trail...

The slack head tube angle will increase the front-center but it will also increase the steering trail, unless a large amount of fork offset is used.

I would NEVER build a bike with untested steering geometry. Here's some info and links discussing trail.


Rake (offset) is the perpendicular distance between two parallel lines, one through the center of the hub, and one through the center of the steering tube. Trail is the horizontal distance between the tire contact point and a line through the steering axis. The more trail, the more stable the bike (slower steering). The less trail, the quicker the steering. Both rake and head tube angle affect the amount of trail. Steepening the head tube angle or increasing rake will decrease trail, reducing stability and quickening the steering. The formula for trail is as follows, where R is the tire radius, and H is the head tube angle. Trail = (R/ tan H) – (rake/sin H). As an example if R = 33.65cm, H=73, and rake is 4.0cm, trail = 33.65/tan73 – 4.0/sin73. This calculates to 6.1cm or 2.4 inches.

http://kogswell.com/trail.php
http://www.phred.org/~josh/bike/trail.html
http://www.kreuzotter.de/english/elenk.htm

 

What is the trail?

Although 71 degrees definitely seems to be too slack for a bike that size (55.5 TT), the real number you should be looking at is the trail. Generally speaking a trail of around 56 to 58 is considered "neutral", or having the ability to have positive steering attributes at both low and high speed. A trail of around 56 or 57 requires little input to change the direction of the bike - A characteristic experienced cyclist prefer. A larger trail number means slower, but more predictable steering. Along with the head angle, the rake of the fork will determine the trail for the bike. Some general trail calculations (may be off by a millimeter or so depending on tire size, fork length etc, but this gives you the idea):
73 HTA with 43mm rake = 57 mm trail
72 HTA with 45mm rake = 61.5mm trail
72.5 HTA with 45mm rake = 58mm trail
71 HTA with 43 mm rake = 69.5mm trail
71 HTA with 45mm rake = 67.4mm trail
71 HTA with 50mm rake = 62.1mm trail

If you were looking for a stable, long distance bike for touring I'd say the 71 HTA with a 50mm rake would be good -- I don't know if I'd even go with the 45mm rake, because the steering gets too sluggish (IMO). If you were looking for a race bike, or even an all-rounder, I'd say go with a smaller trail number. If the builder won't do it, get someone else to build your bike. Anvil bikes has a trail calculator that works fairly well -- keep in mind that the dimensions of the fork can affect the trail number, so these calculations are really approximations.

http://www.anvilbikes.com/

Good Luck!

 

Skip the numbers

If you like your current bike, I'd ask them to build something with similar geometry. If they're going to recommend a change, ask them why they think that's a good idea for YOU. Ask them if there's a way you can try one out with the geometry they're proposing.

It's amazing... we all spend so much time telling newbies and friends to go in and buy the bike that fits, since the component groups are usually the same across a given price point. But I've read so many posts in here about people having custom frames built, and they forget that same advice.

I build custom furniture for a living, and one of the things I tell my clients is that the joy of custom work is that you get to have something built the way you want it to be built, instead of simply choosing from whatever you're being offered by stores and catalogs. It's true that many people don't always know what they really want, and offering options helps... but if you're having them build a bike for you, and they're offering you an option, test that option out before you decide to have them integrate it.

As a custom shop, I'm assuming they know what they're doing, and they should be able to intelligently advise you if you have questions. And it's entirely possible that, after talking to you, they thought that this might be a good fit. But they should be able to explain the reasoning to you, AND be able to let you try it out to make sure. The money is yours, the bike will be yours when it's done, for better or worse, and NOW is your opportunity to have it built EXACTLY the way you want it to be. If you're happy with your current geometry, then I'd stick with it. If there's something you'd like to see done differently, then have your bike built that way. It's possible that the different angle is there to compensate for the geometry of their chosen fork, and the amount of trail will be the same, or maybe it's just something they feel like trying out. But if you want it to be built differently, tell them so.

It will be your bike, in a way that a hand-picked, professionally fitted factory bike won't be. It should therefore be built to accommodate your anatomical quirks and the way you really like to ride. There should be no complaints, because any deficiencies you feel your current ride has, should be able to be remedied in this new bike. It should be the best bike you've ever had, because it's specifically tailored to you, your desires, and your riding style... and THAT'S what makes it cool.

Custom shops have the privilege of getting to work in their chosen field, and I can say from experience that it's a joy to be able to work for yourself, doing what you enjoy doing. But you still have to work for the man with the money, and make sure that you give him what he wants. If I want a nice Cherry trestle table for my dining room, I can build one. But If you came in wanting something else, it would be wrong of me to try to force my vision for a pretty table on you. It's ok to make recommendations if the client isn't really sure what it is he/she wants, but in the end, it's their money, and they have to live (and pay for) with the results, so it's their decision.

 

So what is the seat tube angle?

I am very comfortable with both seat tube and head tube angles identical - possibly a half-degree different in one angle. I see no need for a fork rake of 50 - most modern fork manufacturers don't have a fork rake that's 50. If somehow you break your fork and want to buy an aftermarket fork at a reasonable price, you're screwed.

I have owned bikes with 72/72, 73/73, 72.5/73, and 73/72.5 (traditional), but some boutique frame makers might not agree with that philosophy, I think a fork rake of 50 reminds of touring or audax bikes, or mountain bikes.

 

Seems more shallow than I would use. But there you go.

Two paths to go by. Design your bike and have someone build it. Or go to someone and tell them what you want to do with the machine and let them build it. Either way works. Usually if I design my bike I can find one that works without having to have someone build it.

I'd just ask for an explanation of why they propose what they propose. What it does for you. Compromises in doing it their way.

 

FWIW, I wouldn't trust any bike with less than 60mm trail at high descent speeds.

 

FWIW, I wouldn't trust any bike with less than 60mm trail at high descent speeds.

While one could quibble on the exact number, little trail does seem to introduce some thrilling moments at speed. I recall some excitement with long British touring bikes with rakes of perhaps 60 mm. Trail probably just under 50. These were quite OK at plodding along soaking up junk roads, but their behavior in the mountains led me to generally use a rather more sporty geometry frame! Certainly my current machines have more than 60 mm trail

I notice my fancy Italian bike is very close to 60 mm, carves turns gracefully. Great on drops, but very very sensitive to weight etc. A delicate compromise.

 

will i notice any difference in stering?
will the 71 head angle have any ill effects.

Also will the bike look funny,

1) Yes. Whether that's a good or bad thing depends on desires and what is done with rake/trail.

2) Again, depends. A slack angle with appropriate trail tends to makes a bike that steers more 'stably', that is, requires more input to turn, and stays in an initiated turn more. As a point of comparison, touring bikes tend to be slacker, so that the load doesn't drive the steering as much. TT bikes tend to slack, too, because their front-loaded nature would otherwise make the steering too quick.

If you are looking for a century bike, slack can make sense. If you are looking for a crit bike or pack racer, something nearer 73 is probably a better bet.

3) No one will ever notice.