A quick geometry/trail question

[macaroon]

I'm looking at a new frame, head and seat angles are both 74 degrees. TT length is 570mm horizontal and the fork has a 45mm of rake.

Any guesses as to how this bike is likely to ride?

[christian]

It will steer quickly, sort of the anti-Colnago. Don't sneeze while riding.

[macaroon]

Thanks. I'll either blow my nose before cycling, or consider a different frame I think.

[DfCas]

A 40mm rake fork would tame it down some, or a slightly taller one may also.

[christian]

No, seriously, it will steer on the quicker side - it's sort of that 1980s American crit geometry - but I think if it's otherwise a nice bike and a good price, it's not so far out of the realm of reason that I wouldn't consider it. (Well, I wouldn't, because I like bikes that steer like trucks, but I don't think that's in any way unrideable.) Do you like how early Cannondales steer?

[macaroon]

Not familiar with early cannondale frames unfortunately (although I nearly bought a system six the other day!)

this is the frame: http://www.kinokocycles.com/bicycles/tokyo-fixed/road-rocket.html

The new price is probably a bit steep, but it's been reduced a substantial amount.

[tuscanyswe]

I ride a very similar setup. 58tt 74 degree headtube with a 43 rake fork.
It does feel lively and steer fast but its one of the main things i like about it.

[macaroon]

Thanks for the reply. It's making me think it could be ok, althoguh most of my riding is up hill and down dale, rather than on flat, smooth roads so I'm not so sure whether it'd be ideal.

Any views on the frame I linked to?

[happycampyer]

The measurement that puzzles me the most are the seat tube angles—typically, they are steeper in the smaller sizes and slacker in the larger sizes, in order to produce an optimal weight distribution given expected rider dimensions for a given frame size (and often, hta's go in reverse of this and fork rake changes to keep trail relatively constant, but not here). With a 74* sta, will you be able to get your saddle in the proper position?

[mister]

yes
i have a bike that is 74*/74*
my saddle setback is fine

[nooneline]

The measurement that puzzles me the most are the seat tube angles—typically, they are steeper in the smaller sizes and slacker in the larger sizes, in order to produce an optimal weight distribution given expected rider dimensions for a given frame size (and often, hta's go in reverse of this and fork rake changes to keep trail relatively constant, but not here). With a 74* sta, will you be able to get your saddle in the proper position?

They're not only steeper in smaller sizes to get optimal weight distribution. Many manyfacturers have steep seat tubes for the smallest sizes as a way of faking sizing. Figure this: a bike with a 51cm top tube and a 76deg seat tube; if you push that seat tube back to a more reasonable 73deg, then you have a 53cm top tube (give or take a couple millimeter or so). The difference, for any one rider, is where they set their saddle in the rails.

It's a way of fudging the numbers and making bikes that, on paper, look smaller. Stack and Reach measurements are a way to suss out what's really going on with these. Cervelo's article on it illustrates it nicely (http://www.cervelo.com/en/engineering/thinking-and-processes/geometry-and-fit.html).

http://www.cervelo.com/Media/images/geofit-99-19a36f65-ce8a-465c-810d-60f07bd12c58-0-630x320.jpg.

[Mark McM]

Another reason for slacker seat tube angle on larger frames is that crank lengths don't increase proportionally to leg length.

Although there is some controversy over the applicability of KOPS (Knee Over Pedal Spindle), the reality is that when bikes are fitted, the knee does generally end up very close to being directly over the pedal spindle (when the cranks are horizontal). So, consider: The total horizontal distance from the saddle to the pedal is the saddle set back (BB to saddle) plus the crank length. But cranks are only available in a narrow range of sizes (165mm to 180mm is only about an 8% difference), whereas as human legs vary over a wider range. Therefore, to make up the difference, the range of saddle setbacks must vary more widely than range of leg lengths, with the result that smaller bikes need steeper seat tube angles, and larger bikes need steeper seat tube angles, to achieve the same position of the knee over the pedal.

[macaroon]

Thanks for the further info. My current bike has a 73.5 ST angle, saddle height of 830 and a setback of about 98mm. Currently using a 20mm setback seat post. I put some lines into Autocad and apparently I'll need to move my saddle about 6mm further back on the rails to achieve the same set back, which should be doable I think.