azza_333

I am hoping someone can help me understand and compare 2 different set-ups. Any pros & cons of each set up? how do the 2 steering set-ups compare for twitchiness? Thanks in advance.

Current Bike: Head Tube Angle 71°, Fork Rake 50mm, Tyre 700x35, Stem 90mm 6°

New custom frame: Head Tube Angle 70°, Fork Rake 50mm, Tyre 700x35, Stem 70mm 6°

The bike is mostly for lightly loaded touring, and occasional gravel biking.

dsaul

I wouldn't call either of those "twitchy" The custom bike will have more tendency for the front wheel to "wander" or, steer based on pedaling input, on very low speed climbs. My current gravel bike has the same HTA and rake with a 50mm stem and that is the only negative thing that I have found with that frame. It's a very minor thing and all bikes do it to some extent. It's worth it to me for the increase in stability when descending.

unterhausen

Stem length has very little effect on steering. It changes things just a tiny bit at high steering angles, which almost never happen during forward motion. Stem angle has no effect whatsoever. Draw a free body diagram if you don't believe me.

dsaul

I always enjoy the, often quoted, bicycling publications that claim you can approximate the feel of steering with a short stem by gripping the bars close to the stem. Um..no..that only shows that really short handlebars cause twitchy steering.

Andrew R Stewart 

+1. I see the stem as serving two functions regarding steering. First is for the fit of the rider WRT the weight distribution. Longer stems place more weight on the ft wheel. Generally more ft wheel weight means more traction with the road. Some will claim with more weight (however it's achieved) on the ft wheel the steering is "quicker" (and I have always had a hard time with that descriptor). The second is the "swing" factor. As the stem gets longer the amount of travel that your hands go through for steering inputs grows. But how wide the bars are is a usually a greater distance from the steering axis factor and thus have more swing factor then the stem does. Then there's the shape of the bars and do they place the controls further ahead (as with drop bars) of closer to the steerer axis (as with flat bars and their set back angle/length). I think these handle bar influences are bigger then the stem's. (Of course riding no handed makes all this moot).

With today's removeable face plate stems it's very easy to try a couple of different stem lengths and see for yourself what the steering changes feels like. Then you can report back here and learn us something. Andy

Cyclist0108

I came across a trail calculator. That at least enables you to reduce the problem to two variables (trail, stem). Of those, I think the differences between the stems won't be significant compared to the head tube angle.

Bicycle Trail Calculator | yojimg.net

Your new bike comes out to 74.2 mm trail (and 23.8 mm wheel flop).
Your old bike comes out to 67.6 mm trail (and 20.8 mm wheel flop).

Both are high trail compared to a twitchy racing road bike.

The head tube angle matters a lot. The difference in 1/4° is significant. Are you sure these aren't rounded numbers? If they are, I would use 3 or 4 significant figures, do the calculation, and round (if at all) at the end.

azza_333

What about a 70mm stem on the first bike (Current Bike: Head Tube Angle 71°, Fork Rake 50mm, Tyre 700x35) for would that be very twitchy?

Cyclist0108

I'm no expert, but (apart from the change in effective reach) I doubt you would be able to tell the difference, especially with those high trail numbers to begin with. Also, since your shorter stem (in the OP) goes with the higher trail, the difference would have to amount to more than the difference between the two head tube angles (which is a lot -- play with that calculator link I posted a bit).

dsaul

You could probably put a zero (0) mm stem on that bike and it wouldn't be what people refer to as "twitchy". It would definitely feel weird to you, because the bars would be way closer to you and you would have less weight on the front wheel. I think that accounts for most of the handling differences people feel when experimenting with different stem lengths.

unterhausen

The feel from a shorter stem is definitely different, as in shortening the stem on an already built bike to put the handlebars in the wrong place. I am not sure I believe it really changes the rider's cg much. Just reverse engineering the numbers, I assume it's a drop bar bike. The various positions on the handlebar change the rider's cg far more than a small change in stem length.

But since this thread is in framebuilders, I assume the frame isn't built yet. You can't just independently change the stem length, it needs to put your hands in the right place. If you shorten the stem then the top tube should be lengthened to compensate, and thus the cg will not change at all. If we agree on these ground rules, then I'm back to saying that the stem doesn't make any difference to handling. Trail dominates handling, it's much more important than cg and weight balance. You certainly aren't going to compensate for a major change in trail by fiddling with secondary effects like stem length and cg.

79pmooney

Stem length has a very real effect if you go to extremes. (Says he who has many thousands of miles or 180 stems and currently rides 175, 150. 140, 135 and 120.) Much of this effect is from moving the "cockpit" weight so far from the steering axis. Likewise the weight of the rider's limbs.

dsaul

I should have specified that I was talking about experimenting with stem lengths on an already built bike. Not many people have the opportunity to experiment by making frames with different front centers to allow the varying stem lengths to put the bars in the same place. Therefore, their experience is biased by moving the bars to a less than optimal location for them.

unterhausen

I don't think any analysis, no matter how approximate, would show that to be true. If you need a 180mm stem, then you aren't going to want to reduce the length to change the handling.

I always suggest people do what they want to do and are convinced will have the effect they want. Then confirmation bias will make it so. Feel free. Cycling is so full of received wisdom it doesn't make sense to fight it sometimes. It also helps sell new bikes when the polar opposite of what used to be cool is the new thing.