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 Post subject: Geometry
PostPosted: Fri Nov 09, 2007 9:07 am 
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Joined: Wed Aug 08, 2007 12:15 am
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Location: North Yorkshire
Maybe I understand this more than most, being from a precision engineering background. But castor and axle lead, both play an important role here too. The trouble is every damned fork has a different level of castor and axle reach, not to mention crown reach. It's the frame manufacturers that have the problems to solve.
This affects the handling dramatically. Some forks have no Castor, being parallel or in line with the headtube. others run forward to the axle at quite an angle. Fork crown profiles are widely different too. Some are angled well forward so the stanchion crowns are pitched in front of the crown race at varying degrees. Then there is axle lead, this varies from zero, and in the extreme 30mm plus. A ponderous steer effect is down to the axle being placed too far back, i.e. back nearer the crown, there is an optimum for each frame, accrding to head tube angle, so different forks will work better or worse. Interestingly enough fork manufacturers don't declare these numbers or adhere to a standard, which they should. Changing or upgrading a fork therefore, despite spending a fortune, can end up with a worse handling bike!
The whole subject is a can of worms.


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 Post subject: Re: forking
PostPosted: Fri Nov 09, 2007 11:31 am 
retrobike rider
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Joined: Sun Sep 23, 2007 11:22 pm
Posts: 7306
Location: Hove
Wold Ranger wrote:
Just to throw a complete spanner in the works, many 65-80mm forks were actually shorter at full extension, axle to crown, than many old rigid forks! I am not on about modern suss corrected, genuine OEM rigids, so a 100mm will possibly only add 25 mm! Try measuring an original P2 or UGLI fork, I was surprised. Take off 30mm of sag and your back at original geometry or steeper!


The P2 off my 97 Kilauea is 413mm and 80mm 2003 SIDs are 453mm, both measured in the same way. You obviously like a lot of sag, but I run c20mm, so the SID raises the front end by c20mm (loaded), which reduces the head angle by just over a degree from the 71 the frame was designed for. That makes the steering a bit slower, but then the fork still has another 60mm of travel left, so with more compression under deceleration etc, it can go steeper yes.

But what I'm saying is that the Kilauea now has a head angle of roughly 69 static and 70 loaded, whereas the ideal - or the way Kona design a frame for an 80mm fork anyway - is 70 static and 71 loaded. So it's not far off, but it's not the ideal.


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 Post subject:
PostPosted: Fri Nov 09, 2007 11:38 am 
Retro Guru
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Don't all forks have some sort of castor simply because of the headtube angle? Unless you know of a vertical headtube?


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 Post subject: Re: Geometry
PostPosted: Fri Nov 09, 2007 11:45 am 
retrobike rider
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Joined: Sun Sep 23, 2007 11:22 pm
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Wold Ranger wrote:
Maybe I understand this more than most, being from a precision engineering background. But castor and axle lead, both play an important role here too. The trouble is every damned fork has a different level of castor and axle reach, not to mention crown reach. It's the frame manufacturers that have the problems to solve.
This affects the handling dramatically. Some forks have no Castor, being parallel or in line with the headtube. others run forward to the axle at quite an angle. Fork crown profiles are widely different too. Some are angled well forward so the stanchion crowns are pitched in front of the crown race at varying degrees. Then there is axle lead, this varies from zero, and in the extreme 30mm plus. A ponderous steer effect is down to the axle being placed too far back, i.e. back nearer the crown, there is an optimum for each frame, accrding to head tube angle, so different forks will work better or worse. Interestingly enough fork manufacturers don't declare these numbers or adhere to a standard, which they should. Changing or upgrading a fork therefore, despite spending a fortune, can end up with a worse handling bike!
The whole subject is a can of worms.


Yes, according to the Kona catalogue the Project 2s on the Hahanna and Fire Mountain used to be different from the ones on the bikes from the Lava Dome upwards, with 1.74 inches of offset for the 'entry-level' rider and 1.54 inches for the Lava Dome upwards. So obviously 0.2 inches on the offset must make a palpable difference in steadying up the steering, otherwise a bunch of cheapskates like Kona wouldn't have bothered with it. My general impression is that most (at least xc) suspension forks do have offset around 1.5 to 1.6 though.

Isn't there quite a lot of significance also in the way that the head angle affects 'trail', which is the distance between the centre of the tyre's contact area and the point on the ground that the steerer is pointing at?


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 Post subject:
PostPosted: Fri Nov 09, 2007 12:07 pm 
retrobike rider
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andrewl wrote:
Remember the Kona angles quoted in their catalogues are a bit dubious. I can put a 97 frame next to a 98/01/02/04 in the same size and all the measurements and the angles are identical. This means the catalogue info can't be correct for all cases as it covers rigid forks, 65mm forks and 80mm forks which were all standard fitment over the years and are all different axle to crown lengths. Yes they quote a sag corrected figures in some years but even then it doesn't quite add up.

In some respects I'm surprised that you find the 80mm SID makes the steering slow. Rather than the length of the fork, my bet is that it could be more die to the flexy nature of the SID than the length. ;)


Andrew, I bow my head in shame! One, the angles I mentioned are as measured by me, not as quoted by Kona. Yes, I'm that sad (but it is quite easy!) Second, I've never found SIDs flexy. I put this down to only weighing 65, I mean it couldn't be that I'm a completely feeble rider, could it? Err, well, umm. Oh.

But hey, you're a big guy, you like a heap of travel, you like a particular head angle and with Z1s your Hei Hei gives you precisely that angle you like. And it looks fab, what more could you want? Oh alright, you can have a new Prime Minister. Anything else?


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 Post subject:
PostPosted: Fri Nov 09, 2007 2:48 pm 
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Joined: Fri Sep 22, 2006 1:12 am
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Location: Hobart, Tasmania, Australia
Anthony wrote:
I've never found SIDs flexy. I put this down to only weighing 65, I mean it couldn't be that I'm a completely feeble rider, could it? Err, well, umm. Oh.

But hey, you're a big guy, you like a heap of travel, you like a particular head angle and with Z1s your Hei Hei gives you precisely that angle you like.


By big I assume you mean that mean taller, as at 70kg and a tad over 6' big is not a word most people would use to describe me :lol:

As for the relative rigidity of SIDs, well I'm happy to agree to disagree, but my SIDs are noticeably the flexiest of any late 90s fork onwards that I own.


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