Appropriate Stem Length

[mdeeds71]

I am buying a 58 CDA with a 57.5 top tube that has a 73 HT angle...

My question is what size of stem do I need for a 64cm reach measured c-c...I want to buy a Serotta ITM stem and bar but can only find 120 and 130 length stems and bars at 40 CC...

Any leads and help would be great...as well location for these types of components.

Mark

[Needs Help]

Assuming a horizontal top tube:

Stem Length =

Total Horizontal Reach - Horizontal TT Length + tan(90 - HT Angle)*(Stem Height)
--------------------------------------------------------------------------------
cos(absValue[90 - HT Angle + Stem Angle])

where Stem Angle is a negative number.


If:

1) the center of your stem joins up with the steerer 9cm above the center of the TT measured vertically(=Stem Height), and

2) your Stem Angle is -17 (which is horizontal with a 73 degree HT), and

3) you want a 64cm Total Horizontal Reach measured C-C

then you would require a stem length of:

9.25cm

If your stem angle is -6, you would require a stem length of:

9.42cm

(audited: 1/05 Aurthur Anderson LLC)

[Dave]

The whole idea of a reach value being meaningful with no regard to the location of the saddle or rider relative to the TT/ST intesection point is flawed. The typical defintion of "reach", is merely the horizontal distance from the TT/ST intersection point to the center of the handlebars. An even more simplified defintion is merely the length of the TT plus the stem length. Both of these unrealistic definitions ignores the effect of saddle height and the fact the the saddle location relative to the TT/ST intersection point could easily vary by as much as 7cm depending on the choice of seatpost and the position of the saddle on the seatpost.

The true reach on any bike could be altered by 7cm just by changing from a Thomson straight-up seatpost, with the saddle all the way forward to an FSA K-force with the saddle all the way back. The calculated reach would not change, but the fit of the bike would be tremendously different.

I assume that mdeeds71 is using a reach value obtained from some sort of fit calculator. If he had a real fitting, he would already know exactly what stem length he needs. He should also know what stem angle and spacer combination is required to produce the desired handlebar height. If a real fitting was done and the fitter came up with a 64cm reach, then by most definitions, a 57.5cm TT would be too long. More common would be a 54cm TT plus a 10cm stem or a 53cm/11cm combination.

I can figure the approximate stem length more simply. Subtracting the TT length from the "reach" yields a value of only 6.5cm. Using your assumption of a 9cm stem height, add .3cm for each 1cm of stem height, or 2.7cm, to yield a horizontal stem length of 9.2cm. The minor horizontal length difference for any stem in the 73 to 84 degree range can be ignored. An 84 degree by 110mm stem has a horizontal length of 108mm for example.

All this suggests is that a 9-10cm stem may be required. Both your and my calculations are pretty much worthless until the rider gets on the bike. Depending on his saddle positon, he may find that the TT length is way too long or he could just as well need a 12cm stem.

[flydhest]

Dave . . . while your input is accurate, ...uh, maybe you could tone it down a touch. Your points could be summarized as follows, I believe:

The true "reach" measurement that is needed is a funciton of saddle position and handlebar position. Just using the frame dimensions won't get you the whole story. What the original poster will need to do is to set the saddle where it's going to need to be and, given the drop to the handlebars, figure out the stem length needed _relative to his position_ and not just the frame.

Does that capture what you meant?

[Dave]

Sorry if I offended, but there are times when simple trig can be made too complex and still not yield a meaningful answer.

The complex equation intends to combine the effect of raising the stem on the steering tube (which brings it about .3cm closer to the rider for each 1cm it's raised) with the effect of stem angle.

Stem angle isn't real important until you get to 90 degrees, where the horizontal length finally becomes about 5mm less then the advertised length. If you're using a "flipped" 84 or 80 degree stem with an angle of 96 or 100 degrees, when the length is about one full size shorter.

Yes, the other point is that either of the common reach definitions really don't mean much. I have two bikes setup with the same reach from the saddle to the handlebars and saddle position realtive to the BB, but one has a TT length of 54cm with a 11cm stem, for a combined total of 65cm and the other has a 52.5cm TT length with a 10cm stem, for a combined total of 62.5cm. The big difference is in the seat tube angle; 72.5 degrees on the first bike and 74.5 on the second. The reach calculation that was provided does not take the seat tube angle into account either. A TT length without a corresonding STA is meaningless.

[Ti Designs]

I think what Dave is trying to say is what I tell my customers when doing a fitting - nail down the saddle to pedal relationship, then work from the saddle position, not the seat cluster.

This brings up the next question, why doesn't the person doing the fitting know this??? It's somewhat alarming to know that you can spec out the bike with parts that you want in specific dimentions, and not know how they relate to the fit of the bike. Do yourself a huge favor, find someone who knows what they're doing in fitting a bike, and have them help you set this thing up right. Better yet, have them do the fitting and explain each step and every measurment, and write it down. That way if you change bikes I woun't have to type this all over again.

[BarryG]

This brings up the next question, why doesn't the person doing the fitting know this???

Maybe Mark is buying it used?

[dirtdigger88]

I think he is buying used remember he posted about trying to buy a bike off of the demo page or the buy it now page and missing the chance. I agree though Mark- you are not supplying enough info- everything "stems" from you saddle position- the reach from saddle tip to bars means way more than what you are asking about. Trust me I have just been through this. . .

jason

[Dave]

I've run the height and reach numbers for a lot of common stem angles and lengths. If a bike has the desired bar height, but too much steering tube spacer, then a higher rise stem is often used with less spacer. If the same length of stem is used, but flipped, the horizontal length (reach) will be the same as before, once the appropriate amount of spacer is removed.

For example, with a 84 degree by 130mm stem, flipping the stem will raise it approximately 26mm, but shortens the reach by 8mm. Most folks would just remove a 2.5cm spacer and call it close enough. If you want to get real picky, then it requires the removal of 27mm of spacer, to account for the head tube angle. Once the spacer is removed, the stem moves forward, by 8mm, restoring the reach and the height.

Calculations of the height difference when flipping a stem are usually not quite accurate either. Why? Becasue they all assume that the center of the stem extension intersects the center of the steering tube clamp, which is most often not the case. I've found that the actual amount of height change may be off by a few millimeters from calculated values.

[Ti Designs]

Maybe Mark is buying it used?


My suggestion still stands. I know that fittings cost money, but it's money well spent. We get $50 for a basic fitting which should nail down all these numbers. That's probably less than the cost of the stem, certainly less than the cost of the stem and bars. Hell, just putting a role of cork ribbon on the wrong bars is gonna cost 1/3rd of that...

[PsyDoc]

...might be giving a little too much credit to bike fittings as they are not the end-all-be-all of bike fit. Granted, they are a great resource and a great place to start to give one a good starting point, but as things change (e.g., getting older, flexibility issues, accidents, etc.) the fit must be adapted to make riding more comfortable. My experience with two bike fitters (Serotta trained) are that they bring their biases and preferences to the fitting. As such, one can go to one fitter and get one set of numbers and go to a second and get different numbers. I think people tend to look for the "Right Answer" when trying to fit a bike and will sometimes, for example, end up with a stem too long or short because they followed a formula only to find that they needed one 1cm or 2cm shorter/longer.

[Dave]

I agree that a fitting is only a starting point, but at least it should eliminate gross errors, which may be Mark's problem.

No fitter can measure or look at you while riding a trainer and guarantee comfort or optium power output. All he can do is be sure that the rider falls within a range of "normal" paramters that aren't too far off. After that, fine tuning is usually required to get everything perfect. That's why I'd never buy a custom frame based solely on a fitter's recommendation. If a fitting suggested changes to saddle height, fore/aft position and reach, I'd try them out on an existing bike first.

[Satellite]

Isn't there some kind of rule-of-thumb that says you should be able to see your front hub while riding on the hoods? I have never been professionally fit so I am sure this is a slap in the fact to the certified fitters here on the Serotta Forum, I mean no disrespect.

Can anyone clairfy this crazy rule-of-thumb?

Best Regards,

Satellite

[Dave]

Yes, the bars obscuring the front hub is an often quoted rule of thumb, but it's pretty much coincidental. Depending how you're looking, two or three different lengths might be close. That's why I use arm to knee clearance as a guideline. If my stem is long enough, my knees and arms won't hit when riding in the drop section of the bars, with my fingers in reach of the brake levers and my upper back horizontal. If you crouch even lower, it's possible to create knee to arm interference even using an overly long stem. An overly long stem usually results in upper arm or shoulder pain due to a too-extreme angle of the arm to torso. Use additional stem length to create knee to arm clearance, not to lower the torso. Some folks can't tolerate a stem long enough to avoid knee to arm interference. If that's the case, then flaring the elbows outside the knee is the only option.

If you're truing to achieve a lower torso position, concentrate on the handlebar height, rather than stem length. Bars that are way too low create a knee in the chest problem. Most people will experience too much weight on the hands or lower back discomfort before getting that low.

[CalfeeFly]

No fitter can measure or look at you while riding a trainer and guarantee comfort or optium power output.

The person I go to has an interesting approach to help cover the above. He works you into the ground! I stumbled out when it was all said and done. His philosophy is to get the person tired enough and less concious of "riding perfectly" to have them duplicate as closely as possible how you pedal and ride in the real world. His fits take a minimum of 2 hours and you get some training advice as you suffer along. :bike:

I love the fit he came up with for me. It took some miles to adjust but once I did I've never been more comfortable or stronger. :D

The value of any fit really depends on the person doing it. Serotta trained doesn't mean anything if they didn't learn and retain much. I know another person Serotta trained and when it comes to fit he is next to clueless. I think he either slept through the training or forgot the bulk of it. My guess is both happened! :)

[mdeeds71]

There are way to many people assuming that I didn't get a complete fit...or I did not get my seat to pedal i.e. appropriate knee to pedal relationship done...I was simply trying to figure out without the actual fit data on hand if I should buy a Serotta 120 stem by ITM that is on ebay given just basic parameters...I am going to get the actual fit data this next week but the stem may be gone...Hence the reason I was asking, as well, if anyone had insight on where one could fine the Serotta by ITM stems/bars...Alsol I had to get the fit done (CA) and leave for my flying job (TX) which is no where near the fit location...thus no data since I had to leave right after the fit.

By the way the store is in Santa Clara, CA and I firmly believe they are doing the fit correctly given that I am going after a standard geometry 58 Serotta...with a sloping top tube...and not a full custom spec frame...I also have my data for a Colorado TG fit back in 95....They are getting the same results, frame geometry that is needed...But my data from 96 had the bar/stem (width/length) left blank until they placed the bike on rollars to finalize the fit then. Bottom line I need at least a 57.5 TT but use a 56 ST thus requiring a sloping top tube for standover.

Again appreciate the help...but I am still looking for data on where to get these stems/bars...