Fork dive

[Propellor]

This one cropped up in a three way conversation on Wednesday. I was outvoted 2:1.

My pal says that when he's on track days and braking at the end of a fast straight the forks on his bike (telescopic btw) compress hard. No surprises so far. But the interesting bit is that he says that if he takes some load off his arms and grips the tank harder with his legs the fork dive is less.

I said well maybe you're adjusting the centre of gravity further back. He says no, it's because he's not pushing on the bars as hard.

I don't see it that way. If your centre of gravity is the same, braking force and all else except the way you "attach" yourself to the bike (keeping the same body position), then the fork dive should be the same.

This one crops up a lot, so which side of the fence are you?

[SteveC#222]

Physics at this time of morning!

.... I suppose that of you are riding with your body 'loose', as you brake the whole of your upper body will pitch forward over the forks.  If you grip the tank tightly with your knees you are bracing your pelvis and lower back so you wouldn't pitch forward so freely therefore transferring less mass forward over the forks. 

That ( I think) means that you have shifted the pivot point of your body and therefore the centre of gravity so ( I think) that means I agree with you........but it's far too early for this sort of question let alone an answer as I'm still working on stringing a co herant sentence together so I could well be talking a load of tosh...I'm going to have a cup of tea now......


...by the way the answer is to buy a set of leading link forks then they won't dive!!!

[CrazyFrog]

If you are bracing yourself against the tank with your knees, you aren't going to take all the force in your arms, so maybe it just seems like the forks aren't diving as much?

[timbo]

Is it not just the simple fact that he's not putting so much of his bodyweight down through the forks, which,as said, means his centre of gravity has moved back also. Seems pretty obvious to me, and can't see how it is any more complex than that 

[Propellor]

.........Physics at this time of morning!


...by the way the answer is to buy a set of leading link forks then they won't dive!!!

Welcome to my world! 

......or telelever, or hossack, or fior, or any of the hub centre jobs. Not that any of them intrinsically eliminate dive. Wouldn't it depend on the travel path traced out by the wheel spindle?

[Propellor]

If you are bracing yourself against the tank with your knees, you aren't going to take all the force in your arms, so maybe it just seems like the forks aren't diving as much?

With respect to the conversation in question, that's exactly what I wondered.

[johnr]

also, if you slacken your grip with your arms, then you arent likely to be pulling the brake as hard are you! dive iscaused by the weight transfer when the brake is applied and the momentum of bike and rider are trying to push the bike forwards. if you loosen your grip on the bars, you will get less dive, not becaust the gripping with the knees is moving the load elsewhere, but because you arent holding the brake on as hard. the points of contact with the bike are immaterial. because the handlebars are an unmoving (in the sense that they dont move with the suspension in any other plane but side to side) part of the bike, holding them tight or slack will matter not a jot, a rider weighing for the sake of argument, 10 stone, will exert the same forces on the bikes suspension whether he is holding the bars tight or not at all.

[Propellor]

also, if you slacken your grip with your arms, then you arent likely to be pulling the brake as hard are you! dive iscaused by the weight transfer when the brake is applied and the momentum of bike and rider are trying to push the bike forwards. if you loosen your grip on the bars, you will get less dive, not becaust the gripping with the knees is moving the load elsewhere, but because you arent holding the brake on as hard. the points of contact with the bike are immaterial. because the handlebars are an unmoving (in the sense that they dont move with the suspension in any other plane but side to side) part of the bike, holding them tight or slack will matter not a jot, a rider weighing for the sake of argument, 10 stone, will exert the same forces on the bikes suspension whether he is holding the bars tight or not at all.

Yes I agree that how you attach yourself to the bike won't affect it, unless you also shift your cg. I tend to disagree about applying the lever though. I think you'd be able to ride loose armed and still apply the lever.

I think there's another component to fork compression, added to that caused by load transfer. It's the "mechanical" reaction to the tyre contact retarding the bike.

We are pretty much singing off the same hymn sheet though. 

[manxie]

I`m far from an expert on matters like this, and understand what everyone has said so far but I think I`m with Johnr on this one? I`m guessing it is simply a case of how good your brakes are, how hard you pull on the lever and the thickness of fork oil etc? I had a panic moment years ago and had to brake HARD from a great lick of knots, I was heaving on the lever, my arms were locked straight, I was hanging on with  my knee`s and my a$$ was twitching. The nose of the bike was pointing downwards and I just thanked the lord I had R1 brakes on my new Fazer back in 1999  There was anti-dive devices fitted to the likes of GPZ`s in the 80`s / 90`s, don`t know if they were any good but maybe something needs fitting to the bike in question at the beginning of this thread   

[silver]

for my two pence worth. I think the centre of mass  (the bit that is acting against the braking force) is lower and further back and therefore isnt exerting so much force on the forks. Need a strong grip with you knees though madam!.
I'll check it out on friday as i am doing a trackday at Cadwell. If i sail straight on at the end of park straight i'll blame you lot!
Dave

[Propellor]

for my two pence worth. I think the centre of mass  (the bit that is acting against the braking force) is lower and further back and therefore isnt exerting so much force on the forks. Need a strong grip with you knees though madam!.
I'll check it out on friday as i am doing a trackday at Cadwell. If i sail straight on at the end of park straight i'll blame you lot!
Dave

 

Well volunteered. Field testing is a tough job. But somebody has to do it!

[silver]

Have to confess i didnt do much testing for this theory. It was freezing cold and mostly damp and drizzly, therefore hard braking was not something to be trifled with. Had quite a few rear end slides particularly going over the mountain. Top temp was about 6 deg. Main focus of attention was just staying on.  Next time maybe!
Dave

[Propellor]

Have to confess i didnt do much testing for this theory. It was freezing cold and mostly damp and drizzly, therefore hard braking was not something to be trifled with. Had quite a few rear end slides particularly going over the mountain. Top temp was about 6 deg. Main focus of attention was just staying on.  Next time maybe!
Dave

No worries dave. You kept it right side up by the sounds o things. Hopefully you had an enjoyable day in spite of the conditions. 

Cheers

Andrew.

[silver]

It was nice to ride in conditions where lack of power was a distinct advantage. Only ones passing me were on full wet tyres. Next outing is at Blyton Park on April 17th.