Bike Forums - Stem Length Theory

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Stem Length Theory

I am ~~mildly~~ insane in that I have three fixed-gear bikes, though the newest is an honest-to-goodness track frame (NJS), whilst the others are roadish things (Steamroller and a Wabi). I am experiencing twitchy behavior with the NJS frame and I wondering if it is due to my stem length. (I admit to having anxiety issues when being around other cyclists, and I am noticing that I am not able to just 'work through it' when passing others on the MUP: The bike feels helluv twitchy and weird.)

I installed a 100mm Pearl with relatively no angle on the NJS bike because it felt like a worthwhile default, thought I am noticing I am positioned kinda far over the contact patch of the front tire and am wondering if this might be indicative of a fit issue. The same stem length and bar-type on my other bikes seem to put me a bit further back, obviously due to a slacker geometry, though my education tells me 'shorter stem length equals twitchier feel'.

In this case, does it make sense to shorten the stem length to allow the bike to feel more gentle in its responsiveness?

Please, please, please forgive my ignorance :)

I'd say it's more likely due to the NJS frame having a steeper seat tube angle that's pushing you farther forward, putting more weight on your hands.

Indeed, I would think that is obvious :)

Can this position be compensated for via stem length, or is this purely geometric and therefore psychological and/or adaptive?

Edit... I apologize for being presumptive: In which manner can the weight upon my hands reflect in twitchiness? Is there a signal I can receive from my hands which can identify wherein the problem resides? Because I am just riding my bike... If anything, I feel kinda better riding a dope NJS bike around. It just feels twitchy at speed...

I rode my Wabi for 100km today and I didn't feel weird... There is just a subtle thing about the NJS bike that feels unsettling and I am trying to identify there that something is.

I wouldn't expect a shorter stem to make it better, but you have those sweepy-backy bars and I have no idea how those behave because I put drops on everything.

Quote:

Originally Posted by seau grateau (Post 20859726)

...but you have those sweepy-backy bars and I have no idea how those behave because I put drops on everything.

Indeed, I understand, and I am thankful for your help. All my metrics are from me riding the bars on the 'tops' or 'humps' (95% of my time on a bike) which put me in the same position as a road cyclist on the tops or a 'fixie foo' on risers.

You can get a setback seatpost, and slam your saddle back, to compensate for the steep seat tube angle and push your weight back. You can also raise the bars to take weight off your hands.

Quote:

Originally Posted by phobus (Post 20859746)

You can also raise the bars to take weight off your hands.

While my stem is slammed, I have 650mm of rise in my bar.... I am in granny territory.

Would a differential in drop indicate twitchiness? When *none* of my other bikes exhibit this behavior with near-identical cockpits? I am not complaining about comfort, all these bikes feel great, otherwise I wouldn't ride them... I am interested in concepts related to twitchiness.

In trying to figure this out, scientifically, I acquired 80mm and 90mm stems.

Perhaps I am crazy, though my theory is that in a quest for stability: Steeper head tubes may find compensation by shorter stems, putting the rider in a similar position over the contact patch of the front wheel as one would find in slacker geo.

Edit: I've suddenly realized why all of the NJS bikes I swoon over on Pinterest have wee stems...

Quote:

Originally Posted by ethet (Post 20859757)

I'm not saying your saddle to bar drop is responsible for the difference, just that playing with it could be one way to change how it feels. And even though your bars have some rise, the hand position is still down there below the saddle, and I don't think any Dutch granny would be riding around like that. ;)

You may want to read up on trail and see if you can calculate it for your bikes: Bicycle Trail Calculator | yojimg.net

Without knowing more about the bikes you’re comparing it’s difficult to offer much help beyond idle speculation. Given that caveat I’ll take a stab at this. My suspicion is that your “real” track bike has greater trail and wheel flop than what you’re used to riding. The fork is likely to have 35mm or less rake and with very narrow tires will work together to stabilize the bike at high speed but do the opposite at lower speeds which you might perceive as the bike wanting to oversteer.

Take the bike for a ride and get it up to a fast pace and maintain it for a bit. If you notice less twitchiness at speed then we’ve found our issue.

I know a couple of ways to address this, I’m sure some folks know of others. The simplest and less impactful change is to swap out your front wheel. A wider tire will impact mechanical trail but some folks report the rim itself can play a role in this. The next step is swapping out your fork. Your steamrollers fork should slightly mellow low speed handling.

What to do if this is a rake and wheel-flop issue and you don’t want to mess with the fork? Well, you might look at handlebar width. This isn’t an area where I have a lot of experience so hopefully someone else will chime in. I’ve read that narrower bars can make a bike less twitchy. However, wide mountain bars always make me feel like my weight is better distributed and hence the bike feels more controllable.

I don’t think a shorter stem will make a bike feel less twitchy. If anything taking more weight off the front tire will have the opposite effect.

Try lengthening your chain by a link or two. A short wheelbase could be a contributing factor.