Date: 14-11-18  Time: 23:48 PM

Author Topic: Am I being silly?  (Read 177 times)

tommyardin

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Am I being silly?
« on: 24 October 2018, 10:47:08 PM »
Here some food for though and I would appreciate others ideas on the topic.

I have been thinking about the topic of forks and cornering.

I have heard tell that raising the fork stanchions up through the top steering yoke by 20mm improves the turning in (Steering) on a motorcycle, doing this lowers the front of the motorcycle by that amount, now does lowering the front of the bike by 20mm improve the handling? I can't see it myself, what else it does do is effectively shorten the bike by that amount, now I can see that making a difference to the turn in on a motorcycle, a slightly shorter wheel base.

I have also read that by raising the forks (Lowering the bike) by 20mm causes the bike to be a little less stable on the straights at higher speeds.
If this is true there is a trade off going on here, slightly better steering for sightly worse straight line stability.

Now when approaching a bend we knock the bike down one or maybe two cogs and gently brake to reduce our speed to a safe pace to take the corner, now very gently, and I mean gently hold the front brake a little longer into the bend will cause the bike to dip slightly on the front forks, effectively lowering the front of the bike, or if you prefer shortening the bike as we approach the apex of the bend.

It seems to me that has exactly the same effect as raising the fork stanchions (or lowering the bike) but that shortening it is recoverable as soon as we let go of the front brake lever so there is no compromise to the straight line handling or stability, so as you start to apply power coming out of the apex let the front brake lever go as well
at that point, bike rises with the power and bike is now at it's maximum length and straight line stability is returned in full.

Wot-ch-fink ?
I spent half my life getting drunk and chasing pussy, the other half I just wasted.

Fazerider

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Re: Am I being silly?
« Reply #1 on: 25 October 2018, 10:33:07 AM »

The height of the front of the bike isn’t really what matters, though it’s convenient as a measurement (at least, the amount the forks have been raised with respect to the yoke is).
What’s happening is the steering head angle has become steeper. This reduces trail: the distance between point on the road the steering head axis points at and the tyre contact point.
Reduced trail means a reduction in the castor effect so the resistance you feel at the bars when trying to turn is less..
Anything that compresses the front forks has the same effect so yes, braking, hard cornering or hitting a bump reduces trail and quickens the steering. As a result, raising the forks needs to be done with some caution, what feels good for normal riding may be unpleasantly twitchy during emergency braking.

Grahamm

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Re: Am I being silly?
« Reply #2 on: 25 October 2018, 10:29:07 PM »
Consider an extreme example of a "Chopper" style bike, with very long front forks at a much shallower angle than a regular bike.

It will have great straight line stability, but there's no way you can flick it through the twisties.

Raising the stanchions (or dropping the yokes, same thing, just a different point of view) will make the steering "quicker" but less stable as Fazerider points out.

You could, of course, also lower the back end by a similar amount which would effectively negate the effect, although you might end up having to cut down the stand too or needing to replace the rear sets because you'd be grounding the pegs sooner.

tommyardin

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Re: Am I being silly?
« Reply #3 on: 25 October 2018, 10:46:28 PM »

The height of the front of the bike isn’t really what matters, though it’s convenient as a measurement (at least, the amount the forks have been raised with respect to the yoke is).
What’s happening is the steering head angle has become steeper. This reduces trail: the distance between point on the road the steering head axis points at and the tyre contact point.
Reduced trail means a reduction in the castor effect so the resistance you feel at the bars when trying to turn is less..
Anything that compresses the front forks has the same effect so yes, braking, hard cornering or hitting a bump reduces trail and quickens the steering. As a result, raising the forks needs to be done with some caution, what feels good for normal riding may be unpleasantly twitchy during emergency braking.


Thanks for that Fazerider,


There are things that you have mentioned that I had not taken into consideration, for example the steering head angle change with raising the fork legs. So (Gentle) front wheel braking also does the same thing, I guess there is also a lowering of the front of the bike when shutting the throttle down fast, causing the bike to over run and gain the braking effect of the engine.
I am going to try gently :eek holding onto the front brake longer going into the apex of a bend just to see if there is any advantage gained by the lowering effect or compressing of the forks.
I think I understand the castor effect that you talked about, the closer that pivot point is to the rear of the vehicle the more exaggerated the steering response will be.
For example a building site dumper that is articulated (Centre steering) will turn sharply and on a sixpence while slow manoeuvring on rough ground but give the dumper the beans along a road and its all over the place because the steering is to acute and over responsive.
Anyway thanks for your response and the food for thought.                 
I spent half my life getting drunk and chasing pussy, the other half I just wasted.

tommyardin

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Re: Am I being silly?
« Reply #4 on: 25 October 2018, 10:52:42 PM »
Consider an extreme example of a "Chopper" style bike, with very long front forks at a much shallower angle than a regular bike.

It will have great straight line stability, but there's no way you can flick it through the twisties.

Raising the stanchions (or dropping the yokes, same thing, just a different point of view) will make the steering "quicker" but less stable as Fazerider points out.

You could, of course, also lower the back end by a similar amount which would effectively negate the effect, although you might end up having to cut down the stand too or needing to replace the rear sets because you'd be grounding the pegs sooner.


Surely that would put you back in exactly the same position as you were before you started, with no gain in quickening the steering, but the disadvantages of a lower bike causing other issues that you have pointed out. The only gain there would be if you were short in stature.  :lol 
« Last Edit: 25 October 2018, 10:53:14 PM by tommyardin »
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Grahamm

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Re: Am I being silly?
« Reply #5 on: 27 October 2018, 01:18:08 AM »
The only gain there would be if you were short in stature. 

Yes, I looked into this because, when I got the FZ6, I was uncomfortable with being unable to flat-foot it (the first time I tried it, about 5 minutes after I got the bike, I dropped it!)

Now I've got used to it, but it took time.