Kontact 

I have a motorcycle license and took the course. You steer at low speeds with the handlbar.

There is simply no way to make a corner on a bicycle at 8 miles an hour while being vertical. You would fall to the outside of the turn.

Kimmo 

You mean you've never seen a bike with a damaged head tube? Come on.

Now imagine a bike with a 2" tall head tube. Doesn't that tell you that a 4" head tube is doing it harder than a 6" head tube?

Now consider some of the less robust headset systems getting around these days.

Kontact 

Not really - I've seen a lot of bike with damaged top and down tubes where they enter the much thicker head tube, and they were all larger bikes.

Short head tubes are generally more robust because the top and down tubes a braced against each other, making the head tube stiffer. Moving the headset bearings closer to each other doesn't make anything weaker, either.

And, small bike are ridden by lighter people. So I don't know where you're pulling this from. But I'd love to see pictures of these damaged short head tubes.

rgconner

You might steer with the handlebar... and that might be what you were taught at the beginners course, but that is not how you ride a motorcycle effectively or efficiently. Experienced riders lean the bike and stay upright at slow speeds. Less experienced riders try to horse the motorcycle at low speeds and it is painful to watch.

The hilarious part is I just showed you video of people being trained to do differently than you, and doing it very effectively. And you don't believe your lying eyes.


People's definition of slow is different. 8 miles per hour or 1 mile per hour, leaning the bike is required to make it turn. Motorcycle or bicycle.

YOU don't have to lean, bu the bike does.

Kontact 

I don't know if we are talking about two different things or not.

Having two wheels, two wheel vehicles must shift their weight to the inside of the turn or they will highside if they are going faster than a few miles an hour. This is "leaning", and it can be initiated solely with steering inputs.

At very low speeds, the bike has no real "centripetal force" to counter, so the bike is not leaned into the turn, because it would fall over.

rgconner

The bike has to lean to turn. Physics. Period.

YOU don't have to lean, and since you outweigh the bicycle 10 to 1 or more, it does not take much to stay upright while the bike leans a few degrees under you. Most people don't even realize they are doing it, thus the erroneous idea you DON'T have to lean the bike.

A lot of these lower skilled/low confidence riders get tensed up, and since you vastly outweigh the bike you cannot lean the bike effectively to turn effectively, resulting in a wide, flat, failed turn.

And then they hit the brakes which stands the bike up and the turn really fails.

Kontact 

You have to lean your body to change direction at more than a few miles an hour because you would high side otherwise. Period.

Kimmo 

Granted, the rest of the frame has a harder time resisting braking and impact forces when the head tube is longer; I should have been more specific. But moving the headset bearings closer together most definitely puts more force on the head tube (if the 2" example didn't illustrate it for you, imagine no space between the bearings). I've seen a few steel frames where the bottom of the head tube is deformed to the point the cup just falls out, a couple of which where it was so severe you could see the deformation. And I've seen an aluminium frame that had developed a crack in the head tube behind the lower cup.

Now, these were all old-school frames with 1" forks; a larger diameter head tube would arguably be more robust. And sure, most of the steel frames I saw with this damage were low quality. But modern systems aren't all more robust; I'd say from my experience as a mechanic that threadless headsets are more commonly loose, which can greatly exacerbate the potential for damage, and the style of head tube with integral bearing seats just begs to be destroyed by these sorts of loads; the only thing holding the bearings in place is the preload forcing them into the tapers. I've had to rescue bikes that had damaged seats by adding aluminium tape around the outside vertical face of the bearings so they were a press fit, providing some extra lateral location. Otherwise the frames were a bin job.

It's a similar sort of oversight in rear hubs with floating cassette bodies; with the drive side axle bearing close to the centre of the hub, unless the axle and bearing are beefed up to compensate (which is rare), the durability just isn't there. Just because a lot of companies make and sell a design, that doesn't mean it's a good design - cf. all those creaky BB standards necessitating the aftermarket band-aid fix of a clamping threaded BB.

Kimmo 

As I said, it's only a problem for me on bikes that are too small, so I haven't had a lot of practice dealing with it. But I don't know how you trackstand - maybe you can manage it without having one foot forward? How is that supposed to work?

Kontact 

Thats quite a theory, just lacking in reality.

Kimmo 

The only bit of theory is inferring that shorter head tubes mean more load on the bearing assemblies, which is pretty damn elementary; if you can't imagine the difference changing the head tube length makes to that picture, that's no skin off my nose. I'm sure many others aren't so challenged by simple concepts of leverage.

And as for the actual experiential bits of reality I mentioned, do you have any response beyond willful ignorance?

Kontact 

I've worked as a mechanic on and off since 1990 and ride 50cm road frames. I've never seen anything like what you're describing.

Are you familiar with BMX bikes and their short head tubes? Have you seen what people do with them?

Willful ignorance, indeed.

DannoXYZ 

Also some physics involved in two aspects:

1. static weight loads (weight distribution). If you've got 40/60 F/R distribution and you bring in head-tube to get say... 50/50 F/R. How much extra is that? If you've got say... 200-lbs total weight, initial balance is 80/120lbs which changed to 100/100. How much worse is that extra 20lbs on front-end bearings? Especially compared to hundreds of lbs impacts from hitting bumps.

2. dynamic loading. As mentioned, hitting bumps puts way, way more load on front-end bearings than any amount of weight-distribution can do. Braking is also a dynamic event that puts more load on front-end. Say you've got 40/60 F/R distribution and at maximum braking, what's the load? It's 100/0 F/R with back-end skimming the ground. With 200-lb load and 1G braking, you're looking at 400 lbs on front wheel.

Let's change head-tube angle and modified weight-distribution to 50/50. What's the load on front-end at max-braking? Exact same 100/0 with exact same 400-lb load on front wheel!


You also have to separate force-vectors on headset bearings into vertical and horizontal components. Head-tube length only affects horizontal loads, which is minor compared to vertical loading from weight and bumps. Unless you're rear-ending parked cars on regular basis...

rogerzilla

On a bike with a fixed gear and substantial toe overlap (proper track bikes have it in spades) you learn to tilt your toes up or down when manoeuvring at low speeds. That is usually enough to avoid contact.