Bike Fit - short top tube

[pink monkey]

hello all! I am looking for some advice on bike fit.

I think the top tube on my bike is too short but im not completely sure. Is there anyway to figure out for sure if it is? I know that for a road bike there is the generic "hub test". Is there a test like that for mountain bikes? I am thinking about switching out the stem and/or maybe getting a layback seat post. Is there a disadvantage to having a long stem? Also, How many inches could i get using a layback seatpost? I've heard that changing the stem lengths can affect the way the bike maneuvers... so i'm thinking maybe getting a new seatpost might be the way to go.

[MartianDestiny]

IMO, you do not adjust reach with your seat position (post or otherwise). Seat position should be all about getting your legs/knees in the right place relative to the cranks for best power and to prevent injuries.

Reach you adjust with stems. In theory a shorter stem makes a bike "twitchier" or quicker handling (not always in a good way) and a longer stem does the opposite (slows down steering and stabilizes the front end).

Honestly, I ride a road bike with a 50mm stem and don't notice any adverse steering effects, and have you seen the teeny tiny stems on dirt jump bikes and bmx? They aren't having any problems controlling their bikes either.

I can't imagine any reasonable length stem (even going longer) is going to make *that* big of a handling difference. People ride with 130mm stems all the time with no issues.

The biggest thing could be the redistribution of weight more forwards (with a longer stem). That will help you keep the front end down climbing, but could take weight (and thus traction) away from the rear; you just have to learn how to adjust to get the proper weight distribution. Other issue would be a slightly more difficult time getting weight back on the downhills, but again that's likely just adjusting riding style unless the bike is grossly too small for you.

[Irulan]

I beg to disagree with the above.

The laid out position and stem advice applies to road bikes, not mountain bikes. The issues with being laid out for mountain biking is that you've got no room to maneuver. This is, basically, the old school XC race or what we call the Ned Overend fit. Mountain bike geometry had really changed over years.

Even a 10 mm stem change can make a huge difference for most riders. Just yesterday I was coaching a clinic with a gal on a very nice older style FS bike that was fitted that way. She literally had no room to truly move her body/center of gravity back for a proper descending position because her stem was so long and she was very extended.

Putting your weight over the front as a given position makes you really unstable for any kind of technical descent. Sure, a good rider can handle it, and that's how everyone rode for years unless they were on a downhill bike. But the overall feeling in this position is "wow, I better be on my game or I'll go OTB". The newer prevalent slack geometry puts a rider more upright, which is more stable, and gives you much more ability to be a dynamic rider.

There's another thread in this MTB area,
http://forums.teamestrogen.com/showthread.php?t=32006
and pinkbikes states it really well,


One other thing to carry in the back of your mind is that I think that women who have been roadies (guilty as charged your honour) and more particularly women roadies who have always ridden "men's bikes" often under-estimate the effect that too long a top tube on an MTB will have.

We are used to a more laid forward and stretched out position and may be inclined towards thinking that the WSD bike in the right size is too short for us. I found it weird feeling how upright and over the bars I was on a WSD MTB. But once I started to realise how much you need to be able to be mobile over the bike and shift your weight so far forward and back, I realised that in the stretched out position of a longer top tube I would never have been able to get far enough back and still control the bike!

As I understand mtb fit, KOPS is not set in stone the way it is for road riding. It's a base point that you want to keep in mind, but there's some fiddle room in there. I know many people that ride a set back seat post with zero issues. It's best to work with a shop that really understands how to fit mountain bikes to find what will work for you.

You can see I am really against relying on stem length to get the right fit on a mountain bike. I work with a lot of gals, especially beginners and newbies to the sport. There are two typical scenarios that I see. One is, well meaning SO builds up a bike with an incorrect frame size and tries to compensate for poor fit with changing out the stem. The other scenario is that a LBS sells a bike that is not the right size and again, tries to make it work by fiddling with the stem. Either way, you've got a rider on a bike that doesn't fit right or doesn't handle well, and the rider might not even know how much better their ride could be until they get on a bike that has a correctly fit frame.

sorry for the novel.

[MartianDestiny]

I said NOTHING about a "laid out fit"

KOPS is set in stone for NOTHING (road or mountain). But it's foolish to screw with a comfortable and efficient knee to crank position to get reach right. You can really hurt your knees in either discipline if you get them out of an alignment that works for your body (maybe moreso on a mountain bike where at least I throw a lot of power through my knees to the cranks in short bursts).

I ride with a "set back" seatpost on a mountain bike too. Almost all posts are setback to some extent now (excepting the traditional Thomson straight post); mine happens to be a Thomson that, while it actually isn't set back that far, looks like it is because the post itself is bent instead of just having an offset clamp. That does not mean that my knees and cranks aren't lined up in a comfortable position for me (this happens to be pretty close to KOPS for me, but that is NOT what I'm referring to). So just saying "lots of people ride with setback posts" means NOTHING; many/most are doing so with knee/crank position (as well as weighting the rear wheel) in mind (if they even think about bike fit in the first place). Lots of mountain bikes are built with the assumption that a setback post is going on them and tubing angles are adjusted accordingly; thus you are looking at a setback post as optimal to get your position correct relative to cranks. Just because the angles are different than road bikes to give a more upright position and more stability doesn't mean you should throw position relative to cranks out the window (note, I did not say "KOPS" specifically, and I won't).

I did address the issue that a longer stem would put more weight over the front which would destabilize you on a downhill (depending on degree). Some of that has to do with the accompanying saddle to bar drop and overall rider positioning as well. I still say if you can't handle the bike with a (fairly standard) 100-120mm stem that's giving you a good fit then you need some handling skills (or a downhill bike...). I have a 100, 0* on my XC bike. Fit is still much more upright than the oldschool stretched race position and I can get all the way behind the seat (ribcage behind the seat, "sitting" on the wheel) when I need to and not loose any control of the bike. Obviously it's not "long and stretched out".

I was not saying that the OP's bike was indeed too short for her or that she NEEDED a longer stem. Just that IFF it is TRUELY too short the stem is what should be adjusted, not the saddle. That does not equate to putting a stem on it so that she's stretched out like a roadie in a time trial...

If the bike is so much too short that the stem becomes so long that it's really effecting handling then the bike is too small in the first place.

But yes, I agree, you should not be in a stretched laying down position like you are on a road bike and if you are coming from a road bike then a mountain bike that is actually a good length may feel too short. A "long" stem doesn't necessarily mean you are going too long and stretched though. And I don't think getting the seat back as far as you possibly can without thinking about your knees is productive at all (in fact I think it's dangerous).

PS: I was a mountain biker long before I was a roadie, so I'm not "guilty as charged" according to that article.

[Irulan]

But yes, I agree, you should not be in a stretched laying down position like you are on a road bike and if you are coming from a road bike then a mountain bike that is actually a good length may feel too short. A "long" stem doesn't necessarily mean you are going too long and stretched though. And I don't think getting the seat back as far as you possibly can without thinking about your knees is productive at all (in fact I think it's dangerous).

heh, we are on the same page. I need to stop posting before I've had my full ration of caffiene. ;)

[pink monkey]

Thank you so much for the feedback!! You two have giving me a lot to think about. I am beginning to think that maybe my bike really is a good fit for me but since I have been a roadie for years, I am use to being stretched out more.

Is there anyway to tell if it's a good fit? I know with road bikes, there is generic the handle bar/hub test... (sitting on the bike, holding onto the handle bars, you want the handlebars to block the view of the front hub). Does this test work for mountain bikes too or is there a similar test I could use?

[Irulan]

do you have any neck/shoulder/wrist/back discomfort or aches would be #1.
#2 is, are you able to move dynamically on the bike... that is, can you easily move your hips back behind the saddle without being stretched out? Do you feel stable, or do you feel like if you aren't careful you'll be pitched over the bars?

[pink monkey]

I have been having some mid-back pain while riding. I feel like my nose is in front of the handlebars while i'm going downhill and it makes me feel like i'm going to tip forward. I dont know if that makes any sense. Maybe i'm not scooting back in the seat enough? i'm officially confused now. haha!

[Irulan]

I have been having some mid-back pain while riding. I feel like my nose is in front of the handlebars while i'm going downhill and it makes me feel like i'm going to tip forward. I dont know if that makes any sense. Maybe i'm not scooting back in the seat enough? i'm officially confused now. haha!


It makes total sense. Do try changing your body position on a descent: don't "hover" over your saddle and move back, extend your legs ( not locked) and move back. This should be comfortable, not feel stretched out in the front. This is one way to tell if your cockpit is too long.

The way you'd test this is to extend ( again, not hover) up in the saddle feet at 3 & 9, and then move your torso back. You should ultimately be able to position yourself so that your chest is over your saddle and your butt is over the back tire. Sure, this is an extreme descending position but you never know when you might need it. This is a test is all.

Or, go head and switch out your stem for about 1/2" shorter to start and see if that feels any different.