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Thread: motorcycle geometry specs

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    Member thro's Avatar
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    motorcycle geometry specs

    So, looking to do a few calculations and was wondering if anyone had found a source for (detailed) motorcycle geometry specs?

    What i'm chasing specifically is:

    Swingarm pivot to axle length
    swingarm pivot to shock mount length
    shock mounting angle

    ... basically to make a spring-rate calculator for rear shocks... Also to get an understanding for myself on how the various online calulators work, in order to see what assumptions they're making when you say, select "street use" or "track use" or whatever.

    I could measure, but if the specs are already out there, i can just not.... (and could even embed them into the spreadsheet/app/whatever).

    These measurements don't appear to be in the service manual, at least for a 600RR.

    (i've googled a bit and haven't found anything yet, hence the query...)
    “Crashing is shit for you, shit for the bike, shit for the mechanics and shit for the set-up,” Checa told me a while back. “It’s a signal that you are heading in the wrong direction. You want to win but crashing is the opposite. It’s like being in France when you want to go to England and when you crash you go to Spain. That way you’ll never get to England!” -- Carlos Checa

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    Scott52..?
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    Each bike model is slightly different, you'll get the best results from taking your own measurements for a specific bike.

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    Member thro's Avatar
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    Quote Originally Posted by Water pig View Post
    Each bike model is slightly different, you'll get the best results from taking your own measurements for a specific bike.
    Sure.

    But 1-2mm on swing-arm length really doesn't make that much difference (and isn't going to throw you so far out that you're purchasing a different spring, as they're only available in a limited number of rates anyway) and is within the range of chain adjustment. 5-10mm out on say a 700mm swing-arm pivot to axle length (for example) is roughly 1 percent.

    You can adjust that out with the preload adjustment.

    However having at least ball-park measurements for this will enable you to make a reasonably intelligent spring rate choice to get you in the ball-park at least. i.e., figure out how much sag you WANT, put in your weight and then get a spring rate back.

    In any case, once i get the spreadsheet sorted out i'll upload it somewhere.


    For what it's worth, shock rate was always voodoo black magic that i was going to have to rely on an online calculator for until i found this:

    https://www.qa1.net/technical-suppor...ring-rate-tech


    It's for car suspension but the physics should be the same for a motorcycle swing-arm at least for static/rider sag measurements (as there are the same moving parts involved).



    edit:
    Am aware that some bikes have a rising rate linkage, which is something else to figure out...
    “Crashing is shit for you, shit for the bike, shit for the mechanics and shit for the set-up,” Checa told me a while back. “It’s a signal that you are heading in the wrong direction. You want to win but crashing is the opposite. It’s like being in France when you want to go to England and when you crash you go to Spain. That way you’ll never get to England!” -- Carlos Checa

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    Member Water pig's Avatar
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    While I agree that swingarm lengths are close enough to each other. Bell crank size dogbone length and shock mount points are substantially different to achieve differing rising rates for a given task/ application. Spring rates are a small part of your question

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    Agreed. But have to start somewhere with working this shit out...


    edit:
    looks like the 600RR has 18% progression in rate on the rear linkage out of the box...

    http://www.600rr.net/vb/69-07-13-rr-...k-linkage.html

    Presumably this is something that can be worked out via maths given measurements of the linkage components.
    Also interesting to see how various bikes are more/less linear. Rising rate obviously being a compromise (ideally you'd want linear or closer to it for consistent suspension performance) in order to use softer springs yet not bottom the fuck out as soon as you put a passenger on the back...cater for both fatties and skinny people out of the box, etc.


    Also, according to the guy making linkages, the spring rate shouldn't be affected too much by the linkage. i.e. get your rate correct (the linkage won't fix or ruin that), the linkage will make more difference at the bottom half of the stroke (abnormal events like massive bumps, etc.).
    Last edited by thro; 17-04-2018 at 11:47 AM.
    “Crashing is shit for you, shit for the bike, shit for the mechanics and shit for the set-up,” Checa told me a while back. “It’s a signal that you are heading in the wrong direction. You want to win but crashing is the opposite. It’s like being in France when you want to go to England and when you crash you go to Spain. That way you’ll never get to England!” -- Carlos Checa

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    Member thro's Avatar
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    Quote Originally Posted by Water pig View Post
    While I agree that swingarm lengths are close enough to each other. Bell crank size dogbone length and shock mount points are substantially different to achieve differing rising rates for a given task/ application. Spring rates are a small part of your question

    Oh you were referring to different measurements for different bike models? Yes definitely. I thought you were referring to different lengths/mount points, etc. for different examples of the same bike.

    Aware different models are different. Hence me wanting to find what they are for various bikes rather than just measure my own


    edit:
    primer on linkages:
    http://www.promecha.com.au/leverage_linkages.htm
    Last edited by thro; 17-04-2018 at 11:57 AM.
    “Crashing is shit for you, shit for the bike, shit for the mechanics and shit for the set-up,” Checa told me a while back. “It’s a signal that you are heading in the wrong direction. You want to win but crashing is the opposite. It’s like being in France when you want to go to England and when you crash you go to Spain. That way you’ll never get to England!” -- Carlos Checa

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    Member Water pig's Avatar
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    I’m happy to measure my first gen Daytona for you if you’re interested

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    Quote Originally Posted by thro View Post
    So, looking to do a few calculations and was wondering if anyone had found a source for (detailed) motorcycle geometry specs?

    What i'm chasing specifically is:

    Swingarm pivot to axle length
    swingarm pivot to shock mount length
    shock mounting angle

    ... basically to make a spring-rate calculator for rear shocks... Also to get an understanding for myself on how the various online calulators work, in order to see what assumptions they're making when you say, select "street use" or "track use" or whatever.

    I could measure, but if the specs are already out there, i can just not.... (and could even embed them into the spreadsheet/app/whatever).

    These measurements don't appear to be in the service manual, at least for a 600RR.

    (i've googled a bit and haven't found anything yet, hence the query...)
    Thro have you considered using high-res side on photos and taking measurements in a photo editing program? For more popular bikes you can find frame-only pictures on the googles.

    You might not get upper shock mount locations but certainly swing arm lengths shouldn't be a problem. I believe that's what they use to power cycle-ergo.com
    Quote Originally Posted by devolved
    I just crashed into a schoolbus full of kiddies, killed them all, raped them, set them on fire and now I'm pissing on their charred remains.

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