neeb

I'm getting a custom frame built in Columbus XCR stainless steel tubing.

Firstly, I know my fit very well, so that's not a problem. The issue is how various geometry options that are all compatible with the same fit will affect handling and ride feel for a frame made with this particular material.

I'm looking for a relatively stiff and aggressive ride with a modern feel, not for an old-school steel feel. That said, I don't want the frame to feel completely dead for someone of my weight (63kg). I want something that feels alive when making hard efforts out of the saddle and encourages me to push harder. I can put out about 4w/kg at threshold and over 5w/kg over 5mins, but I'm no sprinter.

What I'm most unsure about is seat tube angle and chainstay length. I need a relatively small saddle setback from the BB, but I don't know whether it is best to achieve that with a steep seat tube angle and a setback post, or a slacker seat tube angle and an inline post.

e.g., I can achieve a perfect setup on my carbon frame that has a 74.5 degree seat tube angle and a Thomson setback post with 16mm of setback. I can also achieve it with a 73 or 73.5 degree seat tube angle and an inline seatpost.

Is there an advantage to the traditional setup whereby the seat tube angle brings you forward and then the seatpost setback puts you back again (seems rather unparsimonious), or for a custom frame is there any reason for not going with the seat tube angle that allows you to use an inline seat post?

I'm guessing that seat post setback potentially provides some shock absorption, but might not a slacker seat tube angle also do that?

As far as chain stay length goes, should I be trying to shorten this as much as possible for a responsive ride (given that my fit allows for a steep seat tube angle in any case), or should I allow a little bit of extra length for "comfort"? My tendency is to prioritise performance over comfort, although only up to a point.

Another thing I'm uncertain about is bottom bracket drop. Most of the carbon frames I have been riding recently have pretty high BBs (e.g. 67mm of drop). I've heard that cornering and handling can be better with a lower BB, although I've found the 67mm drop handles pretty well with a 73 degree head tube angle and 43mm of rake (about 58mm trail).

My frame builder will doubtless have his own opinions on all of these issues, but the more I know and understand myself, and the more diverse opinions I can get, the better IMO.

David Tollefson

Decide on the feel you want (it seems from your write-up here that you're not sure yet. Pick a frame builder you can trust, and TRUST THEM. The more you start monkeying with their interpretation of your wants, the more you'll end up like a monkey ****ing a football on your bike. And they'll be ultimately frustrated.

neeb

I know the feel I want perfectly well, but feel is subjective, so I can't just specify it like I can for something that's quantifiable.

Similarly I'm not about to not trust the frame builder, but the more I know about the process the more I can have a meaningful dialogue about what I'm looking for.

Life has taught me that being well-informed and involved is rarely a bad thing, and I refuse to believe that a frame builder relies on some mystic karma that I'm going to jinx by asking him a few polite questions.

I'm particularly interested in the seat tube angle / seat post setback thing. Maybe that requires a separate thread in its own right. Why would you choose a steeper seat tube angle and seat post setback over a slacker angle and no seat post setback, or visa-versa? What impacts (other than aesthetic ones) could that choice have?

ksisler

OP; Realistically you might want to look for a builder with more patience than I would have... FWIW, I have a few old left over quad butted tubes that have lots of Left Karma, we should use two of those in the design and use tubes with the moderale Top Karma for the rest of the frame to keep it all balanced...[;-)

Realistically, suggest riding several frames until you find one that is pretty close to perfect in the non-measurable areas and take it with you to your session for discussion with your builder. Well before hand put your riding style in writing and at considerable depth then add a list of the component you will add, the required braze-ons, etc. Be descriptive, not directive. I'll note that dictating the tube type and model up front may result in the builder having to make compromises elsewhere in the design and not be able to meet all your needs... Recommend also being aware that the time the builders spends talking with you has an hourly value. FWIW, I normally would expect to spend about 90 minutes in the shop to capture what a customer really wants out of a frame assuming they bring it in with them and stays focused... After that the cost of the frame has to start to go up.

Hope that helps...and please note that I am not poking at you on purpose.
/K

tuz

It's good that you know your fit well. A steeper angle w/ setback allows for a) a traditional road look and b) shorter chainstays. The slacker angle w/o setback might be a bit stronger (which is why you see it a lot on MTBs) but you loose some tire clearance.

For chainstay length, opinions varies, but many experienced builders settle on 420-425. No doubt that shorter stays are stiffer, although 425 vs 400 is not a big difference. One could argue that more confort equals more power.

For BB drop, opinions also vary, but I doubt it makes a big difference since the centre of gravity is pretty high, somewhere above the TT. So +/-10 mm isn't much.

I'm sure you can discuss those factors with the builder, and that he will be receptive to your ideas.

Andrew R Stewart 

I'll toss out some comments that show my biases.

Seat angle has, basically, no effect on bike handling AS LONG AS the seat set back is the same. It has a little bit of tire clearance effect but as I'll mention later i hope you wouldn't be shortening up the chain stays by enough to run out of tire/seat tube gap. The seat tube angle does very much effect where on the seat rails the post is clamped for the post that you're planning. So, and you mentioned this not I, if you wanted more cush and you then thought that a sprung post was the way to go then a straight post/seat tube angle would be best. because there are more sprung straight post options the set back ones. Either way getting whatever post to clamp the seat rails in their center is my goal WRT seat tube angle (and set back being maintained).

Chain stay length has a few effects. It will help control the front/rear weight balance and this is the largest handling contribution CS length has IMO. CS length also effects the tire clearance as said above and it contributes to a good or bad chain run. The shorter the CS the more chain rub, noise and general rough feel the chain will have.

BB height is about cornering clearance (crank arm length) and handling feel. The higher the BB the more it will feel like you're "falling" into the turns. However the stability is another thing and has many disagreeing about BB height and it's effect here. I would say that other geometry aspects have FAR greater effect of stability and cornering then BB height.

The Op not having a real good understanding of all this is all the more reason to follow the builder's lead in most all the design aspects. He says he "knows the feel" but not what has contributed to it. The advice to list/refer to a few bikes that met his goals is spot on. While putting in writing what he wishes, handling wise, is not wrong it is going to be full of descriptive judgments with little actual fact to grasp and discuss.I always thought that some of the bike reviews in consumer magazines are questionable. When two bikes with the same core set of geometry dimensions are described so differently I tend to question the subject description process. Hence having actual bikes to reference is very important, including bikes that the OP has found to not be what he wants.

In the end some of the OP's job will be whether he wants a subcontractor or a custom builder. While you do need to mention what's important, the builder also has their standards and wishes of customer control that might not match yours. (As an example- I would never build a bike with a sub 41cm chain stay). Better to discover this before the builder feels the need to charge consulting time.

neeb

Thanks for all of the advice.

Yes, I'll take the builder's advice in all aspects except where I feel that there's something about me and my riding I haven't communicated well enough.

For the record, it now looks like he wants to go with a slacker seat tube angle and an inline post, which is fine by me. I actually think that this looks better on metal frames if you can de-condition yourself from what has historically been the norm. And as pointed out, the angle in question isn't going to restrict the chainstay length because I/we wouldn't want it any shorter than this angle allows in any case. It should put the saddle right in the middle of the rails when using an inline post.