RoadBikeReview.com's Forum Archives - General


Archive Home >> General(1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 )


What size is your Stem? (no joke)(13 posts)

What size is your Stem? (no joke)RCole
Jul 21, 2002 9:22 PM
Moving from a traditional frame to a compact and was in between sizes. So my options were to go with the smaller frame and run a longer stem or go with the larger frame and run a shorter stem. I am leaning towards the larger frame and running a 100mm stem and was wondering if that was getting a little to short. Any thoughts? How short is too short?

Little background: I'm riding a 57cm frame (with a 56.5 top tube) now with a 120mm stem. Moving to a "large" compact frame with a quoted 57 inch virtual top tube but will run a 100mm stem. I know - the math doesn't add up. It must be in how the 2 manufacturers measure... Thanks for your help!
short stem, faster handlingweiwentg
Jul 22, 2002 1:40 AM
and 100mm is OK for me. the handling will get quite a bit faster, I think. how long a stem would you have to run if you were using the smaller frame?
are you sure about that?Starliner
Jul 22, 2002 6:44 AM
A shorter stem will tighten up the steering arc, giving a quicker steering feel, but the bike shouldn't handle any faster assuming nothing else is changed but the stem length.
Might pay to check those sizes yourself, but...hayaku
Jul 22, 2002 5:56 AM
I don't think there is much of a problem in riding a 100mm stem for your size. The handling may feel twitchy but you learn to control any bike you ride. The benefit in my opinion is that you can add a longer stem, move your saddle foward and get a good TT possition on one frame. You get a more versitile bike. I don't believe that there will be a noticable difference in frame stiffness.

If you can test ride the two options, that is by far the best option.
M.
don't forget seat tube angle...C-40
Jul 22, 2002 6:05 AM
If the seat tube angles of the two frames are not the same, then the difference between the published TT lengths cannot be compared directly.

A one degree increase in STA effectively increases the TT length by about 1.2cm.

Also compare the head tube length. If you are using steering tube spacers, the larger frame would generally have a longer head tube that would reduce or eliminate spacers.
don't forget seat tube angle...jp2
Jul 22, 2002 7:26 AM
"A one degree increase in STA effectively increases the TT length by about 1.2cm."

for a 66 c-c frame size going from 73 to 74 degrees. a better estimate is ~1cm per degree. i know 2mm isn't much, but relatively speaking it is 20 percent.

it's a rest day so we need something to think about.
wrong....C-40
Jul 22, 2002 8:21 AM
A better estimate is 1.5cm. The change in effective top tube length must take into account the saddle height. The accurate formula is: saddle height x (cosA-cosB). A huge 66cm frame measured c-c would have a saddle height of about 87cm. 87 x (cos73-cos74) = 1.46cm.
oh, so....jp2
Jul 22, 2002 10:50 AM
now when people talk about top tube lengths, they mean if the top tube was at saddle height. well that is interesting. kind of throws all of those geometry charts right out the window doesn't it. does this take into account the headtube angle and stem length too?? it seems those will affect your effective toptube also. by your definition, every frame has an infinite number of effective toptube lengths and kind of eliminates the whole concept altogether ;)

if you are going to compare framesets based on seat angle, use 1 cm per degree, everything else can be adjusted(saddle height/fore/aft/stem height/length)
don't get insulted.....C-40
Jul 22, 2002 1:59 PM
Perhaps the following detailed explanation will clarify the subject. The concept of "effective" top tube length does not throw out the geometry chart. It also does not take into account differences in the head tube angle, because this amount would rarely exceed 1mm. Hopefully you are aware that only the stem should be used to adjust the reach to the bars. The saddle is adjusted to change the position of the knee relative to the bottom bracket. Stem length is selected only after the optimum KOP position is set.

The effect of seat tube angle (STA) on top tube length can be expressed simply. The idea when comparing frames is to maintain the same body position relative to the bottom bracket while comparing any difference in the reach to the handlebars. If two frames have the same STA, the actual TT length will be an accurate comparison. For instance, if the both frames have a 74 degree STA, but one has a 55cm TT and the other has a 56cm TT there is a difference of 1cm in the (horizontal) reach to the bars. If the STA is not the same, this comparison would not be accurate. Here's why:

On a frame with a 74 degree seat tube angle, the saddle must be moved further back than a frame with a 73 degree seat tube angle, to achieve the same position relative to the bottom bracket. This movement of the saddle must be accounted for. Since the TT length is the standard reference value for comparing the reach to the handlebars, the difference in SADDLE POSITION is added or subtracted from the actual TT length to produce an "effective" TT length. The effective TT length will accurately predict the difference in reach to the handlebars between any two frames and allow you to determine the difference in stem length that would be needed to maintain the same reach. When comparing frames with different STA, add TT length to the frame with the steeper (74) angle or subtract length from the frame with the shallower (73) angle, using one of the following formulas: 1.32 x (cosA-cosB) x frame size, or an alternate formula, saddle height x (cosA-cosB). An average amount is 1.2cm per degree for a midsize frame.

The explanation for the 1.32 constant is simple. It's merely a way of expressing (approximate) saddle height in terms of frame size. Frame size (c-t) is .67 times inseam and saddle height is .883 times inseam. Expressing saddle height in terms of frame size, you get .883/.67 =1.32 frame size.
i won't.....jp2
Jul 23, 2002 6:19 AM
we are completely on the same page, however, to me effective toptube means effective toptube at the toptube.

ie. if comparing a 55cm cc frame both with a 55 cc toptube, only one is at a 73 sta and the other a 74 sta and one is going to get the same position in switching from one to the other.(new frame) one will hopefully only change stem length by 1 cm, because that is all the effective toptube will change. if you go up to the saddle, take you 1.5cm, and change you stem length to that, guess what, you have changed your reach.(increase or decreased by 0.5cm) going from 73 to 74 shorten, 74 to a 73 longer stem)(0.92cm on a 55cc frame) kops positions would be identical, only the saddle rails would be clamped your 1.5 cm fore or aft. (but this is not the value for change in reach)

your method(1.21cm for a 55cc*1.32=72.6) only works if the saddle is clamped into the same exact postion on the seatpost for both frames. oops, that changes your knee position relative to the spindle/bb/whatever else, and goes against your statement that you don't change that because it needs to be moved fore/aft as you state above. you have contradicted yourself.(you are taking the change in distance to the center of the seatpost clamp as the change in effective toptube, clearly not what you wanted)

i'll make it easy for you. if you want to set the two frame up identically, you will want the nose of the saddle to be the same distance behind the bb. height of saddle will not matter(it will be the same on both bikes as will the setback). now if you use the same stem on both, the change in reach will be......~1cm. coincedently the same distance as measured along the toptube. pretty simple, really.

instead of playing with simple math formulas only, draw yourself a fbd, better yet, use a cad program to draw it up and see the difference.
140 mmTig
Jul 22, 2002 7:23 AM
I'm used to riding 120 mm stems in the past, but am now using a 140 on my new bike for a better fit. The top tube on the Merckx is long enough, and the relaxed seat tube angle makes for a longer effective top tube. I still needed a longer stem since I feel better stretched out.

The handling is a bit more stable, but the low bottom bracket and long chain stays keep the bike stable in the first place. In the close quarters of a well organized paceline, stable steering is a benefit. What surprises me is how easy this bike is to steer in tight corners while in a fast group. I would feel comfortable in a crit even.

Your larger frame will have a longer wheelbase, so a shorter stem might not feel too twitchy. If the chain stays on the compact frame are nice and long, I'd go with the smaller frame and a longer stem.
9 cmlaffeaux
Jul 22, 2002 11:17 AM
I ride with a 9 cm stem and would not consider my bike "sketchy" at all. It handles nice and smooth.

I have a 59 cm effective TT on a compacy frame. I have longer legs to the larger frame gets the bars up high enough for me to be comfortable.

10 cm is fine for a stem.
110 for me.look271
Jul 22, 2002 11:28 AM
Went from a 120 to a 110. Still as stable as a rocket sled on rails.