Girvin Geometry vs Typical Forks

[beeeerock]

Until today I had assumed that the Girvin fork with the wheel mounted behind the fork produced essentially the same geometry as a 'generic' fork, with the wheel mount just to the front of the fork.  However, after looking at the photo in this recent thread http://idriders.com/proflex/smf/index.php?topic=4041.0 and the photo of my bike here: http://idriders.com/proflex/smf/index.php?topic=3994.0 I'm thinking there might be as much as an inch of difference in hub position.

I'm wondering if anyone has ever compared the geometry side by side?  During compression, the Girvin fork is going to push the wheel slightly forward too, as the parallelogram does its thing... if I'm visualizing this correctly.  That would change the effective angle of the headset... even more so as the remainder of the frame pivots down around the rear wheel.

[fyrstormer]

This sort of thing is REALLY hard to determine visually. The Girvin may start with the hub further forward, but the J-path shape of the suspension travel means the hub will end up roughly in the same position at maximum compression as it would if it were attached to a telescoping fork. It also means that most of the rearward movement of the hub will happen during the first part of the suspension travel, which gets used-up just by putting your own weight on the bike. Unless you run your Girvin fork with zero sag (which sounds painful if not impossible), the position of the hub when there's no weight on the bike is not indicative of the position of the hub when you're actually riding.

[beeeerock]

Unless I'm not visualizing this correctly, I'd say that the fork assembly will move forwards relative to the rider as you add weight from the completely unloaded position.  Assuming 20% is used up by static rider weight, that probably doesn't put the assembly fully forward.  Without seeing how far this fork can travel before reaching the upper limit, I'm going to assume that the 50% loaded position is fully forwards, so extreme loading beyond that will start to bring the hub backwards.

However, I've seen reference to the 'J-path' a few times so maybe I'm missing something in the way this works... I would have assumed something closer to parabolic.  But, the upper arm looks shorter, so there is going to be something a little different about the path, right?  Perhaps this is where the 'J' comes from?

[Spokes]

The j path term is kind of right. Under no load imagine the wheel is at the bottom of the J. On hitting a bump the wheel starts to move in towards the bike( shortening the wheelbase slightly) then move up the J as the suspension compresses.  It makes for super sensitive(comfy) and stable over small hits as the wheel is moving back and forth. Bigger hits then use the 90mm travel.

I've often wondered myself if the wheel base is altered when using a Tele fork but not measured it. I can say that I have 90mm teles on a 957 and Girvins(90mm) on my 857 and the bars on the 957(teles) sit about an inch lower. Because Girvins have the ulm, which sits on top of the headset,  the bars sit at least an inch higher. Makes for a more comfy riding position.

Chris

[Colin]

beeeerock, the axle definately moves backwards and up along the "Holy Girvin J-Path" movement.....
..............(also known as the "Holy Sh1t" J-Path movement as you go over the handlebars!) <GRIN>
If you don't believe me, remove the shock and work the fork through it's travel, in this way you will achieve enlightenment.........

I believe the J-Path is more accentuated (but adjustable) on the Vectors than the Crosslinks (not adjustable), but is still evident on both.

On all suspension forks, I've always felt that "dive" feeling as you go through a big rut as it compresses and almost pauses so that it that makes you feel that you're going over the handlebars, which never happened with a rigid fork, which just felt that it was "driving" through, but I have accepted that as the trade off of front suspension. It seems to be the same feeling for a few people I know who have started out on rigid forks.

and Chris, I'd definately say that the wheel base shortens on all suspesnion forks, after all they are at an angle and so any upwards movment also results in a movement back towards the rear wheel.

See my exceptionally boring posting previously on a similar subject.
http://idriders.com/proflex/smf/index.php?topic=3827.msg28980;topicseen#msg28980
Here's another oppurtunity to rip my logic to shreds!

Col.

[Spokes]

Unfortunately Col your link seems to be in a foreign language I can't read, ehem. 

My wheelbase query referred to whether Tele forks shorten the wheelbase before they are compressed. Ie does just swopping Girvins out for Teles on a  Proflex shorten the wheelbase of the bike?
I know under load all forks shorten the wheelbase as you say due to their angle.

Holy Girvin J-path - 

Chris

[beeeerock]

I'm having troubles visualizing the Holy Girvin J-Path... so I sketched it based on the approximate proportions in the photos.  Not exact by any stretch, but I wanted to understand the way the two lever arms work together.  The attached image is what I came up with.  It suggests the motion is actually forward as the suspension compresses, relative to the headset.  Note that I'm not factoring in any rotation around the rear axle because I'm more interested in what the front one is doing.  If the suspension is working optimally, the bike should stay level and the suspension move up and down relative to it.

I'd love to see a diagram from Girvin to see if I'm even close, but Google wasn't cooperating today.  I did find an interesting patent document though, which suggests the lever arm can be adjusted in length to modify the path we're talking about.

If my sketch is correct, then I can understand why my regular Marzocchi forks might give me more of an 'over the bars' feeling... because they move in line with the headset and have no forwards/backwards motion other than from the angle of the headset.

I don't have Girvin forks and have never seen them in the flesh, so this is all based on diagrams and photos! I might have to try again with some different proportions to see how that changes things...

[fyrstormer]

You've got the right idea, but the two links don't start at the same angle. The lower one is mostly level while the upper one is slanted sharply downward. This causes the upper link to push the top of the fork legs forward as the suspension compresses, while the lower link pulls the bottom of the fork legs backward.

My experience with my Girvin Vector was, it felt good in a straight line, but when I was cornering, the J-path would cause the front hub to move backward AND outward relative to my cornering line, which made me feel like I was going to lowside the bike. A telescoping fork doesn't move the front hub inward or outward relative to the cornering line, which makes it more stable in corners.

[beeeerock]

You've got the right idea, but the two links don't start at the same angle. The lower one is mostly level while the upper one is slanted sharply downward. This causes the upper link to push the top of the fork legs forward as the suspension compresses, while the lower link pulls the bottom of the fork legs backward.

Looking at the photo at the start of this thread http://idriders.com/proflex/smf/index.php?topic=4041.0 gave me the impression that both links were at approximately 45 degrees from the line of the headset.  Is the one in the photo not typical of the shock system, or perhaps the photo is misleading?

[Colin]

ooooooh, you went and got me hooked beeeerock!, I just had to check this out, despite knowing it as a "fact".

So, I went to one of my spares hidey holes and got out a pair of standard length Crosslink ELT's with no shock installed. I then set the "critical measurment" of 153mm between the ULM and the bottom steerer post flange. I then measured the pivot point locations and lengths and produced this diagram:

http://idriders.com/proflex/smf/MGalleryItem.php?id=523


(Bearing in mind I measured everything just with a ruler, and so a potential error of +/- 1mm is possible.)
My own findings is that the J-Path is very minimal on the Crosslink, but does exist and it's definately in a rearward direction!

Important points to bear in mind are that the lower link is longer (90mm) than the upper link (65mm) and the ULM throws the upper pivot point 35mm forward of the Lower pivot point.
The only real imponderable is the angle of the upper pivot that the movement starts from at rest, and I have shown this as 45deg in the diagram, but is more likely to be steeper than this, more like 50-60 deg from the horizontal. PLacing the fork legs roughly parallel to the steerer tube.
But importantly the lower pivot is always at a much shallower angle and so you can see from the arcs of movement that the upper fork pivot point rises quicker and is thrown further "forward" than the lower fork pivot point, thus creating the slight rearward movement of the axle point.

Go and have a play with these measurments Mr beeeerock!

I do recall that someone else once produced a diagram of the "Holy J-Path" but I couldn't find it anywhere.

One other interesting finding was that whilst the axle looks like it trails the fork, in fact it is in direct line of the two upper pivots beacuse the upper pivot location on the fork arms is forward of the centre line of the fork shaft.

(<sigh> I'm going for a lie down now!)

P.s. "spares" safely hidden away again from prying wifey eyes in the spider infested garage.

Col.

[beeeerock]

Good to know someone else suffers from the same Just-Gotta-Knowitis as I do!    If we'd grown up in the same neighbourhood, just think of how much stuff we could have taken apart together as kids!! 

When you refer to shallow angles and 'horizontal', do you mean when the assembly is in the approximate orientation as on the bike?  That is, if your diagram was rotated 90 degrees clockwise?

I can see already where my error is... my sketch was based on the photo with no firm measurements and if I set the links in my sketch to both be horizontal, the lower total horizontal length of the link is greater than the upper.... just the opposite of what you have drawn.

I will play with this a little more when I get a few minutes and post the curve I generate here for posterity... 

Thanks for taking the time to indulge my curiosity! 

[w2zero]

Just cut out some cardboard facsimiles of the links, locations and a fork leg then stick pins in the appropriate places to determine what may happen.  As far as the axle path, a slight rearward movement also happens with tubular slider forks since the legs bend back ever so slightly on impact with an object more or less depending on the size of the hit.

[fyrstormer]

Ah, but the Girvin fork will bend backwards AND pivot backwards, so the backward travel of the hub is still more pronounced than it might be with a telescoping fork.

[Colin]

Good to know someone else suffers from the same Just-Gotta-Knowitis as I do!    If we'd grown up in the same neighbourhood, just think of how much stuff we could have taken apart together as kids!! 

When you refer to shallow angles and 'horizontal', do you mean when the assembly is in the approximate orientation as on the bike?  That is, if your diagram was rotated 90 degrees clockwise?

Ha ha, yes I love taking things apart and creating new things from them!

yes all angle references are to the bike as it sits on the ground.

Col.