Spacers vs Stem Rise?

[Ti_on_Steel]

Hi all, I'm trying to understand bike fit a bit more by comparing my current bike to some other stock bikes. My Serotta has a 210 head tube and I run a 100mm -6 stem slammed. Say I wanted to replicate that fit on a Tarmac with a 170 head tube, theoretically I could use the same stem with 4cm of spacers to get back to the same place.

What is the ratio between stem angle and spacers? Say I used a 0 degree stem, how many fewer spacers could I get away with?

Just trying to get a better sense of how this works!

[DRietz]

It's just geometry. I'd do the math for you, but I think there are calculators on the web designed for this purpose.

To give an example, I remember doing some calculations for myself with a 110mm stem on a 73.5degree head tube and the difference between a 0degree and -6degree stem was around 1.3cm. I think.

[nighthawk]

This might be helpful:

http://yojimg.net/bike/web_tools/stem.php

[pdmtong]

I find this one very useful
http://www.brightspoke.com/t/bike-stem-calculator.html

[lhuerta]

You need to figure out stack and reach of both frames and then figure out where your cockpit contact points will land. Here is some interesting reading on the topic:

http://www.cervelo.com/en/engineering/ask-the-engineers.html

http://velonews.competitor.com/2011/01/bikes-and-tech/technical-faq/technical-faq-with-lennard-zinn-lz-suggest-some-new-years-resolutions-for-the-bike-industry_154565

http://www.slowtwitch.com/Bike_Fit/Choosing_a_Tri_Bike_via_Stack_and_Reach/Stack_Reach_Primer_Chapter_One_95.html

[Dromen]

Not sure if there is a documented standard but my mechanic + a couple others have always strongly recommended never mounting a threadless stem more than X amount above the headset with a carbon steerer tube. Same principle as max insertion point on a seat post. Obviously variables like stem length/angle/HT angle will increase/decrease force(leverage?).

Again, no expert here but logic would suggest you would want to have a stem too high on a carbon steerer. My logic anyway.

[oldpotatoe]

Not sure if there is a documented standard but my mechanic + a couple others have always strongly recommended never mounting a threadless stem more than X amount above the headset with a carbon steerer tube. Same principle as max insertion point on a seat post. Obviously variables like stem length/angle/HT angle will increase/decrease force(leverage?).

Again, no expert here but logic would suggest you would want to have a stem too high on a carbon steerer. My logic anyway.

Pretty much 4cm under the stem is the recognized maximum..at least by us on a 1 1/8inch carbon steerer.

25mm on as 1 inch.

If ya need lots more than that, I suspect something wrong with the frame size-headtube too short.

[tuscanyswe]

As been said allrdy, remember to take bb-drop and different headset solutions (if any) when you are figuring out how much drop you want. If you know stack height of both frames you can skip bb drop but not that many older frames list that measurement (stack that is).

Im guessing the serotta have an ordinary aheadset whilst the tarmac runs an integrated headset. That diff alone could be as much as 2.5 cms.

[Chance]

Hi all, I'm trying to understand bike fit a bit more by comparing my current bike to some other stock bikes. My Serotta has a 210 head tube and I run a 100mm -6 stem slammed. Say I wanted to replicate that fit on a Tarmac with a 170 head tube, theoretically I could use the same stem with 4cm of spacers to get back to the same place.

What is the ratio between stem angle and spacers? Say I used a 0 degree stem, how many fewer spacers could I get away with?

Just trying to get a better sense of how this works!

For what you describe above it's 1 cm.

And if you take your -6 degree stem and flip it over so it's +6 degrees, it will be the same as adding 2 cm of spacers. Actual number is about 2.1 cm but you get the idea. It's very easy to make large changes in bar height with stem angles.

[merlincustom1]

This might be helpful:

http://yojimg.net/bike/web_tools/stem.php

+1. All you need.

[Mike748]

What Chance said. With 73-ish HTA and 100-120 stem -6 to zero is ~1cm and zero to +6 is about 1cm in spacers. And the handlebar end up at the same horizontal extension as well as vertical height.