Assuming a steerer/fork/brake/cable setup that does not rely on the stem facing forward: is it safe to point a stem backwards as a way to reduce seat-to-bar reach (i.e. as a way to 'fit' a too-large bike to a smaller/shorter person)? If not, why not?
No, it would not be particularly safe. The steering would get extremely twitchy and difficult to handle.
In the BMX world, they use minimum offset or zero offset necks for flatland. The minimum offset options allow you shorter reach without causing problems being backwards. These are some options (the neck/bar combo might not help, but it was shown as examples of options that exist)
In the picture you see a zero offset zero angle bar-neck combo, a minimum offset S&M neck and a Kore neck on a Morales:
Short answer: probably. As long as the stem and handlebars are metal, or composite that can handle the stress of the reversed position you're not going to have mechanical problems.
Long answer: Unless you are riding with your hands off the handlebars what matters is the position of your hands relative to the steering axis. As long as your hands are a reasonable distance apart, most people can adapt to anywhere from about 20cm in front of to 10cm behind the steerer tube without much effort. That's the difference between drop bars and M bars. Look at the variety of positions that cycle tourists use, for example. If you're comfortable holding the handlebars that matters a lot more than minor changes to how the steering feels.
I've ridden a wide variety of bikes over the years, and played with handlebar position at times to get comfortable when I've been injured. I ride a folding bike with the stem reversed when I had a broken arm, and while that was somewhat twitchy the very upright position made it much easier to ride with one arm in a sling. The extreme positions tend to be on recumbents because they have more variation in their designs.
You could in theory use a long stem pointing backwards as long as the bars were long enough to put your hands in a sensible position. Some recumbents do this. Others have seemingly ridiculous forward extension and equally ridiculous backward extension on the bars. Below the silver stem is long as well as tilted down, then the bars bring the riders hands back to a comfortable position (the reason the bars go so far forward is to clear the riders knees)
I rode a touring bike somewhat similar to that, but with a very different handlebar:
You can see that the "stem" is about 500mm long and extends back from the steerer tube. There was a big tiller effect (handlebars swing a long way side to side when turning), but the bike was very comfortable.
With upright bikes, I've used a reversed stem to get clearance from the load area:
On that bike the fork had a vertical steerer and about 10mm of trail, so it was kinda twitchy. But the handlebars were 2m behind that and geared up slightly, so the overall effect was more "land whale" than "twitchy mess". Albeit having the rider over the rear axle also meant that there was no way to ride the bike without dealing with the weirdness. I liked it, but other people found it hard to ride. "does not ride like a normal bike"... ya think? :)
What matters is that the bike is comfortable for you. If you can afford to buy a short stem and try it, I suggest doing that. Stems are relatively cheap compared to bikes, and if you have a bike co-op or tip shop near you it might be possible to get a second hand one cheap or free.
The handling issues would depend on the rider's common hand positions vs the steering axis. Generally the hand positions for most stability would have the hands forward of the axis.
Steering issues aside, I would think that even if you got the reach aspect of the bike to work like this, there would be many other problems with the fit of the bike. Could you even stand safely over the top bar? Could you drop the seat low enough to reach the pedals?
Also, the headtube and stem are usually slightly angled. On a mountain bike or especially a road bike, you probably wouldn't have much movement available, as I think the handle bars would meet up with the top bar.
Assuming, as the questioner did, that we are not considering cabling and such, Whether or not to put the stem facing forward or backwards depends on the handlebar shape and placement as well as where you hold the handlebar. I think the physics of bicycle stability is pretty complicated, in any case.
There may be analogies between holding an automobile steering wheel at the "10 and 2" o'clock positions versus the "8 and 4" o'clock positions (or even one-handed at the 6 o'clock. In the car, though, we generally don't use the steering wheel to support ourselves, except during cornering. If you are making a hard turn to the right in a car, then your body will shift to the left. If you are holding the upper part of the wheel (10/2), then that body shift will help pull the steering wheel back towards the center. But if you are holding the bottom of the steering wheel (6 position), then the harder you turn, the more your body will cause the steering wheel to turn even more to the right. So, holding the upper part of the wheel would be more "stable" than the bottom, especially in an emergency.
On the bike, we do put weight on the handlebars a whole lot more, and emergencies can throw us from the bike!
Without holding onto the handlebars (or very lightly), the weight of our body and the bike serves to help keep the front wheel tending to point straight forward because of the way the fork is at an angle and the way the wheel axle is off the axis of the steering tube/headset. At the same time, there is a force of, say, road or hub friction that pushes the front wheel to twist towards the back. The angle and weight should overpower the push back.
Let's now suppose that we have a somewhat standard stem, facing forwards. Now also suppose that instead of riding without holding the bars, you ride by holding onto the end of the stem that clamps onto the handlebar, right in the middle of the bars. If you are putting your weight on the bars, as it a road bike I think, then you are probably pushing forward. This would basically make a force to make the wheel face straight forward. However, if you reversed the stem and held it in the same way, then when pushing, it would tend to swing the stem around making it turn either one way or the other.
When braking, this force of pushing on the handlebars would become even greater, even if you are riding a "cruiser" (where more of your resting weight would be on your butt).
Of course, we generally don't ride single handed (or no-handed). And so each hand can serve to balance the push of the other. If we have a backwards-facing stem, and our hands very close together, as in those stylish messenger handlebars, then braking would be very dangerous because it would be almost like riding one-handed and the bike would suddenly turn whenever you braked:
In all diagrams, the ^ forward direction is UP, | looking down on bike. O <- "O" means steering tube axis. | | ---o--- <- "o" is the stem clamp to the handlebar.
On the other hand, even with forward-facing stems, we do see hand placement that is behind the steering tube in cruiser or beach bikes or dutch bikes (as mentioned by user Kibbee), yet they are somehow safe (motorcycles also often have the grips behind the steering axis):
____------o------____ -- | -- / | \ / O \ / \ / \
This (above) situation is, I can only guess, safe because the hands are so widely spaced from each other that they balance out. Additionally, maybe it's because you put less weight on your hands in bikes that use these.
With those narrow messenger bars, they are actually relatively difficult to change direction compared to mountain bikes because you don't have much leverage on the turning and because the hands serve mostly to keep you aiming straight:
---o--- | | O
Normal mountain handlebars are just wider, but that gives you a lot more leverage and control:
________------o------________ | | O
And the most "maneuverable" and "twitchy" bars would be something like this:
Some other handlebar setup, for reference, are below. These others also offer multiple hand placement, so the twitchiness and handling would be different depending on where you hold them. These tend to have grip locations ahead of the steer tube.
Drop and/or mustache handlebars, where it's been a bit "squished" so we see the drops:
_ _ / \ / \ | | | | | | | | | \----------o----------/ | | | | | O
Trekking / butterfly handlebars:
_--------_____o_____--------_ / | \ \_____---- | ----_____/ O
There does not seem to be a strict rule of whether hands forward, at, or rear of the steering axis makes a particular situation safe or dangerous. In general, there seems to be a mix of influences on handling (and danger from turning-when-braking) depending on such things as:
- how forward/backwards the hand placement is,
- how wide the hand placement is,
- geometry of the bike (angle of steering tube, axle location for front wheel, etc.),
- whether you have more resting weight on hands or not,
- riding one-handed as when signalling a turn (or no-handed),
- braking using one-hand (this almost threw me from my own bike), using coaster brakes, etc.
- braking front vs. rear wheel (may depend on which hand in single-handed riding)
I have reversed the stem to bring the bars closer. I found no difference in steering or safety. If a reversed stem is needed, use it without worry. My answer is specific to "cruiser bars," or "cow horn bars" that do not interfere with the top tube.
The real answer here is that reversing the stem changes the handling very little. 90% of the handling comes from the fork length and curve (trail) and the head tube angle. These things control the inherent stability of a bicycle. The stem length is more a matter of comfort, although there is an important matter of knee clearance, especially if you ever intend to stand on the pedals while riding.
If it makes the controls more usable, do it. You need to use a minimimum offset stem backwards. Anything longer and the drops will hang in the top tube from the swing. I have two kids road racing bikes (650c and 24) but the bike manufacturer seems to forget how short a kids reach is. I use a minimum offset stem on each (in the forward position) and even swapped out the drop bars with an MTB flat bar on the smaller bike until my daughter can reach the drops. The one in the picture below turned out to be very light with the cutouts on each axis. They come in 25.4 and 31.8. In my opinion, a zero offset stem in reverse would still put the drops in front of the steerer tube. I plan on going reverse stem next year on a bigger racing frame (kids road bikes are too raked out for good speed and we need to get past 30 km/h). Besides, my kids can ride a Razor scooter and no bike is as squirrely as that. ... I forgot to mention to check the reach of your drop bars. Go to a big LBS and look at short reach womens specific design (WSD) drops. It brings the reach back even more. You can add brifter wedges to shorten the reach to the brake levers if you have to move your palm forward to reach the brake lever. Its crazy what I had to consider for a 7 year old.
Do it, and find out!
Seriously: flip it and take it up the park and find out the fun/hard way.
All other responses regarding the effect of this are speculation. The actual effect will depend on the bike's existing geometry and the height of the rider.
Assuming the bike is too big for the rider, the switch will probably be workable. It may even be more comfortable than riding bike that is too long.
I just went out and did it. It handles differently, but not twitchy or unpredictable. Yes, you can safely reverse the stem on your bike.
I tried this on a 26" beach cruiser with a 18" frame, and it was quite herky-jerky to steer.
My knees hit the bars so it's not safe for me that way on this bike.
The same change worked well for me on the 29er with a 20" frame size.
Putting the stem on backward won't make a bike twitchy. Think about riding a bike no handed. Some bikes do ride twitchy no handed, some are very stable. How it behaves is determined by the fork and the head angle and geometry of the bike. None of this changes if you were to rotate the handlebars or take them off entirely.
There is an instructional video on YouTube of a guy teaching how to ride a manual (a wheelie without pedaling) and one of his tips is to reverse the stem to move the handlebars way back -- it makes it easier to leverage the front wheel of the bike off the ground. He not only reverses the stem, but replaces it with a longer stem so the handlebars are really way closer to the saddle.
Here it is:
Just in case anyone comes looking for answer (like I did), here's an extreme example of not just inverting the stem but raising the handle bar too. Apparently for comfort.
I added a 8 inch riser bars and a layback seat post reversed the stem comfort is paramount to me added a split seat for the family jewels dept. handling is 95 % the same maybe a little quicker,I'am old but love to ride Should add the frame is a 24 in. stem has an up angle so when I turned it still had an upward angle. top tube slants down. bike as I bought it was too long and tall.details did not say frame size.Mods moved all back 5 1/2 inches and two inches lower.
I forgot to mention the bike is a genisis 6160 29 er have no unsafe feelings when riding the fastest was 30 mpg ,going slow uphill, it always has been quirky stearing like any other bike I have ever riden No Problem
If you do not try this, you will never know how much more comfortable it can make your ride.
Danger is part of driving and riding anything.