Safe Speed Forums

Sorry if this sounded like I was teasing, but it's a very serious point. The tenet "NEVER brake in a bend" is trotted out as being beyond any question, but it should be taken as advice, not a rigid rule. Indeed, as you say yourself (when gently pushed...):



...and...



Now I can absolutely agree with you! But the point remains that we CAN brake in a bend, albeit not much and very gingerly. Just like driving a car it's all about remaining within the friction circle and dealing with weight transfer and any suspension / steering geometry issues. (and additionally gyroscopic precession if on 2 wheels).

But knowing that we CAN brake in a bend opens up safety options that would otherwise be closed, and that can be a lifesaver. Once we destroy the "NEVER" belief we might even start to experiment and practice techniques such as trail braking, This all increases our skills and therefore our abilities as a safe driver or rider.

I'd much rather know what the cornering / braking limits are than falsely believe they are zero, as I naively thought when I first started riding bikes all those years ago.

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CSCP Latin for beginners...
Ticketo ergo sum : I scam therefore I am!

I worked for the company that designed the package, my part was optical testing of the CCDs used for beam acquisition, tracking and data reception. I designed the optical test bench and spent 3-4 months in the basement, in a clean, dark room running the tests. It was in winter too so I only saw daylight for about half an hour at lunchtime.

This was all at our site in Chislehurst, Kent, the launch did not happen until after I had left the company. Very small cog in a large project, still, something I have touched and worked on has ended up in space which is about as close as I will probably ever get.

The original intent was to launch two satellites and communicate between them and the ground, cost cuts eventually meant just one flew so the link was just ground to orbit.

Working through the wheel rotation thing was just for my own interest really as I have little call on my physics knowledge now, reminded me of the sort of things I used to think about while cycling to school.

A frustrated space cadet

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Driving fast is for a particular time and place, I can do it I just only do it occasionally because I am a gentleman.
- James May

Snap. We designed some high speed fibre optic links for the ALMA radio telescope in Chile under contract to ESO. Some of the RF guys here have designed low noise amplifiers for the Planck sattelite which is due to launch on Ariane 5 this year. I am glad not be involved in that - the prospect of five years work going up in flamea at Kourou is too awfull to contempalte

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When I see an adult on a bicycle, I do not despair for the future of the human race. H.G. Wells
When I see a youth in a motor car I do d.c.brown

Toltec. The wheel rotation thing. A valid question fom an intelligent listener would be: if angular momentum is conserved why does the car just change its attitude rather than continuing to rotate slowly? I can only answer that by invoking conservation of energy.

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When I see an adult on a bicycle, I do not despair for the future of the human race. H.G. Wells
When I see a youth in a motor car I do d.c.brown

As a teenager, I was not renowned for caution. If I had believed that the cornering / braking limits were zero, instead of finding the limits myself "experimentally", I would have saved myself the cost of many indicator stalks and brake levers!

You need to control your speed do that you don't have to brake sharply if confronted with the unexpected. It's not hard. But if you brake hard with the front brake in a bend, you're going to come off. Some token gesture braking with the back brake might be OK, but you could end up skidding. Besides, on some modern machines, the brakes are linked - applying the back brake applies the front one as well, though not as hard as if applied directly using the front brake lever.

Wind resistance possibly, thinking more of the aerodynamic change of area rather than pushing air out of the way though that may contribute. Then again the time in the air is usually pretty brief so does the rotation always actually stop before touchdown?

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Driving fast is for a particular time and place, I can do it I just only do it occasionally because I am a gentleman.
- James May

agreed.. in principle the transfer of momentum would change the pitch rate of the whole vehicle.. ie all things being equal it should continue rotating.. although as previously discussed the difference in inertia means you're only likely to induce a very small pitch rate.... so you may not see a massive change before gravity (should it be looking at the time) does its thing.

also agreed aero forces may slow/halt/prevent the pitching (i refer you again to various cars on the straight at le mans that year) before touchdown (if it ever occurs i mean).

Yes, in theory at least speeding the wheels up should cause the car to begin to rotate in pitch, then slowing them back down should stop the rotation. But in reality it wouldn't be that clear, as flying cars tend to be pretty unstable in pitch. Watching videos of the more spectacular fliers you generally see that the car is steadily rotating one way or the other in the pitch axis (and often in roll axis too!), so how nose down or up it lands depends very much on how long it spends in the air. My understanding is that the engine revving / braking effect is used as a means to moderate unwanted pitch rather than to try and achieve any fine degree of control.

In other words the typical scenario is that when the car takes off it starts to rotate nose down, which is very undesirable, so the driving wheels are accelerated as much as possible to reduce this. Even if they only affect it by a few degrees this might make the difference between a safe landing and wiping the nose off the car! Which is why when you look through rallying videos you may sometimes notice that a flying car has it's engine revving right up to the limiter, when you would normally expect a top driver to be trailing the throttle to preserve the engine while the wheels are off the ground. What he's actually doing is trying to stop the car nose-diving, and trusting the limiter to protect the engine.

[edited to add]
I agree with ed_m, presumiung of course that the driver hasn't accidentally knocked the "gravity on/off/manual" switch during a bumpy take-off...

_________________
CSCP Latin for beginners...
Ticketo ergo sum : I scam therefore I am!

As you say JT, I think the effect is small in cars and I was only really aware of it being used with bikes. All of the rally driving techniques I have seen explained concentrate on the approach and what to do with the throttle and brakes to compress the jump and land square and level.

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Driving fast is for a particular time and place, I can do it I just only do it occasionally because I am a gentleman.
- James May

I don't know as much about it as you do, but the opposite rotation caused by accelerating a momentum wheel would not be an angular increment, but a continuous rotation until the momentum wheel decelerates to stop the rotation. I don't know the maths, but I think the car/momentum wheel form an inertial system. The acceleration imparts a force on the car, which cause indefinite rotation because of the first law. Perhaps that's how it works, anyway. I forget, to be honest.

sounds good.

That should certainly be the case and is easily demonstrable mathematically for simple models. The real thing is rather more complex with other external influences/forces so I would be tempted to take the word of someone who drives a rally car about what really happens.

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Driving fast is for a particular time and place, I can do it I just only do it occasionally because I am a gentleman.
- James May

Ah good, so someone now takes my word for it