Inertianinja

If a bike's top tube is too long for you (let's say by 25mm), but you are positioned correctly over the rear, would that cause the front end to be unstable / wobbly?

roadwarrior

A loose headset for one thing. But your description could be better. Try closing the front brake and rocking the bike back and forth to see if the headset is loose.

Inertianinja

Headset is tight.

Looigi

Not sure what exactly your post is about, but FWIW, IMO, a rear weight bias generally helps improve stability.

Inertianinja

Question's pretty clear.

Assume the saddle position is correct w/r/t the rear wheel

YOJiMBO20

No.

Phil_gretz

What is the frame material? The bike in question, and particulars on its geometry? At what speeds? Is there a resonance at a particular speed? Does it vary with road surface/condition?

Try clamping your knees on the top tube?

njkayaker

Don't tell this guy that!

https://davesbikeblog.squarespace.com...tribution.html

achoo

Death wobble?

If so, you need to change the resonant frequency - shift your weight, loosen your grip on the handlebars, clamp your knees on the top tube.

Hunt-man

Try a different front wheel. I have had stability problems, changed wheels and problem went away.

kv501

How long is your stem?

If you're top tube is too long for you (you said you were positioned correctly over the rear), that makes me wonder how short your stem is. I ride a 58cm frame, but with an 80mm stem. Put thousands of miles on that way and I'm very comfortable. I do live with it being semi-twitchy as a side effect, however. I took a big jump all at once from the stock 110mm stem, and the first time I descended at 35mph+ it was...invigorating.

mtn.cyclist

Are your tires out of round? Are your wheels true? Just a little will have a significant effect at higher speed. Sometimes wheels can be unbalanced like an automobile wheel. Might be worth a check.

rpenmanparker 

I don't know the answer, so I am just sort of stating ideas in a way to elicit more information. I have read the above article and was not convinced. Moulton is a big name and should know what he is talking about. On the other hand all his information is anecdotes, observations, etc. none of it is scientific. An arrow has nothing to do with a bicycle, for example. The pros could have adopted the newer downhill position for aero effect not stability. The newer position could be more stable because of the rider's mass in contact with the front end to dampen it, not because of weight distribution. This may sound contradictory, but it is really not. Why for instance is it easier to ride no hands sitting up right over the saddle? And then there is the gyroscopic effect, which is what keeps bikes travelling in a straight line. If you make the gyroscope itself heavier (the front wheel), don't you increase the rotational momentum, i.e. the gyroscopic effect? But if you put weighted panniers on the front or shift your weight to the front, aren't you increasing he forces that can pull (if unbalanced) on the gyroscope and change the direction of the bike? What do others think? Is more non-rotating weight forward better or worse? It seems worse to me.

Having said all that, OP never said anything about speed wobble, only instability and a undefined wobble. He may just be talking about steering that is too responsive due to inappropriate fork trail for his riding style/skill level. We really need more information to be able to help.

Robert

hyhuu

In the touring section, there are a lot of discussions on the topic of front/rear weight distribution of the loads. It seems like there is no consensus but people who put their heavier load up front like it for high speed stability. They found the heavier rear load makes the bike squirely on a descent (arrow analogy?). But since touring bike geometry is different (and the load is also smaller than the rider's weight), the whole point may be moot.

njkayaker

Are you "Looigi" too??

Anyway, Moulton's position is the fairly-standard one, currently (it seems). (Looigi's position appears to be unusual and unsupported as well.)

https://en.wikipedia.org/wiki/Bicycle...cycle_dynamics


The OP wasn't completely clear but I wasn't thinking he was talking about "speed wobble".

Looigi

Interesting. I do sometimes descend sitting on the top tube (yesterday was the last time), and IMO it's less stable than biasing the weight rearward.

wphamilton

I think putting more weight on the handlebars and less on the pedals (or saddle) hinders your control of the bike.

No question, moving weight forward increases stability with respect to aerodynamics. However, there is another piece to that: the lateral center of pressure. https://www.nar.org/NARTS/TR13.html Put simply, the center of pressure needs to be behind the center of balance. Moulton kind of fudges that by referring to "the center point between the wheels" but that's really irrelevant. It's probably close to the center of pressure when the rider is in the saddle mostly balanced above the pedals, but not when he's moved forward. The center of pressure moves forward with the rider.

It wouldn't be that hard to figure out which is moved more, the center of gravity or the center of pressure. I haven't done it, mainly because I estimate there isn't much if any difference. It is not a given that it's more stable given the change in shifting weight. It could be less stable, with the center of gravity closer to the center of pressure than before. I suspect that the main reason for scooting up forward and low on the top tube is simply to present less surface area to the front.

carpediemracing 

Although you point out accurately that, for example, an arrow has nothing to do with a bicycle, some of your examples are somewhat irrelevant too.

It's easier to ride no handed sitting upright because to lean forward you have to use your glutes to support your body. Since using the glutes exerts force on the pedals you accelerate. At some point you get tired or you're going too fast. A humorous example is when Gilbert Duclos Lasalle, horsing around in the Tour du Pont, goes off the front of the field riding no handed (and leaning forward). You could see him accelerating briskly just to stay upright. It would take a very skilled rider to ride no handed at a regular/slow pace while leaning forward.

The importance of the gyroscopic effect on a bike's stability has been proven to be insignificant. One can ride a bike with virtually no gyroscopic effect (very small wheels). Two factors come into play with "a bike that rolls forward on its own".

One is trail. A bike with momentum will go in a straight line as long as it has trail. So will other vehicles. A shopping cart has trail just like a bicycle but it has a 90 degree "head tube". Likewise the special derny bikes for high speed motopacing are made with virtually vertical head tubes and backward forks (negative rake). This combines to create massive trail, just like a shopping cart (vertical head tube, negative rake). If you turn the front wheels of a shopping cart backwards (so they now have positive rake and therefore negative trail) the cart will not go forward smoothly. The front wheels will turn around first before it rolls forward.

What allows a bike to stay upright is the ability to steer. Without steering it's virtually impossible to ride a bike. This is why it's so important to have control over your bars, front wheel, fork, stem. It's why when Hincapie's steerer tube broke he couldn't just coast to a stop. He could no longer steer and that prevented him from staying upright. As an experiment lock your fork in place or at least make it hard to steer - you can tie your each side of your bars to your top tube so you can't turn the bars (use tape or some string or something). Now try and ride your bike. Nothing else has changed on the bike except you can't steer - gyroscopic effect, trail, etc. The bike will be virtually impossible to ride. I tried it as part of a bike education thing as a kid. It was very illuminating. It's like trying to ride a long board with a wheel at each end. Without steering there's no reasonable way to ride a bike.

rpenmanparker 

Informative. Thanks.

cyclezen

yes

gregf83 

No. Otherwise moving back 25-50mm on a properly fitted bike would cause instability and that doesn't happen in my experience.

carpediemracing 

To return to the original question...

In my experience the bike will feel less stable.

The Missus has longer legs, shorter torso. Long before we were dating she asked for help in getting a road bike. Having a lot of contacts in the industry etc I felt it most economical for her to just get a frame and to build it up using essentially cast off parts. Although I initially thought she could get away with a normal men's frame we ended up putting a short 90 mm stem on her frame. Since the frame is similar in size to mine I looked forward to riding a different bike to see what it was like. Since she was using one of my wheelsets the overall feel of the frame should have been similar to one of my bikes.

I was shocked to find the bike tricky to handle at speed. In fact I had been thinking the Missus wasn't trusting the bike because she said she got nervous once we hit about 37-38 mph. Well I was petrified at that speed so I totally empathized with her thoughts. Unfortunately it would take a custom frame to fix things and she declined the offer.

I can't point to any particular physics to explain it but I've since taken note with stem lengths and how they relate to bike handling.

Short stems lend themselves to slow speed handling. An extreme example would be a BMX stem - it has virtually no reach. A BMX bike is fine at lower speeds. At 30 or 35 mph, if you're not blasting through curves or sprinting out of the saddle, it's a real wiggly ride. The Missus's bike is similar. It's fine at lower speeds but once you get into the 35-40 mph range it feels less stable. Short stems are good when you turn the wheel to steer. The bars turn very easily.

With longer stems (110+ mm) it seems that the bike feels sluggish (in terms of handling) at slow speeds but more stable at high speeds. Long stems don't turn well, meaning when you turn the bars. They work better when you need to lean more than steer, i.e. when you're moving along at a decent pace. They are very stable at speed.

I have a friend/teammate who'd been racing a couple years. After some discussion I radically adjusted his fit. The new fit worked out - in a couple days he won his first training race, he won the next two (three total) and in there somewhere he got third in a target race. Prior to that he hadn't won a training race. His fitness didn't change much in the few days so the only difference was the fit.

Anyway I had him increase the length of his stem by 20 mm (and drop it 25mm). We went for a shake down ride immediately after the fit. I warned him that the bike would steer slowly in the driveway but it would feel better on a 30 mph curve. Sure enough he almost stopped the ride in the driveway because his bike felt so foreign to him. He took a leap of faith and kept going - he ended up doing a sprint or two, went blasting around a few curves, and returned to the house looking forward to his next race.

He'll tell you that on an otherwise identical bike the longer stem really affected the bike's higher speed stability (25 mph and over). It negatively affected its low speed maneuverability, like if he had to turn around on a sidewalk, but at slow speeds you can always put a foot down. You don't have that option at 35 or 50 or whatever mph. He had a professional fit done a few years prior and he very carefully made sure nothing changed. Then I changed the fit. His equipment was the same except for the stem (and we had to replace cables I think). Suddenly his bike handled differently.

I can't offer physics equations or someone else's proof etc. I can only speak from what experience I've had and what I've seen other riders experience.

carpediemracing 

No problem. Actually I'm impressed with your response

abhirama

Really informative, CDR. Thanks!

99Klein 

Op doesn't say anything about wobble but I agree a short stem will "liven" up your ride at speed. Experienced it myself trying to change my fit.