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  1. #1
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    Default Springer and Girder setup #'s and behavior

    Hey all - I recently purchased a piece of software written for the purpose of analyzing the characteristics of various pivot-based motorcycle front ends. The designer intended it for stuff like the BMW K1300, Yahmaha GTS1000, and so forth, but it can equally be applied to traditional and custom springers and girders.

    I'd like to build up a database of such designs and the results the program spits out, and compare them with how people subjectively feel they handle. I think it could help confirm some "rule of thumb" notions, and maybe improve on them.

    To do that, I'd need to get some measurements from bikes that have springers and girders, to enter into the program. If anybody provides me the numbers, I'll run 'em and post the analysis the program spits out, which is basically a set of charts showing the dynamics of fork dive and rake / trail variation due to bump travel. Figured it might be fun for folks with such bikes to see, especially if considering changing the rockers or some such.

    It doesn't tell anything like the whole story, but I think it would be really interesting to use this to compare (say) the dynamics of a stock springer to (say) a Sugar Bear style fork. Might also shed some light on why some of the old designs were bad... or point up some as being potentially rather good.

    Post to follow with a form for the numbers I'd need.
    Last edited by 53Bash; 12-26-2013 at 6:25 PM.

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    Default measurements

    measurements:
    These need to be pretty precise, and there's a lot of them. Its probably simpler to get them all from a profile picture. The picture would need to be taken as a square are on side view, from more than 30 feet away, with a yard stick placed upright against the front tire so I can use that as scale for measurements. If you want to take the measurements yourself, its still easier post such a picture, and I'll draw in lines for the measurements I need; describing them verbally for various styles of fork would be a cluster fuck.

    weight distribution (optional)
    There's some weights I can enter, but are really only used to figuring out anti-dive, so could be left out. Combining these numbers, the program figures out the bikes center of gravity and uses that to figure out dive ratios (and no, I'm not entirely sure what 'dive ratio' means, but if your bike brakes really nice or wallows when stopping, even if not using front brake, it could be a factor).

    weight on front wheel (with bike level):
    weight on back wheel (with bike level):

    weight on front wheel (with bike front elevated):
    weight on rear wheel (with bike front elevated):
    elevation of front wheel from level in previous measurements:
    Last edited by 53Bash; 12-26-2013 at 7:03 PM.

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    That software have a NAME or it is some NSA secret?

    What activates it? Does it "phone home"?

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    Program is "Software for the analysis of alternative front suspension" from http://www.tonyfoale.com/

    Copy protection requires you to email Tony a key generated during installation; the program won't run until you enter the counter key he emails back. Other than that, it can run offline, etc. He hasn't given me any hassle about doing a couple new installs after a single purchase (which is required whenever I switch to a new computer). Its a pretty good program, but only useful for the single purpose of analyzing pivot based front suspension.

  5. #5
    SamHain
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    heres some decent webb dimensions.

    http://velobanjogent.blogspot.com/20...k-at-some.html

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    Intresting, thanks, just came back looking for this after long delay. Might be fun to run those.

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    How does the 'program' figure out the center of gravity if you don't enter 'moments' about an axis of some sort. Seems to me you need to know the weight of various components and distance from some established control point to come up with any valid database from which to derive CG, both vertical and horizontal. I know this is an 'optional' data-set but I can't see how you can do any real fork simulations without knowledge of CG. Without this the braking and acceleration calculations would be almost meaningless.

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    Quote Originally Posted by Smiley View Post
    Without this the braking and acceleration calculations would be almost meaningless.
    Yeah ... and it doesn't do any braking or acceleration calculations. It also can't handle any fork that uses any sort of sliding motion (conventional telescopic, tele-lever, saxon). All it calculates are changes in wheelbase, rake, trail, and anti-dive character for various degrees of suspension compression of pivot based front end suspensions (springers, girders, various hub steering setups, Hossack, etc). Pretty special purpose, but also pretty damn useful if building such a system from scratch (as I am).

    COG and wheel moment don't impact the first three any, and it does account for those two items when figuring dive. The manual includes a guide to calculating them using some fairly simple measurements, and there's some tools in the software to help. For example, to calculate COG, you don't need to know anything like various component weights; you just measure the weight on the front and back wheel, then raise one end up a known amount, and weigh again. Enter the 4 weights, the amount you raised the end by, and the wheelbase into a form, it spits out the COG. If not known, reasonable approximations can be used without throwing the numbers to far off. For the setup on my bike, for example, moving the COG up or down by 12 inches only changes max / min dive by maybe 20%. Moving the pivots by less than 1 inch potentially makes a bigger difference.

    Anyhow, my interest in looking at some existing fork setups was largely in qualitative comparison, not hard figures. For example, how would a Sugar Bear style long springer (wheel axle much higher than pivot) differ from one where the pivot is level with the axle?
    Last edited by 53Bash; 08-12-2014 at 7:01 PM.

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    Quote Originally Posted by 53Bash View Post
    Anyhow, my interest in looking at some existing fork setups was largely in qualitative comparison, not hard figures. For example, how would a Sugar Bear style long springer (wheel axle much higher than pivot) differ from one where the pivot is level with the axle?
    It gives a more linear travel and less of an arc, since its essentially a longer link. Arcs are a bitch to figure out dynamic numbers, as they are constantly changing. Static numbers mean nothing.
    In the case of a springer though, the front and rear legs are much more important than the wheel though when it comes to the rocker.

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    Yeah, obviously a longer rocker would produce a less arced wheel path, but there's also factors that come into play w/ the wheel axle position relative to rocker pivot location. For example, your springer has a pretty long rocker, but the wheel axle is about even with the pivot, which (I'm guessing) means you get a large increase in trail during bump, which would create a difference in character vs a fork where the wheel axle is well above the pivot that otherwise offers similar handling. Neither's really better that I know of, but it seems hard to even find any information that mentions why you might want to build one either way.

    Compared to springers (which use a single pivot to control wheel path) the dynamics of 4 bar linkages (eg, girder forks) seem really nasty to figure out, which is exactly why I bought this program; for building my own, I'd have been shooting in the dark in terms of even very rough dynamics without it. I suspect that the people who built vintage girders either knew some math I don't (likely, engineers at the time routinely designed some impressive linkages) or built a number of mock-ups and refined the ones that seemed best in terms of basic dynamics (what I would have been forced to do).

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    Yes and no. Static numbers might be easier, but dynamic numbers should be easier with a girder, as they should stay pretty close. I'm shit lousy sometimes at explaining, but I design 4wd suspension every day, this isnt that far off, just differant measurements.

    I get what you're saying about my rockers being level, but theres more to take into account than that. I thought about the changes in trail, but I was more worried about the change in wheelbase. With an angled rocker, the change in wheelbase as the front end compresses and extends is pretty drastic, and those are the times that you dont want the wheel speeding up and slowing down extremely fast, it causes a loss of traction and control, and if you happen to be on the brakes would change the braking forces on the wheel. At least in my head of course, and I've been told I've been wrong before, but I have to run with what makes sense to me.

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    Hmm, the wheelbase issue makes sense. Setting brakes up on a floating caliper with their own linkages (which it seems a lot of long springers do) may help deal with that when running a 'high axle' springer, but is more than I'd know how to account for. Probably a lot off different setups can work equally well, maybe for different reasons.

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    Driver and terrain has a lot to do with it as well.

    Once you figure out what the numbers mean, you can build to suit. When I build suspensions at the shop the first question is how do you drive, and where do you drive. You can tune these front ends so much, a very agressive rider, constantly on and off the throttle, and hopping curbs, would benefit from a differant setup than the guy that never gets over half throttle and is just cruising to curch.

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