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My girlfriend and I are looking at a new bike for her. The one we're looking at has a 70 degree head tube angle while her current bike has a 70.5 degree head tube angle. Unfortunately, none of the local shops has one in stock for her to try out. So my question is:

How much does the head tube angle affect the fit and feel of a bicycle? I'm sure that it has at least some effect over the fit of the bicycle, but how much? And I'm guessing it has an even bigger effect on the steering, but again, how much?

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A superset question: bicycles.stackexchange.com/questions/12374/… –  cmannett85 Oct 21 '12 at 19:54
    
Is this a mountain bike or one with suspension? The angle difference you mention is very small, you'd need to be a very sensitive rider to actually notice the effect. IF this is a bike with suspension fork or a double suspension bike: Consider that the mere fact of having a working suspension produces head tube angle changes larger than 0.5degrees while riding, an no body seems to have problems with that, so, I would'nt worry over such a small difference. –  Jahaziel Jul 21 at 22:18

4 Answers 4

up vote 10 down vote accepted

there's a good write-up about this here: http://www.calfeedesign.com/tech-papers/geometry-of-bike-handling/

In general the steeper the angle the more agile the steering. If you like steeper you might say "quicker, more response", if you didn't like steeper you might say "twitchier".

A 1/2 degree difference is probably not going to make that much difference by itself. The other factors, which headset, installation of headset, stem length, handlebar type and setup, etc., will mask the differences here. As the Calfee page mentions, people will adjust very quickly.

Happy Riding!

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Also, handlebar reach (as adjusted by stem height and length) have a very significant effect on bike handling, for any configuration of angle/rake/trail, since it affects the torso-weight distribution over the handlebar (via each hands), thus making the self-centering effect on steering more or less pronounced. –  heltonbiker Oct 19 '12 at 14:14
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It should be noted that "steeper" means closer to 90 degrees -- vertical. And generally head tube angle is adjusted in concert with fork "rake" -- the degree to which the fork curves forward. There is a complex relationship between the two that determines stability vs responsiveness. –  Daniel R Hicks Oct 19 '12 at 15:28
    
With a recumbent, it's even possible to have a vertical or past vertical headtube, which won't be twitchy with an appropriate fork: fastfwd.nl/assets/images/nieuws2.jpg Obviously if you tried that on a conventional frame, the front wheel would hit the downtube. –  armb Jun 12 '13 at 11:14

When the head angle is steeper, the bike becomes more nimble and twitchy. The bike reacts faster to steering input making it prone to oversteer. Road bikes tend to have "steep" head angles in the 72 to 74 degree range. A half degree is more noticeable on bikes with steeper head angles, especially at higher speeds. However, if you're a casual rider than a half-degree probably won't make a difference.

When the head angle is shallower/ slack, the bike becomes smoother and more stable, but prone to understeer. Insofar making half-degree increments almost indiscernible. These angles are commonly associated with mountain bikes and beach cruisers. Slacker angles are sometimes associated with longer wheel bases as well; Boutique bike manufacturers commonly offer 'chopper' style bikes with long wheel bases and super slack head angles. This makes the bike fun to cruise but it's not very nimble.

Mountain bikes tend to have a wider range of head angles available depending on the type of riding you're interested in. XC (Cross Country) tend to be steeper, measuring in the 70 to 71.5 range, and on the other end are DH (Downhill) bikes raking back as much as 64 degrees on popular models. In between those model types are Trail and AM (All Mountain) bikes that have respective head angles.

Another thing to understand is that bikes with rear suspensions have a noticeable affect on the head angle when the rear suspension activates. When the rear suspension squashes, the head angle slackens. It gives the rider more stability while also increasing ride comfort, which is why full-suspension bikes are so popular. There are other benefits but that's another topic :)

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As you already suppose, the angle does make a difference.

To your first question: If you take the distance between the wheel axles as a constant measure, the handle bar will get further away from the saddle the higher the head tube angle gets, therefore the geometry gets "longer". This can for a certain amount be compensated by a shorter stem.

The bigger effect is - you guessed it - on the steering behaviour. A flat head tube causes the steering to get more stable, so its tendency to go straight is higher. This means that if you compare a bike with a flat head tube to one with a steep one, the flat one will feel like driving on rails, meaning you feel like you have to "force" it into curves as it tends to go on straight.

The most interesting question is if the difference of half a degree really would make a difference but I think it won't. I can remember that when I bought my last bike some time ago that the different bikes that made it to the final decision had some spread over some 1 or 2 degrees in head tube angle, but they were all from the same price and purpose class. So I would say that such a small difference will be more due to technical decisions of the manufacturer than due to better or different handling.

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Head tube angle is one of 3 factors that together determine bicycle tendency to stay in the same direction. The other two are wheel size and fork rake, and with head tube angle they give value named trail .

Bicycles with larger trail tend to be stable and hold direction, while bicycles with smaller trail react faster and with smaller input.

Given the wheel size and same fork rake, smaller head tube angle will give larger trail.

While most of the bicycles with smaller head tube angels tend to have matching forks such that they have larger trail, true answer can be found only by inspecting all elements.

Or even better by trying out the bicycle.

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