front-center numbers and frame sizing.

Discussion in 'Downhill & Freeride' started by dhkid, Jan 13, 2008.

  1. dhkid

    dhkid Turbo Monkey

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    i was just calculating front centers on exel out of interest, i found out that different manufacturers have very different front center numbers for bikes of similar size. it also goes to show how bad top tube length and even effective top tube is as showing frame size. (and this isn't on bike which have interrupted seat tubes which make eff tt measurements even less useful)

    on my short size (17.3") commencal, i have a 17.4 front center, 22.2 eff top tube
    a medium (17") sunday , it has a 16.6 front center, 23.5 eff tt
    a large (19") sunday, it has a 17.6 front center, 24.5 eff tt
    a medium (16") dhr, it has a 17.2 front center, eff top tube not known)

    i am only 5'8" and ride what (now) seems like a bike way to big for me, i knew its was long and like it like that, i actually want it a bit longer. so does that mean if i was to get a sunday, i should be getting a large frame? which ih recomends for >6' riders.


    edit: more numbers:
    17" v10, FC-15.3, TT-21.9
    17.5" v10, FC-16.7, TT-23.2
    19" v10, FC-18.2, TT-24.8
    15" 224, FC 17.1, TT-22.8
    17" 224, FC 17.9, TT-23.6
     

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  2. buckoW

    buckoW Turbo Monkey

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    Front center is very useful for dirt jump bikes to see if you will have xup/barspin clearance with the toes but for DH/FR bikes, I think the distance on a flat line from the bb to the top center of the head tube is more important. I like to think of it like a ratio or a lever between the contact points...... from rider (hands & feet) to bike to ground or more freeride oriented would be hands to feet to back wheel. I think the priority should be in other places than the distance between the hands to butt, IMO. BB height is also worthless. It should be BB drop or rise. Helps compare apples to apples.
     
  3. dhkid

    dhkid Turbo Monkey

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    that is what is measure right now. the horizontal distance between the bb and the bottom of the head tube. i was thinking it should be the top of the head tube, but turner measure their bikes this way and i though i should just stick to whats out there already.
     
  4. buckoW

    buckoW Turbo Monkey

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    Sorry, I thought you were talking about bb to front axle, which is also useful, IMO. The front to center that you are talking about is gaining popularity and I saw that Canfield is using that now as well as Scott with the Gambler. Because it is a bit new there are a couple different ways that it is being measured but the bottom line is that geo for the bikes we ride is getting more thought.
     
  5. Dogboy

    Dogboy Turbo Monkey

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    Front center is - center of BB to center of front axle.
     
  6. Steve M

    Steve M Turbo Monkey

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    Downtube length is a better gauge for sizing than real front centre (bb-front axle) or the faux-pas front centre (horizontal reach from BB to head tube) anyway, because the distance from the bars to the BB is ultimately the determining factor in how stretched out you are on the bike.
     
  7. dhkid

    dhkid Turbo Monkey

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    i was following this for front center measurements. 'B' in the pic.
     
  8. banj

    banj Monkey

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    A more useful measurement in my opinion would be to measure b from the bottom bracket to the top of the headtube, giving both a horizontal and vertical distance. Basically what you want to know is the position of your hands relative to your feet, so take the guess work out of it and measure the distance between the two points on the frame that are closest to your contact points. The only variation then is handlebar and stem.
     
  9. dhkid

    dhkid Turbo Monkey

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    yea, that would be the best. coordinates of all the important contacts on the bike. but i dont think manufacturers are ever going to do that. loads of people wont care about it too.

    the point of this thread is that i was surprised how much difference there is from manufacturer to manufacturer on frame sizing. and further more how misleading top tube measurements can be.

    go back and look at the numbers i posted, the bike i ride now has a shorter tt then a m size sunday, but to get the front center that i want, i would need a l size sunday which has a top tube over 2 inches longer then my current bike.
     
  10. banj

    banj Monkey

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    I wouldn't compare bikes by pure numbers. Everyone seems to measure the frames slightly differently. You really need to sit on a bike to be sure
     
  11. flymybike

    flymybike Monkey

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    DHKid- It's scary how much difference there is from bike to bike. I think alot of them are trying to tighen up the wheel base by sacrificing hands to feet distance.

    I've been including both BB to top center and BB to bottom center of head tube. I think the top center to BB on the horizontal is the best of all the ways mentioned. If you measure from the bottom, the head angle will lengthen or shrink the tt measurement. It's hands to feet that's important. The downtube measurement would be good for a consumer that has no actual numbers on the bikes being compared. A good SWAG measurement.
     
  12. dhkid

    dhkid Turbo Monkey

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    banj- the numbers here aren't by the manufacturers, they are the calculated front center numbers. and i think the numbers are important, they are only supporting what i feel when i tried the bikes.

    flymybike- i get what you mean by the bb to the top of the head tube being more accurate, i just did it this way so that its easier to calculate it for any bike just given the wheel base, chain stay, head angle and fork. most of the time its hard to find head tube lengths.

    nice work on the geo for your bikes. its got loads of information up there, is it just me or is the wheel base missing? thats going to get messy with all the different forks tho.
     
  13. joelsman

    joelsman Turbo Monkey

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    don't forget about the seat tube angle when looking at eff top tube, slacker seat angles stretch it out on paper, but may feel cramped when riding,
     
  14. dhkid

    dhkid Turbo Monkey

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    the whole point about the front center measurement is to that you dont have to 'take into account' anything else. that number is an accurate way know what the actual frame size is.
     
  15. pelo

    pelo Monkey

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    Yea, it´s a good point. But depending on linkage-type and chainstaylength combination, you might want to have a different length TT (front center)?
     
  16. dhkid

    dhkid Turbo Monkey

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    maybe, actually, thinking about it thats a very good point. so are you saying that it should be left to the bike designers to decide what sizing will work for that particular bike?
     
  17. pelo

    pelo Monkey

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    I guess so. I´ve had different dh-bikes through the years. Depending on design, I´ve had quite different cockpit setups/lengths. Vpp/Dw/Maestro rides more neutral than singel pivots, I think, and in that matter you might want to shift your body weight different in some situations. I´m not saying that multilink is better than sp (to leave out that discussion), but yes I do think sizing can be a little different. Especially when it comes to front center length.

    Although numbers are good guidence, I think it´s more in to it than that; linkage path etc.
     
  18. dhkid

    dhkid Turbo Monkey

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    what exactly do you mean when you say rides more neutral?

    anyways, back to sizing. i have tried a m size sunday and i would have preferred if it had a longer front center, it had a long top tube, but i jst felt like i didn't have room to move around on it like i do on my bike.
     
  19. banj

    banj Monkey

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    But what numbers are you using to calculate the front centers? I'm assuming you're using the manufacturers head angle and such right?

    Have you ever heard the saying garbage in garbage out? If the numbers you're using to calculate are flawed, because of either different ways of measuring or even the rounding off of numbers than the calculations won't be worth much right?
     
  20. pelo

    pelo Monkey

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    This is my personal experience, people might have other experiences and it might come down to suspension setup/tuning as well.
    However, first I think you can run a "plusher" rear setup on a dw/maestro if you like than on a s.p., without the feeling of riding a "boat". On a single pivot, I feel there is less room for failure, a multilink is a little more forgiving, but not nessecarily faster. It feels like dw/vpp/maestro is more neutral to braking forces and especially body weight shifts (front, rear). Kind of like riding a hardtail with suspension when needed. Don´t know how else to explain it. But I do like most bikes, you get used to them soner or later.

    You ride Commencals dhkid? They´re pretty big I´ve heard. I guess you´re right in between a M/L sunday. The Sunday is a pretty "lively" bike, so going up a size for extra stability is probebly a great idea.
     
  21. flymybike

    flymybike Monkey

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    I think anyone should be able to get a good idea of the size they have verus the size they want/need.
    To measure the "DH Size" or "standing size" take a string with a weight tied to the end (plumb bob) and hang it from the top center of the HT (actual frame, not stem or headset). Position the bike on level ground standing straight up and with the front wheel straight, you may need someone else to hold it, and measure parallel with the ground from the BB to the string (horizontal). This number will change depending on tire sizes and fork length but this will get you really close to knowing the size of frame your looking at. Then if the bike manufacturer gives you the same dimension, you can compare.
    For standup riding, this is the best way to judge the true frame size. The bike designer should make sure the seat ends up in the right spot regardless.
    pardon my kindergarden sketch:nerd:
     
  22. buckoW

    buckoW Turbo Monkey

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    To me, this seems like a shift from traditional geometry measuring borrowed from road bikes where the distance from your butt to your hands is important. With these "newer" style bikes and piloting styles comes the need for newer ways to design and measure frame geometry. A rider like Sam Hill seems, to me, to be riding more over the front, a bit more moto style than the older styles of riding where you are centered or off the back with longer chainstays and steeper head angles. I think certain styles of pilotage will work better on some designs than others because of where the rider places their weight with respect to the wheels.

    The front to center measurements aren't yet standardized but many people are thinking about these ideas. For example, look at the shift in geometry on the new Turners. Shorter CS and slacker HA than the older ones requires that you shift your weight foreward and ride a bit more moto style than the older Turners where you are a bit back and lean them over more, IMO. Interesting stuff though. I wish we could forget about the standards that we borrowed from road bikes and get on with the evolution.
     
  23. dhkid

    dhkid Turbo Monkey

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    pelo, i used to ride a vp free, and the best way i could put it is that the bike was lazy and blew through its travel.hence why i would never touch them without a ten foot pole these days. the limited time i had on the sunday, i liked. yea, i might have to get a L size sunday if i get one. but then thinking about it being more stable then m might be fine?


    quoted for truth.
     
  24. pelo

    pelo Monkey

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    Yea, it´s got over 45" wheelbase, so it´s probebly stable enough for most of us.
    I´m a little shorter than you (5'7) and will ride medium, coming from a small.
     
  25. buckoW

    buckoW Turbo Monkey

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    The unwanted side effect of the L is the high seat tube, IMO.

    I think the "lazy" characteristics of the VP Free come from the linear suspension and the long chainstays coupled with the rearward axle path of the VPP = a bike for passengers and not pilots, IMO.

    I think the M should be fine for a Sunday.