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  1. #1
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    Noob Guide to Geometry, Bike Fit and Handling

    Geometry: A lot of people make a big deal about a frame's geometry and for good reason - because it effects bike fit and handling. Changing a bike's geometry can make a bike more capable at climbing or descending; turning; more or less stable; more or less comfortable; and more or less efficient.*

    There are a lot of variables to consider when designers develop a frame and, like chaos theory, changing one little variable can drastically effect the outcome of the overall fit and handling of a bike. The good news is, as long as you are within the ballpark of frame size and type, you can change what I'll call the "effective geometry" of a bike without cutting up the frame and welding (or gluing) it back together. I call it the effective geometry because you are not changing the geometry, just the angles in relation to the rider. This is accomplished by changing or adjusting some of the parts that bolt onto the bike. I'll touch on those areas as I explain them in relation to the frame.

    For beginners, disecting a frame's geometry can be a little too far in the weeds when choosing a bike. View the following as an extremely generalized guide to help you understand why one bike may feel different than another and give you an idea as to what people are talking about when they throw around esoteric terms while discussing the mystery of frame design.

    For the most part, certain categories of bikes have a specific type of geometry and each is within a pretty close range of one another. Cross country bikes are pretty close to other cross country bikes' geometry and same goes for all mountain vs AM and down hill compared to other DH bikes. But cross country bike geometry is a lot different than a down hill bike's geometry. Manufacturers tweak their designs within each category to give specific types of fit and handling because some people prefer a certain type of geometry over another because of their body dimensions or riding style.

    There are four basic areas of a frame's geometry that will effect the way it rides: head tube, top tube, seat tube and chainstay. See diagram below:

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    Let's start with the front of the bike.

    The head tube. There are two measurements to be concerned with: the head tube length and the head tube angle.

    Head tube length can be important if you want a really aggressive (low) handlebar position or a really relaxed (high) handle bar position. You can alter the handlebar position with more or less spacers on the steering tube, but only within limits (the steering tube is the part of the forks that goes through the head tube and connects to the stem and handlebars). So, for the most part, a short head tube, with no spacers, will allow you to get lower than a longer head tube with no spacers.

    Since the number of spacers you can use has limits, some people need or want a longer head tube to get a sufficiently upright position, just as some prefer the short head tube to get a sufficiently aggressive riding position. Some bikes with a short head tube ship with a long steering tube and a lot of spacers to allow you to determine how aggressive of a riding position you want. Others will cut it low assuming you want an aggressive riding position.

    Note: You cannot lengthen a steering tube once it has been cut - you must buy a new set of forks, so, make sure you like the handlebar position before you go cutting off that steering tube! If the bike comes with a short head tube and steerer tube and you want a more upright position, you can buy a set of riser bars. Or, if it's too high, you can go the other way and cut the steerer tube or buy a set of flat bars. Because 29ers' head tube sits higher than a 26ers' head tube, some people are slamming the stem and/or turning it upside down with flat bars to get the aggressive riding position they are used to having with a 26er.

    This begs the question, "what's the difference between an aggressive and relaxed riding position?" In short: aerodynamics, leverage and comfort. The lower you are, the more aerodynamic with greater leverage to pull up on the front wheel (and arguably less comfortable and less stable) compared to more upright. So, it's a trade and depends on what is more important to you.

    Head tube angle. For the most part, the more a bike looks like a chopper, the more stable it will be while riding in a straight line. This is why downhill and all mountain bikes have "slack" HT angles - for stability. The less chopper-like the bike gets, the faster it will steer and turn. Careful though, not enough head tube angle and the bike will be twitchy (difficult to ride with no hands) and you'll also likely to be over the bars often.

    Depending on the category of riding a bike is designed for, the angle of the head tube will be within a certain range. XC bikes have a steeper HT angle compared to All Mountain (AM) and Downhill (DH) bikes. Generally, entry-level XC bikes will be a little more slack than race XC bikes.

    However, one can change the "effective" HT angle (slackness/stability and steering speed) by increasing or decreasing the length of the forks. Not all forks are adjustable. However, some are designed and built to allow a rider to change the fork length (travel) by +/- 20mm. This is typically accomplished by altering the travel, or, the distance a suspension fork can compress. A segment of adjustable forks even have a way to adjust the travel (and therefore fork length) with a couple twists of a knob. When you increase or decrease the travel, you are raising or lowering the head tube. In doing that, you are increasing or decreasing the "effective" head tube angle and changing the angle of the forks. Comparatively longer travel will make a bike more slack. Be careful though - if you change the travel more than +/- 20mm, you will void the warranty because you are putting additional stress on the head tube and you might cause it to break.

    Another way is to buy a special "adjustable" headset. The headset is the compilation of bearings and spacers in the head tube. An adjustable headset allows one to adjust the steerer tube angle within the head tube +/- 1.5 degrees. It might not sound like a lot, but 1.5 degrees in either direction can really change the way a bike rides.

    It is also possible to speed up or slow down the steering or twitchiness a couple of ways without changing the travel or HT angle. Putting a longer stem on a bike and/or widening the bars slows the steering down just as a shorter stem and/narrower bars speed it up. This is like the difference between a small steering wheel on a sports car and a big steering wheel on an 18-wheeler. It doesn't actually change the ability of the bike to steer quicker or slower, it just feels like it.

    Changing either the stem or handlebars, or both, will also change your effective reach and put you in a more or less aggressive riding position. Longer stem means you have to bend over farther to reach the grips. Wider bars have the same effect. This may or may not be what you want. You can counter this by moving your saddle forward or backward to compensate for the change in reach. Again, you have to be careful making changes because moving your seat position also changes your effective seat tube angle and, in an essence, also changes the effective top tube length.

    Which brings me to effective top tube length.

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    They call it that because the actual top tube length is the measurement of the top tube from the middle of the seat tube, through the middle of the top tube, to the middle of the head tube. The effective top tube length is the measurement of the middle of the seat tube, parallel to the ground, to the middle of the head tube. The reason these two measurements are different are the same reasons Pythagoras has his own Theorem. More on the importance of top tube angle in the discussion about seat tube length and stand over height later.

    A shorter or longer ETT will dictate how cramped or stretched, or how upright or aggressive the cockpit feels. If you are a person with long legs and a short torso, you'll likely want a shorter ETT. This is also desirable for people who prefer a more upright seating position. The seat tube angle also plays a part in the ETT measurement. The more relaxed seat tube angle, the longer the ETT. Generally, an entry level bike tends to have a shorter ETT and a more relaxed seat tube angle because this allows a more upright seating position. This makes the front triangle of the frame shorter than a race bike which typically has an upright seat tube and longer ETT to give an aggressive riding position.

    Nevertheless, you can customize cockpit length by moving your seat +/- 1.5, inches, which is a common adjustment. In extreme cases, some people opt for a "laid back seat post" which is a seat post with a backward bend in it to give a little more room in the cockpit. This can give another inch or two of rearward seat adjustment. *I've also seen people use a laid back post to bring the seat forward, but I don't recommend you do it.

    When moving your seat forward or backward, you are changing the "effective" seat tube angle (a term I think I made up a long time ago) which is responsible for the position of your butt in relation to the bottom bracket. This measurement is important because it will dictate the weight distribution of the bike while peddling.

    Moving your saddle forward shifts your weight forward, which can make climbing a little easier by keeping the front tire from popping up on you. However, it can also reduce traction during climbs and, on the flats, it will add a little more pressure on the wrists and hands and makes popping a wheelie a little more difficult. Moving your seat back shifts your weight*back, which, conversely, *puts more weight on the rear wheel and increases traction. This can also make climbing while in the saddle a little easier, but it can also cause you to loop out on steep climbs in low gears.

    Lastly, a more relaxed effective seat tube angle is a little more comfortable as it will soak up bumps a little better by letting your seat post flex instead of sending the force of a bump straight up the seat post and into your back.

    When adjusting your seat, make sure your "sit bones" rest on the back of the seat. This will take some pressure off your pelvis and make riding more comfortable (There are different width seats for custom fit) This will also give you some room to adjust your weight forward by sitting on the front of the saddle when climbing.

    Some bikes have a top tube that is more parallel to the ground than others. This is mostly determined by the seat tube length.*

    The extreme examples are XC bikes compared to trials and jump bikes. Trials and jump bikes have a steeper top tube angle to give more clearance between one's groin and the top tube. This allows the seat to be low*and out of the way (or eliminates it in the case of a trials bike). XC bikes have a top tube closer to parallel with the ground to support a raised saddle. In XC, you want to have your saddle in a position for maximum peddling efficiency when seated. This is achieved by having your leg almost at near full extension when the pedal is in the full downstroke position.

    Note: The best way to find the right seat post height is to sit on the bike with the right-side crank in the five o'clock position. With riding shoes on, put your heel on the pedal spindle. If your right knee is bent, your seat is too low. If you are stretching or reaching to touch the spindle with your heel, your seat is too high.

    For XC, if your seat tube length is short and you have long legs, you will need a long seat post. If your seat post sticks up too far past the top of the seat tube, it will break, or it will break the frame. Seat posts are marked with a maximum height. Do not exceed this height. If you must exceed the height, you need a larger frame.

    If you are short and your seat tube is long, you won't be able to stand over the bike without your crotch painfully straddling the top tube - which is where the "stand over height" measurement comes from.

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    A lot of attention is given to stand over height, which is important, but, arguably, less important than ETT; because, hopefully, you spend more time riding your bike than straddling it. It gets the most attention because most manufacturers sell frames in sizes according to seat tube length and stand over height. A general rule of thumb is to have two or three inches between "you" and the top tube. But this doesn't always give the best fitting frame. Worse is when they recommend a bike size according to height. Most times this is okay, but, sometimes it's really bad.

    Remember the rider with long legs and short torso needing a shorter ETT? Well, paying attention to height or stand over only would possibly recommend a frame too large. Conversely, having short legs and a long torso will require a longer ETT. Sometimes this will result in a larger frame choice that will be grazing the rider's crotch when standing over. However, the cockpit*will be more comfortable when riding compared to a smaller size that gives two or three inches stand over clearance and a hump back. Of course, this can all be "fixed" with a laid back seat post, a longer stem and a set of wide bars - but it isn't ideal.

    The last area of discussion is chain stay length.

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    This area effects stability, agility and weight distribution. The shorter the chain stay, the easier the bike will be to wheelie and the more nimble it will feel. Too short and it will loop out on climbs. Longer chain stays make the bike track in a straight line better. Taller guys riding larger bikes with a relaxed seat tube angle and a long seat post typically need longer chain stays to keep from having too much weight on the rear wheel.

    Those are the four basic areas of geometry and how they relate to handling and fit. As you probably noticed, the frame is just a starting point and the parts you bolt to it can change the handling and fit.*

    Now, let's take a minute and imagine an interactive graphic of a rider on a bike that allows you to*drag each of the areas we discussed and how it makes the bike and rider look. Drag out the front wheel, pull the top of the seat tube toward the rear wheel, make the seat tube longer, make the top tube shorter. You see how changing one changes almost all?

    I made sure to stay away from including angles and inches because different sizes, different constructions and the parts bolted to a frame can move some bikes into different categories. Some XC bikes have slack head tubes. You can make an AM bike a decent XC bike.

    The short of it is, geometry is relative. It's impossible to understand a frame's geometry without putting it into context. It's more of a comparative analysis. Get a bike based on your height and standover and category. As you ride it, you can customize it. If you find yourself wanting your bike to perform a way that can only be accomplished outside the adjustable limits of your current bike, look for a frame with that geometry. If you see a bike that you want, let us know what you plan to do with it and we can help you understand if it is appropriate. For the most part, you can just look at a bike and understand what the category is.
    Last edited by wmac; 05-15-2015 at 01:42 PM.
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  2. #2
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    Thank you once again sir. Mods, please sticky.
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    I'm not a noobie, but reading this made me feel like one. Excellent info that will help me identify "effective" changes I can make to my bike now and what to pay close attention to when scoping out the next one. Thanks!
    Please don't mistake my enthusiasm for mountain biking as an indication of my skill level.

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    Absolutely a post hit subject! thank you!
    Now if only it was as easy to do the try outs needed to get things right without buying a lot of "bits" ...

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    Good read.

    Im in that fuzzy area of most bikes between M-L, chose the medium so it would be a bit more comfortable on long rides... and weighs less.

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    Dang this should be stickied, great job

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    Thanks - Rep it if you like it
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    Very useful information wmac. I'm getting my bike fixed. Front and rear suspension are going to be new. This information comes at a great time. thanks

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    Thanx wmac

    Reading this has been time well spent.

  11. #11
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    Good job wmac.

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    "You must spread some reputation around before giving it to wmac again"

    I'll get you eventually, thanks for taking the time.
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    I'm not experienced in any skin but mine, but it took me 20 years of trial and error to learn most of what was covered in a 5 minute read. I am pretty close to knowing if I could be happy on a bike just from looking at the geometry numbers, but I doubt if I could help another without taking a few rides with them.

    Where were the guys like this 30 years ago?

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    Quote Originally Posted by Oldfatbaldguy View Post
    I'm not experienced in any skin but mine, but it took me 20 years of trial and error to learn most of what was covered in a 5 minute read. I am pretty close to knowing if I could be happy on a bike just from looking at the geometry numbers, but I doubt if I could help another without taking a few rides with them.

    Where were the guys like this 30 years ago?
    I was racing BMX
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    *Excellent post. Helped me a lot.

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    Good info

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    another..."You must spread some reputation around before giving it to wmac again"

    but man, noob or not, that is good information. I learned a lot.

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    Thanks everyone.
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    Anyone have anything to add to this?
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    Do you mind putting the "over mountain" geometry up for comparison. I'd rep you again but was told to spread more love before I can do it.


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    I won't be able to rep you for a while, but I'll make sure to do it.

    Quote Originally Posted by wmac View Post
    Anyone have anything to add to this?
    BB height. Good comparison would be between DJ/slalom bikes and trials bikes. The geometries are pretty similar with the exception of noticeably different BB height. DJ/slalom bikes usually have BB drop with 100mm suspension forks, whereas trials bikes have BB above the rear axle with short rigid forks.

    Lower BB, lower your center of gravity = better cornering.

    Higher BB, easier to pop the front wheel up by weight transfer (bunnyhop, manual), better clearance on obstacles, etc.

    There may be other effects I'm not aware of.

    EDIT: Higher BB also makes the bike more stable when you are standing on the rear wheel. Obviously this is irrelevant unless you are actually into trials.

    reference: Bike Trials Geometry Guide - at WebCyclery|WebSkis|Bend, Oregon


    For typical riding style (XC, AM, etc), it's better to have the BB as low as practically possible. This obviously is affected by the amount of rear suspension travel you have, crank length, as well as the type of terrain you ride.
    Last edited by Katz; 01-12-2013 at 07:11 AM.

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    Ah, nice add Katz!
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    great read put things in perspective.

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    Thanks Sno!
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    It won't let me rep you right now. Thanks for the info. Is there anywhere that I can find a chart or something that has general geometry numbers for the different types of riding disciplines?

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    Go to specialized website and look at the geometry of the different bikes.
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    Hey all.
    Quick question regarding the bike I've just bought. On the advice i received from my LBS i got an 18inch frame bike, after reading the manufacturer's manual it recommends that i have 7.5-10cm of crotch space. At the moment I have about 2-3cm.
    The bike I bought is the 2013 Kona Splice. I'm 5'10 with a longer torso. I ride mostly trails and mild/moderate XC.
    My question is, would the 16inch frame be a better option for me, or does it not really matter?
    Thanks in advance.

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    Can't say for sure, but if you have a long torso, you'll want the bigger bike. Standover isn't as important as effective top tube. It's explained in the guide above.
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    You really have to think twice in ajusting the seat forward or backward. It is usually the handlebar you move either way and not the seat. The seat possition is really importent in Body fit, so you sit perfectly on the bike according to you body geometry.

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    Frank - can you explain this a little more please?
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    Well, if you go to a specialist an have him make a bike fit on you. One of the first things hi does is to have you seadet in the right way, so your pedalstroke is correct. A guideline is to have your knee is right over where the pedal is mounted when the pedalarms is in horrisontal position (you can find this on the web). Ofcause it is not 100% correct setup for you, but nearly. From here it comes to the lenght to the handlebar. This is depending on how you want to ride your bike (aero or comfort) this is ajusted by the lenght of the stem. You are not supposed to ajust this by moving the saddle. The saddle position is very importaint in getting all out of your pedal stroke, and preventing injuries. Both the position in forward/backward and in up/down.
    Hope you can understand my poor english :-)

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    Re: Noob Guide to Geometry, Bike Fit and Handling

    Mark, for later reading.

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    I just test rode:

    Trek Mamba 29er (medium)
    Specialized Hardrock 29er (medium)

    There's a clear difference in geometry between these bikes.

    I thought the mamba had good "trail" geometry and found it to be easy to flick around. I also knew it had I think the longest wheelbase (111.6cm) of the bikes i've been looking at and yes you can feel that. It was also a slacker bike (headtube angle) for sure at 69.3 degrees. I hated pedaling on it though. It seemed like the BB was too far forward. As soon as I started pedaling i thought about my knees which ain't good. I tried raising the saddle but I dunno, i couldn't get over it.

    It seems like with the Trek's longer wheelbase and slacker HT angle that it would be a good descender.

    The Hardrock. As soon as i got on this bike instantly I could feel that the wheel base was shorter, I mean right away. The hardrock wheelbase is 109.0cm which is 2.6cm shorter and felt obvious to me. I found the pedal position to be way better, i was instantly happy pedaling on it. I could also feel that was the HT angle was more aggressive (71 degrees).

    Given how much I can't get along with my 26er Hardrock i was shocked to find how much I liked the 29er. I mean instantly it felt more fun than the Mamba and I remember thinking "Lets go on the trails!!!!" while riding it around on the parking lot. It just felt more aggressive and fun which i like.

    Unfortunately the Hardrock component level isn't for me and they didn't have a Rockhopper in the correct size for me to try. I really wish they had one though since i know the geo is a lil different than the Hardrock and the Rockhopper is now setup with 100mm fork with a tapered headset. Not totally sure on the 100mm part because their specs aren't clear on that.

    EDIT: I should correct and say that the Rockhopper has a tapered Headtube....what that means for us i don't really know.
    Last edited by zephxiii; 03-08-2013 at 07:28 AM.

  34. #34
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    Quote Originally Posted by Frankbs View Post
    Well, if you go to a specialist an have him make a bike fit on you. One of the first things hi does is to have you seadet in the right way, so your pedalstroke is correct. A guideline is to have your knee is right over where the pedal is mounted when the pedalarms is in horrisontal position (you can find this on the web). Ofcause it is not 100% correct setup for you, but nearly. From here it comes to the lenght to the handlebar. This is depending on how you want to ride your bike (aero or comfort) this is ajusted by the lenght of the stem. You are not supposed to ajust this by moving the saddle. The saddle position is very importaint in getting all out of your pedal stroke, and preventing injuries. Both the position in forward/backward and in up/down.
    Hope you can understand my poor english :-)
    Frank - you are correct! Horizontal position of the seat is to get your knee directly over the pedal spinal with your shin vertical when the crank arm is in the three o'clock position. Someone with long legs may need to move the seat back to get their sit bones on the widest part of the seat whereas someone with shorter legs would have to move the seat forward. Thank you for adding this good information!
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    Very informative post, thanks for taking the time to post it.

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    More info regarding seat position:

    The Myth of KOPS - An Alternative Method of Bike Fit
    The Myth of K.O.P.S.

    "The most important argument I came up with to establish a basis for believeing that KOPS is only an accidental relationship, only has a weak statistical basis and is not physically siginificant is the discussion of the recumbent. In that case, the location of the rider's knees over the pedals is completely different, entirely unrelated to gravity, but the position is as effective in pedaling. This gives you a very simple way to see that the KOPS method has no basis in physics or physiology."
    No, YOU don't understand. You're making an ass of yourself for all of eternity.

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    Thanks for the info

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    I'm printing this out. Great info here thanks for taking the time.
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    Thanks for info. Gave me an idea for what to look when I go back to re-test ride a couple of bikes that I'm looking to get.

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    You're welcome!
    No, YOU don't understand. You're making an ass of yourself for all of eternity.

  41. #41
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    Comment on this every so,often to keep it relevant so the n00bs can take advantage of it.
    No, YOU don't understand. You're making an ass of yourself for all of eternity.

  42. #42
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    Quote Originally Posted by wmac View Post
    Comment on this every so,often to keep it relevant so the n00bs can take advantage of it.
    Sure, I've had this post bookmarked since I first came across it and I reread it from time to time. Solid post built with some good info. Bike fit is about as important as it gets. I find the better the bike fit, the more enjoyment the rider receives which has the added effect of improving ability more quickly and increasing time in the saddle. Thanks.
    I don't know why,... it's just MUSS easier to pedal than the other ones.

  43. #43
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    Thanks!
    No, YOU don't understand. You're making an ass of yourself for all of eternity.

  44. #44
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    Any noobs got any questions?
    No, YOU don't understand. You're making an ass of yourself for all of eternity.

  45. #45
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    Quote Originally Posted by wmac View Post
    Any noobs got any questions?
    Actually, I do have a question.

    Would trying a shorter stem (to shorten the ETT length) make it easier to do manuals and stuff?

    I went to a skills clinic last weekend to learn more finesse with front wheel lifts, rear wheel lifts, manuals, etc. Right now I enjoy riding fast, weaving through trees, and climbing hills. I sorta lift/unweight the front to get over logs, etc., but mostly rely on the 29" wheel and shock to keep me moving forward.

    We got into a brief discussion of bit fit. Instructor suggested that I could consider trying a shorter stem to get me into a more bent elbow position riding position which might make some of these skills easier to execute. Thoughts?

  46. #46
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    Technically it would, but you'd want to move your seat back as well. This would also shift your weight back - which may or may not be good. However, If Knee over pedal is important to you, then this might not be an option. Shortening the chain stay length (which you could only do by buying a new frame) would probably give you the ride feel you're looking for. This is why some people (like me) prefer a 26" bike. Everything requires trade off decisions.
    No, YOU don't understand. You're making an ass of yourself for all of eternity.

  47. #47
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    Wouldn't moving the seat back stretch the cockpit back out, undoing the effect of the shorter stem?

    Not really sure what bike fit or ride feel I am looking for. I would have said that I am perfectly comfortable on my bike now (2013 Race Pictures...here we go!!!).

    The instructor for my group at the clinic kept wanting me to bend my elbows more. Later, we were all comparing bikes and I mentioned that I'm on a men's medium. Then I got to thinking about cockpit size.

    She suggested n+1, have a bike for each speciality. :-) I was thinking of saving $$$ and just changing my bike back and forth. For racing… leave it as it is now. For "tricks"… lower seat, switch to flats, maybe shorten stem?

  48. #48
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    ^ Yes. It would. Moving your seat back would also shift some weight back a bit, which would help "lighten" the front wheel. Depending on your frame and torso length, you might be able to get away with just a shorter stem. Before you adjust or change anything, practice wheeling/catwalking in the dirt or short grass.
    No, YOU don't understand. You're making an ass of yourself for all of eternity.

  49. #49
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    I'm going to keep practicing all the techniques and drills they gave us AND I'm going to keep in mind everything from your article. I'm glad the coincidence worked out that I get both these at the same time.

    Thank you for your help!

  50. #50
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    Glad I could, sort of, help!
    No, YOU don't understand. You're making an ass of yourself for all of eternity.

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