Forward Geometry - The Results Are In.- Mtbr.com
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  1. #1
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    Forward Geometry - The Results Are In.

    So a few months ago i posted this https://forums.mtbr.com/frame-buildi...s-1116077.html

    And have been slowly but surely gathering data since. A very brief re-cap is 2 identical DH bikes a medium and a large, the medium with a 35mm reach stem, the large with a 10mm reach. both bikes ridden back to back at various different tracks, timing all recorded with freelap.

    So with almost 80 runs worth of data i figured it was time to analyse, within this sample I've binned 14 runs that were clear something had gone wrong, a crash, a mechanical, maybe the rider stopped to admire the view? Just looking at the numbers there was definitely no clear pattern or winner emerging, only after plotting them on a graph and rearranging a few times has it become apparent that the medium (with the 35mm stem) has managed a vague pattern of being 1% / 1.5% quicker. Realistically this is way to small a sample size to draw conclusions from, but it's as far down this road as i'm willing to go.

    So for now at least, the supershort stem is back hanging on the garage wall, and I'm scratching my head mulling over the results.

  2. #2
    pvd
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    Can you show a print of each bike including handgrip and saddle position? That's important here.

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    Quote Originally Posted by pvd View Post
    Can you show a print of each bike including handgrip and saddle position? That's important here.
    Err, i could make 1 for a complete bike on bikecad I suppose, i haven't needed to draw an entire bike. I think you are coming at it from a different angle to me, my work has been trigonometry to replicate hand position of a medium, onto a large. So i have basically resolved a few triangles to move handlebars on a large, rearward, until they match the medium. Basically a short stem with different height. Does that make sense? The rest of the bike, rear axle/saddle etc,,,,, is of no concern as i can't change it, nor do i want to. I just want bar position to be very similar to the medium with a 35mm stem.

    I suppose for you you want the rest of the info to put the rider position into some sort of context. The difference being I am making the most of the frame Ive got, you making the frame from scratch. I will draw the medium up on bikecad when i've a minute.

  4. #4
    pvd
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    Without a pair of prints, there's little to discuss or conclusions to make.

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    Quote Originally Posted by pvd View Post
    Without a pair of prints, there's little to discuss or conclusions to make.
    Why would you say there is nothing to discuss without? Or maybe it’s my fault because I’m quite good at visualising within my own mind. Anyway, ill not get sidetracked, I’ll get some size data and draw something up.

  6. #6
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    Well,

    Peter has a point here. Without a pair of prints it will be quite hard to communicate your opinions… (Mind you that this remark comes from an artist, that is quite able to visualize his imagination, that’s what I do for a living).

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    I'm not a bikecad whiz, so i don't know how to get dimensions from the grip, this is from handlebar centre, but seeing as they are identical bars set to the same angle, I think its a good starting point.

    I hadn't bothered with any saddle based dimensions, its a downhill bike. The manufacturer states that for
    medium Reach 402 Stack 607
    large Reach 426 Stack 607

    My trig calcs don't quite match bikecad, maybe this is headset and spacer height, or decimal points, (or i'm just sh1t at bikecad) anyway i think the 407 figure on the large should be 408.1 and the 668 should be 668.9.

    Forward Geometry - The Results Are In.-screen-shot-2019-12-02-20.43.49.jpgForward Geometry - The Results Are In.-screen-shot-2019-12-02-20.44.06.jpg

  8. #8
    pvd
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    I'm not sure if you understand what 'forward geometry' means. These bikes are totally antiquated and really not worthy of comment. You should conduct a test on a modern bike.

    My all-road bike has a longer front center (785mm) than your longer DH bike (782mm).

    Airspeeder! | Peter Verdone Designs

    My trail bike dwarfs the longer one with a front center of 850mm.

    I don't make bicycles. I make weapons systems. | Peter Verdone Designs

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    I wasn’t aware that “forward geometry” had a defined numerical value. Please can you point me towards this info? If I could be bothered I’d re-name the thread “1 bike slightly longer than the other but with a shorter stem” Would that help?

    Not all riders are the same size, and clearly, as this little experiment proved, a longer front centre doesn’t always equate to a faster bike.

    Quote Originally Posted by pvd View Post
    These bikes are totally antiquated and really not worthy of comment
    Auw, did poor ickle PVD wake up in a grump-grump again? Listen to mummy and have an early night, nobody likes a moody child, try turning your Xbox off and getting some sleep. Or was it another “accident” that woke you up? There there, I’ll go find your dodie for you.

    Glad you found time to comment on what wasn’t worthy of comment. I wish there was a kissing emoji.

  10. #10
    pvd
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    Now you know why it's important to be able to produce a quality print. Just one dimension told me so much. Imagine if you had the other important ones.

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    The front centers on these bikes are much shorter than I would consider to be "forward"

    To get the "forward" thing, as Peter points out, you will want to try something at least in the 790's-810's, and with a FC as short as the ones drawn, and the HT's so slack, getting a short stem to fit nice (i.e. place your weight correctly) is going to be tough. hard to tell without all of the dimensions on there though.

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    I'm also confused by the prints. That's so short! My road bike has a longer reach.

    I'm not totally on board with the concept that the cockpit dimensions shouldn't change when the frame size does.


    Not a downhiller though, so it's all academic.
    "Things that are complex are not useful, Things that are useful are simple."
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    You guys are all heightist! The rider is (like myself) a little vertically challenged. these 2 frames are the manufacturers medium and large, the rider did very briefly try an extra large, but immediately decided it was too long for him.

    Maybe i'm just not explaining myself very well, I'll try again.

    So we had a choice of M and L frame, the M seemed to fit the rider best (his descision, based purely on feel) after a while we experimented with the L, but the cockpit of the L felt to long for him, but the increased stability at speed was noticeable (presumably from the longer wheel base that was entirely added to the front centre, everything else remained the same).

    So i made a shorter stem, to recreate the cockpit layout from the M, onto the L. That is all i'm changing, nothing else.

    For any experiment you need to limit the number a variables, ideally down to 1, unfortunately it is impossible to apply a shorter cockpit to longer bike without effecting something else, in this case as well as shifting bars rearwards (on the L) i will also have changed the weight balance between the 2 axles.

    If Forward Geomtery is the wrong name, then please educate me. I thought that the principal of forward geometry was to ride a longer wheelbased bike, but still have the rider and bars in the same place relative to the BB. That is what i have achieved.

    Maybe for you the front centre is short, maybe not. It is entirely irrellevant, we had a choice of frames, he picked the 1 that fitted him best, he's the 1 who rides it, he's the 1 who can regular top 30 at UCI world cups, and has also top 10 at an EWS stage or 2. If he tells me that frame X is the 1 for him, then frame X it is.

    I'm not trying to tell him what works or doesn't, and this whole experiment has also proven(ish) that the frame he initially chose, (the medium) has been faster than the large.

    If you want more dimensions for the frame, just ask, i can go on the manufacturers website and add more. I have only included what was neccessary for me. Wheel axles, BB, steering axis and the important bit, bar position.

    Just out of interest i've done PVD's RAD triangle for said rider/bike combo.
    Attached Thumbnails Attached Thumbnails Forward Geometry - The Results Are In.-img_1296.jpg  


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    Could the entire frame be re-designed? yes it could, but thats not what this is about. So without getting any further bogged down with the existing frame geometry or sizing or something else i can't change. I was just going to concentrate on what i've learnt from the change i've made via the shorter stem.

    He found the large with the short stem "felt" more in control on the REALLY steep parts, but less confidence in grip on the front wheel in off-camber and flat corners. We ended up spending more time chasing set-up on the front of this bike, constantly making small changes (damping, spring and tyre pressure) to try and find more front end grip. I think this is down to the fact that with the short stem there is less weight on the front tyre, hence less grip. The bike felt more in control on super steep sections, but this didn't show up on the stopwatch, this might be down to "feel" not equating to actual speed, or that on even the steepest of steep tracks, there are still more area's that require a planted front i.e. flat turns and off-camber, that they outweigh the steeps. Maybe if we could go back in time to Champery circa 2007 we could record a difference?, but even a track as daunting as that in its heyday still had a large amount of off camber and flat corners.

    The results are not clear cut, and I suppose thats no surprise, this isn't motoGP, all the tracks he's ridden the bike on have a lot more than 12/14 corners to memorise, and each time you arrive the condition of them may have changed. We did try to test on a wide variety of styles of track, again, no clear obvious winner was found, it was only after carefully sifting through the results to find runs that could be compared did we spot the slight advantage of the medium frame.

  15. #15
    pvd
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    Quote Originally Posted by Cord View Post
    For any experiment you need to...
    Define what you are working with and what has actually changed. You're totally resisting producing an actual drawing of the bikes in question. You're referring to manufacturers stated 'geometey' rather than actually measuring what you actually have. You don't show the hand grips or saddle, everything else seems to be a guess.

    Why don't you learn to draw a bike before explaining to me the nuance of design? Why is this so hard for you? Drawing can be done quickly and easily but what you've shown is almost useless.

    The PVD/RAD that you show has a very rearward bias. That explains why the front tire grip is gone. Try an actual forward configuration.

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    pvd
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    #imadeastem

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    #magicnumbers

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    Quote Originally Posted by Cord View Post
    I was just going to concentrate on what i've learnt from the change i've made via the shorter stem.
    Have you tried measuring the change in weight distribution? It would be interesting to see if it matches what you'd expect from calculating it.

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    Quote Originally Posted by pvd View Post
    #imadeastem
    Finally you get it, I'm not redesigning an entire bike, all i'm doing is using a different stem to replicate handlebar position from BB, onto another bike to see what effect the longer wheelbase/shorter reach has and trying not to alter anything else at the same time. If that is not the basis of forward geomtery, then what is? You say "try an actual forward configuration" please tell me what you mean?

    I have measured a good few aspects of each frame to see if they matched the book figures, to the best of my measuring equipment, they were spot on. What makes you say I've guessed at anything? I haven't done anything with saddle, it's a downhill bike, its not used. What is of importance outside of the only points where the rider connects to the bike, which is handlebar and BB?

    Explain design nuance to you? I've stated a few times that this specifically isn't about re-designing a frame. It's about modifying 1 aspect to try and compare 2 frames. All I've done is use a different stem so that i can have the same handlebar relative to BB on 2 frames that are as close to identical as i can get, other than length of front centre. I'm just trying to single out stem length/front centre and record the results. I'll say it yet again, I'm NOT re-designing a frame. I'm modifiying 1 aspect.

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    Quote Originally Posted by dr.welby View Post
    #magicnumbers
    ?????

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    Quote Originally Posted by dr.welby View Post
    Have you tried measuring the change in weight distribution? It would be interesting to see if it matches what you'd expect from calculating it.
    I've tried, and failed miserably. Balancing a bike and rider on 2 sets of bathroom scales did not produce anything other than fits of laughter and a few strange looks form the kids! Obviously the rider needs to be stood up, we just couldn't get any repeatable results. I'd like to measure it, but not sure how.

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    Quote Originally Posted by Cord View Post
    ?????
    That there's a "magic number" that is the boundary between forward geometry and regular geometry that is independent of the rider's size and preferences.

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    Quote Originally Posted by pvd View Post

    The PVD/RAD that you show has a very rearward bias. That explains why the front tire grip is gone.
    PvdRAD is almost identical on both bikes, thats the whole point, I've kept rider position and contact points as identical as i could manage. I really don't think you understand what i've done.

    PVDRAD is unchanged, so that can't explain where the "grip has gone." The only thing to change is the front wheel is now further away from the rider. Obviously with the front wheel being further away, there is less weight on it, hence less grip. Is it as simple as that?

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    Quote Originally Posted by dr.welby View Post
    That there's a "magic number" that is the boundary between forward geometry and regular geometry that is independent of the rider's size and preferences.
    I thought it was the idea of a longer front centre, counteracted by zero or very short reach stems, to end up at the same rider position. If it has nothing to do with stem lengths, then is it just getting stretched out on a bike thats a size bigger than you need?

    #everybodyneedsmorelength

    I thought that going from a 35mm stem to 10mm would give a noticeable change in steering characteristics, because going from 60 to 35 certainly does, even to a very average rider like me. I certainly struggled to tell any major change, as did my rider. ????? Is stem length a law of ever diminishing gains?

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    I think a key part of the forward geometry equation is being able to move the bars forwards and down to a more aggressive position. Doing this normally would move the weight distribution forwards and feel worse in technical situations. But with a longer front center fixes the weight distribution. This is also something we couldn't do pre-dropper.

    Looking at the hinge diagrams (and accepting the limited sample size) if we say that your rider has a "good" arm angle at ~34 degrees but is too upright at 88 degrees, the only way to change this is to rotate the whole triangle forwards, moving the bars forwards and down. With a short rider it's going to be harder to do the latter. Using bigger frames might also make getting the rider to ride "lower" tougher to do, even if the seat seems low enough.

    Getting the rider to adapt to a whole new position isn't something you can do mid-season anyway.

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    Yep, i know where your coming from, and a mid season change was something we avoided, and only tried this after the main season had finished.

    On paper maybe the 88° is too steep? In the flesh he appears to be an aggressive rider who certainly works the front of the bike very hard, doesn't look stood up too much, but obviously thats quite subjective. (this is from watching re-runs and footage from world cups on the Medium with a 35mm stem, we have very very little footage of the Large with 10mm stem being ridden at 100%)

    The longest frame, the XL, was dismissed very soon, and the Large was tried for quite a while with a 35mm stem, but that too was eventually dismissed as feeling to long.

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    Quote Originally Posted by pvd View Post
    Define what you are working with and what has actually changed.
    I have, handlebar position.
    The medium with a 35mm stem is my datum.
    It started at 409 mm in X, 672 mm in Y from the BB.
    On the large with a 10mm stem my handlebar has moved -0.9mm in X, -3.1mm in Y
    It is now 408.1mm in X, 668.9mm in Y from the BB

  28. #28
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    Good grief, even Minnaar doesn't ride an 850mm front center.

  29. #29
    pvd
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    Aaron Gwin does. That's about what his large frame choice uses. We're about the same height.

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    Quote Originally Posted by pvd
    Aaron Gwin does. That's about what his large frame choice uses. We're about the same height.
    Did you calculate his RAAD? What if his head isn't as big as yours?


    Joking aside, the M29 XL has an 852 FC...the large is 4 cm short of that.

    Few manufactures have gone that long out front; I know of 1 production frame in large that is that long (I don't doubt there are more). Several XL and XXL's are growing that long; but many small, medium and large 27.5" frames are still hovering in the high-1100's to mid-1200's mm wheelbase with FCs under 830mm.

  31. #31
    pvd
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    If you are talking about production geometry, you're missing everything. Production geometry is not composed by very knowledgeable people. Product managers are usually just marketing majors a year or two out of school.

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    Ah, so you measured Aaron Gwin's bike. I hear ya.

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    Quote Originally Posted by VegasSingleSpeed View Post
    Good grief, even Minnaar doesn't ride an 850mm front center.
    True. He rides a LONGER bike. The XL V10 has an 865 front Center, and that’s what Minaar is riding.

    https://www.santacruzbicycles.com/en-CA/v10 published geo

  34. #34
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    Quote Originally Posted by VegasSingleSpeed View Post
    Good grief, even Minnaar doesn't ride an 850mm front center.
    I'm 6'3 and i have a trail hardtail with a 850mm FC. It sounds enormous, but you can't see the number when you're riding. It shreds, and felt immediately natural. The bike made my old (782mm FC) hardtail completely redundant, which is a bit of a bummer. There are very few trails where it's inferior. I'm convinced that these super long bikes are going to become normal in the next 2-4 years. I've been 2 years ahead of frame geo now for so long that i just don't know when it's going to land.

    Or maybe us tall folk have been on cramped bikes all along. I dunno, i was born this tall. My poor mother.

    Quote Originally Posted by Cord View Post
    Maybe i'm just not explaining myself very well, I'll try again...
    I follow you now.

    For me forward geometry means leaning the rider forward so their center of mass is in front of the BB, lengthening the front-center, and then adjusting the rest of the bike to play nice like that. Details differ on how exactly one goes about that, though. By that definition you didn't have forward geometry. You pushed the front end further out while preserving everything else, which really changes how weighted the front end is.

    I've never tested anything so rigorously, but your results totally jive with what testing i've done. It would be interesting to test it the other direction- keep the stem the same and adjust the chainstay length/saddle position/bar height so the bike lofts the same way. That would be forward geometry, and i would anticipate a measurable improvement. You kinda get the advantages of both configurations you tried.

    It's too bad your rider is short enough that cockpit setup constrains what he can try.



    I'm speculating, and i appreciate your sharing your data. It's really cool to read what riders at the top of the sport are experimenting with.

    #armchairengineering
    "Things that are complex are not useful, Things that are useful are simple."
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    M29 “clue in the name” is for 29” wheels. “Hey apples, come meet these oranges” Kind of a pointless comparison I think. And Jeff Steber a production manager a few years out of school? hmmm. PVD, your all encompassing cycle knowledge is not quite as good as you think. Ha ha!

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    Quote Originally Posted by scottzg View Post
    I'm 6'3 and ............
    Show off, i'm about 3'6" in my cuban heels and no longer listening!!!!! Whats the weather like up there!

    I think you're right about taller riders, for years and years bike lengths have not gone up in a matching percentage to rider height. As a short arse (not quite 5'8) it's always been easy to find a bike that fits, just go up a size or 2, it's only now that smaller sized bikes are starting to have adequate length.

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    Quote Originally Posted by scottzg View Post

    I follow you now.

    For me forward geometry means leaning the rider forward so their center of mass is in front of the BB, lengthening the front-center, and then adjusting the rest of the bike to play nice like that. Details differ on how exactly one goes about that, though. By that definition you didn't have forward geometry. You pushed the front end further out while preserving everything else, which really changes how weighted the front end is.

    I've never tested anything so rigorously, but your results totally jive with what testing i've done. It would be interesting to test it the other direction- keep the stem the same and adjust the chainstay length/saddle position/bar height so the bike lofts the same way. That would be forward geometry, and i would anticipate a measurable improvement. You kinda get the advantages of both configurations you tried.

    It's too bad your rider is short enough that cockpit setup constrains what he can try.

    I'm speculating, and i appreciate your sharing your data. It's really cool to read what riders at the top of the sport are experimenting with.

    #armchairengineering
    He is short, and that will always be a limiting factor. Bar height is critical, we run lowest stack headset available, and minimum spacers. Frame has a shortish headtube at 115mm, but a 200mm fork is long, end of story.

    The frame does have 10mm of chainstay adjustment, but we are already on the longest setting. I did very briefly try the medium frame with the shorter stay, compared to the longer with the longest stay. But on the medium the rider didn't like that shorter stay, so that left no where else to go.

    On the large with 35mm stem (and very briefly XL) the riders weight was tipped further forward, but he didn't get on with it. Maybe because he is used to that position, maybe because the rest of the bike needs adjusting in some way to counteract it? Unfortunately i only have a certain amount of changes i can make, and at some point the rider just wants to hone what he can do, on what he's got. In other words, "p1ss-off making me test stuff"! The rider really is 80%? of the package.

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    Quote Originally Posted by armyofevilrobots View Post
    True. He rides a LONGER bike. The XL V10 has an 865 front Center, and that’s what Minaar is riding.

    https://www.santacruzbicycles.com/en-CA/v10 published geo
    Sorry; my info is dated. His XXL 29'er from the 17'/early'18 seasons had a 1300mm wheelbase, 462mm stays...sub-850mm front center. At least that's the information his mechanic divulged.


    Quote Originally Posted by scottzg
    I'm 6'3 and i have a trail hardtail with a 850mm FC. It sounds enormous...
    It's the context that matters. The OP is discussing the subtleties of changing the FC on a downhill MTB, and gets blasted because the frame geometry isn't as long as PVD's hardtail.

    "Your bike must be this long in order to engage a discussion"...Seriously?


    Quote Originally Posted by scottzg
    It's too bad your rider is short enough that cockpit setup constrains what he can try.
    So, going back to anecdotal observations. If Minnaar's '19 ride is indeed longer in the FC than his '17/18 bikes, notice that they were playing around with lengthening the chainstays mid-season in '19.

    Cord, you mentioned in the original thread that the chainstay-length is adjusted to its longest setting; any way you can machine/fabricate an extension? Perhaps shift some weight forward on the large frame by moving the rear wheel back an additional 15mm?

  39. #39
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    Quote Originally Posted by VegasSingleSpeed View Post
    It's the context that matters. The OP is discussing the subtleties of changing the FC on a downhill MTB, and gets blasted because the frame geometry isn't as long as PVD's hardtail.

    "Your bike must be this long in order to engage a discussion"...Seriously?
    No. My point is that the OP is confused about the changes he is really making to the system and what those changes due because he hasn't learned the basics of bicycle geometry. He hasn't learned to even draw a bike properly and has resisted for months on this board. He can't understand how to use a basic CAD program, but also can't make a paper drawing. Rather than do any of that he swaps a stem and claims to be a tuner.
    He's trying to talk about 'forward geometry' with no understanding of what that is and is running tests on bikes that are antiquated. He hasn't even built a bike frame.
    Its message board silliness. Learn to draw a bike. Learn to draw a bike. Learn to draw a bike.

    That's the problem.

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    Quote Originally Posted by pvd View Post
    He hasn't even built a bike frame.
    This doesn't matter.

    They're willing to share a season's worth of useful data for a Pro level rider, and the experimentation has a very limited set of variables.

    We could take the hinge diagram they provided and try to use it to suggest where the bars should be on the large frame, and winter would be a perfect time to test it out with the rider.

    But no, let's just tear this person down.

    Piling on this level of abuse is disappointing, doesn't the Facebook forum provide enough fodder to get this out of your system?

  41. #41
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    The only thing "pro" in this case is the skill of the rider. Everything else seems pure guesswork.

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    how can one be such a know it all with about a dozen bikes in existence?


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    Quote Originally Posted by .WestCoastHucker. View Post
    how can one be such a know it all with about a dozen bikes in existence?
    When you're a "legend in your own mind" its easy.

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    Rooting through my notes and found a sketch i made when i set off with the shorter stem malarkey. There are dimensions on here that are from BB to grip, bonus, saves me having to work it out again.

    PVD, is going to have a field day!! Shock horror i used paper, how very 70's of me, and also my sketch/notes/scribbles are not to ISO9001 drawing standard. It is a random mess, and i may have to explain my scribbles, please ask, well, if anyone is actually interested anymore.

    Forward Geometry - The Results Are In.-img_1303.jpg
    Thats pretty unreadable, didn't realise how bad till I'd uploaded it. Ah well,,,,

    From BB to bars
    medium X 388.8, Y 694.0 with front centre of 758
    Large X 387.9, Y 690.9 with front centre of 782

    Also I've dropped a right clanger, my hinge diagram was done using the wrong data, that was the older higher rise bars and headset. Glad i double checked, i'll be more fastidious in future. So 2 new hinge diagrams for the VERY slightly different positions on the 2 bikes.

    Forward Geometry - The Results Are In.-img_1304.jpg
    The medium with 35mm stem

    Forward Geometry - The Results Are In.-img_1302.jpg
    The large with 10mm stem
    Attached Thumbnails Attached Thumbnails Forward Geometry - The Results Are In.-img_1302.jpg  


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    ooops

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    Quote Originally Posted by .WestCoastHucker. View Post
    how can one be such a know it all with about a dozen bikes in existence?
    Thanks, but it's only actually half a dozen i've made. I'll take it as a compliment.




    Don't worry, i get you really.

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