner. And yup, I pretty much do it for most corners pacier than a pootle.

[Grub]

Try it. Walk beside your bike. Now, lean it over (I suggest toward you), but hold the bars straight ahead. Keep walking. Are you going in a circle? Thought not. It would be quite hard to push the bike to the gas station if it were not so..

No mystery here I think. If you "hold the bars straight" of course the bike will go straight.

Lean it instead by holding onto the grab bar at the back and the bike will turn. We know this because we have all walked along pushing our pushbikes by the seat and leaning them to turn.

Remember that this thread isn't about "countersteering is bunkum" even though it has been construed as such.. It's about using body position to give a natural impetus to the turn. It helps, it is controlled and it does work.

[Ixion]

Yes, but my point was that leaning the bike does not of itself provide any cornering capability. As you yourself say, leaning will cause steering movement and THAT causes cornering.

So it is not possible to corner just by leaning. You MUST provide steering input , or at least allow it to happen. And that is, by definition, countersteering, if the speed be great enough. Whether you countersteer by pushing the bars or forcing the bike to lean over and move the bars , as in your bicycle example.

[dipshit]

The countersteering action is used to lean the bike over - not to steer it. When the bike leans over the steering geometry takes care of the rest

This is the way I think of it and can see why some people are getting confused.

Countersteering causes a gyroscopic effect with the spinning front wheel that causes the bike to lean over. This youtube video shows it best perhaps, but imagine the guy facing down towards the ground while doing that. The effect will be to lean the bike over.



When a bike is travelling in a straight line, the centre of gravity is in line with the contact patch of the tyres. (fig 1) When the bike leans over, the centre of gravity is moved to the inside of the turn and the bike will travel in that direction. (fig 2) To demonstrate that lean angle alone does not make a bike go around a corner, imagine a bike leaned over but with the rider completely hanging off on the outside and moving the centre of gravity back over to the contact patch of the tyres. (fig 3) In such a case the bike will still travel in a straight line even though it is leaned over.

Countersteering is a good way to lean the bike over... then the large change in the centre of gravity causes the bike to go around the corner. A rider that only shifts their body weight is only making a small change on the centre of gravity, so the bike will only slowly go around a corner.

[dipshit]

And watch this one for how controllable the effect is.

[swbarnett]

All this talk of the wheels gyroscopic effect but I dont think anyones mentioned the other gyro -the crankshaft - yet.

I think the crankshaft is too small a diameter to possess much angular momentum involved. The wheels, having a large diameter, have a far greater effect.

[FROSTY:Me]

Instead of sitting behingd a friggin puter screen why not get on ya bike go for a ride and find out?
find a quiet bit of road and experiment ?

[Sully60]

I think the crankshaft is too small a diameter to possess much angular momentum involved.

Adrian Gorst and Colin Edwards II they may have a differing opinion on that matter.

[MSTRS:Me]

.... To demonstrate that lean angle alone does not make a bike go around a corner, imagine a bike leaned over but with the rider completely hanging off on the outside and moving the centre of gravity back over to the contact patch of the tyres. (fig 3) In such a case the bike will still travel in a straight line even though it is leaned over.

Rubbish. Unless the bars are fixed in the straight-ahead position. We've been over this already. How does 'dirtbike/motard-style' work then?

I think the crankshaft is too small a diameter to possess much angular momentum involved. The wheels, having a large diameter, have a far greater effect.

Then can you please explain why a bike is more stable in a corner at higher revs...

[swbarnett]

I think the crankshaft is too small a diameter to possess much angular momentum.

Adrian Gorst and Colin Edwards II they may have a differing opinion on that matter.

I'd be happy to stand corrected on this. I based my comment on nothing more than apparent logic.

[swbarnett]

Then can you please explain why a bike is more stable in a corner at higher revs...

It seems I spoke too soon. Just seemed logical at the time. What you say makes a lot of sense.

[Ocean1]

Rubbish. Unless the bars are fixed in the straight-ahead position. We've been over this already. How does 'dirtbike/motard-style' work then?

Think that's what he was getting at, the bars need to move AND the bike to lean.

Actually, just to chuck in a wee red fishy thing, lean angle is only relevant at countersteer speeds, you can turn a trials bike the opposite way to the lean, simply by compensating with body weight to maintain the CG over the contct patch.

Edit: I think that's be possible at higher speeds too, if you could get enough mass inside, but I ain't trying it.

[Bass]

I am lagging behind the discussion yet again. However, being a pedantic SOB, I think the following is worth a reply

You say you disagree - except it doesn't really seem so to me.

Oh yeah, and please do continue to argue against gyroscopic stability induced by rotational mass!

I don't think anyone here is arguing against the existence of gyroscopic forces.

At least I hope they aren't!

Forest has it right.
I have never argued that precession was not a factor in countersteering. What I have contended is that it is NOT THE MAJOR FACTOR.

As evidence, I made mention of a bike that had been created with contra-rotating counterweights (which would negate gyroscopic forces) but which reportedly countersteered just fine.

You contend that such a bike is unlikely because it is Gyroscopic stabilisation which makes it easy to ride a bicycle and so getting rid of it would make the bike difficult/impossible to ride.

My comment on this was that one only needs to annul the angular momentum of the front wheel to investigate the part that precession plays in countersteering. This leaves in play, the gyroscopic forces generated by the rear wheel and the motor and so such a bike would not be particularly difficult to ride at all.

Your move I think.

[dhunt]

A forced experiment on this subject that I had at Manfeild a few months back when my front tyre started going flat on me certainly emphisised (sp?) the necessity of counter steering to me.

What happened is the bike had a huge tendency to standup and to actually get the bike to navigate the corner required large amounts of force in the opposite direction (Counter steering) to where I wanted to go. No amount of leaning off made any difference due to the tendency of the bike to standup. As the tyre got flatter the more force was required to get it to turn.

Now while I don't suggest any one go out and try this experiment it certainly proves the point.