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Discussion Starter · #1 ·
I'm putting together some plans for my custom cx and wanted to get a little feedback on HT angles. The numbers as they stand now are ST 60 cm eff, TT 59.5, HT 73 deg, ST 73 deg. Any one else running similar angles?

e
 

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escamillo said:
I'm putting together some plans for my custom cx and wanted to get a little feedback on HT angles. The numbers as they stand now are ST 60 cm eff, TT 59.5, HT 73 deg, ST 73 deg. Any one else running similar angles?

e
Never something I have really paid too much attention to regardless of discipline I guess primarily as I never had the $$ for custom!, sure 73/74 for road/crit bike but cross mine is 73 ST and 72 HT, I think most important is seat position for CX as far back as comftable (remember bony lump on knee ideally discect pedal spindle) to aid in traction and generate max power from glutes and hamstrings etc.
This can be achieved from layback seatpost or obviously slacker ST angle to a point. What does the frame builder recommend? and who do you plan to use, can you get ideas from Richard Sachs etc as I believe he builds awesome CX frames.
 

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Discussion Starter · #3 ·
dreww said:
Never something I have really paid too much attention to regardless of discipline I guess primarily as I never had the $$ for custom!, sure 73/74 for road/crit bike but cross mine is 73 ST and 72 HT, I think most important is seat position for CX as far back as comftable (remember bony lump on knee ideally discect pedal spindle) to aid in traction and generate max power from glutes and hamstrings etc.
This can be achieved from layback seatpost or obviously slacker ST angle to a point. What does the frame builder recommend? and who do you plan to use, can you get ideas from Richard Sachs etc as I believe he builds awesome CX frames.
Currently I ride a 59 cm LeMond and like the way it rides...mostly. I was going to build on what I liked on the lemond and tweek a few things ie head tube, raise the bb (i ride 180's), slightly longer tt and st. The lemond has a 72.5 ht and 45 rake on the fork. The Q fork I plan on getting has a 47 rake (though I am a bit dubious of the epoxied nut for the stem, no adjusting down the road). I am going down from many bikes to just two: a 29" mtb and a cross rig and will be using it as a traning and racing bike. No more road bike. I considered the 73 ht to speed up my steering on the two or three crit/road races I "might" do this year. Just for kicks of course.

I plan on having Seven build the bike (tsunami steel), would love to have a Richard Sachs but the wait is something like a year or so, sorry Richard can't wait. I also wanted the sloping top tube. It will be interesting to see what the Seven spec program will spit out.

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escamillo said:
I plan on having Seven build the bike (tsunami steel), would love to have a Richard Sachs but the wait is something like a year or so, sorry Richard can't wait. I also wanted the sloping top tube. It will be interesting to see what the Seven spec program will spit out.
Just thought if you spoke to Sachs and told him you are interested he might have good info for you, he charges over $2000 for frame and wait is I believe closer to 2 years.
You have a Poprad Lemond or a road version? I had a Poprad and loved how it handled (steel not alloy) and am seriously thinking of getting another for training CX, too heavy to race on. If you are putting on a shallower rake fork you may be able to get away with a 1degree steeper HT.
 

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72.5/72.5

escamillo said:
I'm putting together some plans for my custom cx and wanted to get a little feedback on HT angles. The numbers as they stand now are ST 60 cm eff, TT 59.5, HT 73 deg, ST 73 deg. Any one else running similar angles?

e
I have a custom curtlo cross bike 56cm size 57cm tt I'm 6ft was ridding a 58cm road bike, based my angles on my ss cross check but with a 1/2 degree steeper ht angle, it handles nice and doubles as my only geared bike and only road bike, 3 wheelsets. But I should have had the bb dropped a little, cause it is a little bit twitchey at times, the 72.5degree st angle is really good for seated climbing, running a thomson w setback. My bb is about 11.25in, both the lemond poprad and bianchi sanjose have 72.5degree ht angles in similar sizes, but with a lower bb, 10,75 or so I think, but not totally sure. I think that 73 degree ht angle on cross bike, even a large one, with a higher bb, would be too twitchy to want to cross race on. Talk with seven for a while, they should know what works and I would expect should steer you in the right direction or give you reasons for possibly not going so steep on the ht angle.
A custom bike is a great thing, but it is a lot of money, time(waiting) and anticipation, The builder should be a great source of info, and be quite knowlegeable, also asking other builders or racers on custom bikes should help also.
 

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HT and ST angle doesn't mean very much unless you also have the fork rake you have in mind, the front center and chainstay length calculations. One important element in crossbike design is the proper fore-aft weight distribution. I think it's quite possible that a 73 degree head tube, on a square frame, with most forks might put too much of your weight on the front unless you increase the chainstay length.; i.e. your front to back weight distribution is not high enough for a cross bike. I'll bet someone must have made a decent spreadsheet or frame geometry calculator. If you can find one, you might want to put in some numbers some reputable frame companies and see if they come up close to your design. If I were going custom, the things I would consider are:

1) TT length (fit)
2) Seat angle / setback (fit)
3) Chainstay length (for tire clearance and rear weight distribution (which is also affected by SA)
4) Head tube angle and fork rake (for the proper trail, front center and front weight distribution for handling speed and traction in turns)
5) Overall wheel base (for handling)
6) The bottom bracket drop, (with the kind of tires you are using in consideration), for clearance and stability.

That being said, I've raced for 18 years and I really couldn't tell you how to design a bike from scratch. I can tell if a bike rides badly if I ride one, or the geometries that make a truly poor handling bike, but I don't think I can articulate and combine all the variables into a good design. I think the best idea when going custom is to trust your builder and tell them the kind of fit you need, your riding style, the characteristics you want, and then trust their knowledge to come up with a good design.
 

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There are 2 ways to go about ordering a custom frameset. One is to specify every detail, including frame angles, tubing lengths down to the millimeter, even tubing types and gauges, and then the builder, essentially acting as a technician, makes it to your specs. If you have owned many bikes, have a firm understanding of bicycle geometry, and know exactly what you want, this is a fine approach.

The other way is to choose an experienced framebuilder with whom you have a good rapport, tell him your body dimensions and biomechanical/medical issues, what you want, how you want your new bike to be similar or different from bikes you have ridden in the past, what kind of riding or racing you plan on doing with it, and leave it up to him to create your dream bike. This is how Richard Sachs works. He has definate opinions on bike fit, geometry, and aesthetics, and will not compromise his values to meet the oddball request of a customer. Read this fascinating interview of Richard Sachs by Grant Peterson, wherein Sachs explains why he doesn't "really make custom frames": http://www.richardsachs.com/articles/rsachsriv.html . I am a fan of this second approach to ordering a custom frameset, where you choose the framebuilder, and then trust him to build you a fantastic bike.

Getting back to your question about head tube angle, in my opinion it is an almost meaningless number when considered in isolation. It is a gross simplification to say that bikes with "steep" head tube angles are "fast," "agile," or "twitchy," while those with angles like 72 degrees are "unresponsive" or "sluggish." How a bike handles in response to steering inputs is a function of many factors. An important variable is fork trail, which can be calculated from the outside tire diameter, fork offset (rake), and head tube angle. Kogswell Cycles has a handy online trail calculator: http://www.kogswell.com/geo.php . But trail is not the end of the story. Fore-aft weight distribution, wheelbase, chainstay length, BB height, frame and fork tubing characteristics... they will all affect how a bike "handles." And the handling characteristics that one person likes might make the bike unrideable to another person. So, the answer to the question of "what is the best head tube angle for a cross bike?" is, "it depends..."

I don't mean for this reply to be condescending. If it comes across that way, sorry. It's just that I frequently see posts to internet forums like MTBR that read, "Hey guys, I am ordering a $2000 custom Ti 29er frameset, here are the geometry numbers I am thinking of... Do you think I should raise the BB by 1 cm, or maybe shorten the chainstays to 16" because I heard that's what Jeff Jones rides... And what about the top tube--should I keep it 24" or add another half inch?" I think that most people would get better results trusting an experienced framebuilder to make these decisions, rather than soliciting opinions on internet forums from people who may or may not know what they are talking about, and who almost certainly know nothing about your individual body dimensions, riding and racing history, fitness, flexibility, strengths and weaknesses on the bike, preferred components for things like saddles, bars, and shift/brake levers, medical problems like knee or back pain, pedalling style, etc., etc.
 

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73 HA with 47 mm rake will give a short trail and in my experience be way to nervous. My roadbike is more much more stable (riding no hands) than my cross bike. It will be OK for 45 minute race but you won't like the bike on the road.
I have a 1980,s Italian roadframe (Columbus SP 66 cm 75 HA) that almost killed me because the severe shimying.
If I would have a custon it would have 72/72 angles, not too high BB, I would not use 180 mm cranks in cross. The frame would be a great bike to tour/train on too.
 
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