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Screwed-up geometry on small frames!?!?(5 posts)

Screwed-up geometry on small frames!?!?Nebuladds
Sep 26, 2003 7:08 PM
It just occurred to me: a 50cm frame is most likely going to have a more slack headtube angle than a 56cm frame in order to maintain a similar wheelbase.

Is this correct?
re: Screwed-up geometry on small frames!?!?rwbadley
Sep 26, 2003 7:41 PM
Possibly. Some compromises might be made in some smaller frame sizes. The main problem of toe overlap has to do with the fact as the bike gets much smaller, the foot and pedal size and position need to be taken into account.

Toe overlap is one to watch out for in a small frame with steep head angle. The seat tube and head tube angle can be adjusted to counter this. Framebuilders know enough to watch out for and mostly have the situation in hand.
Toe overlap on small frames - You Bet!GeoCyclist
Sep 26, 2003 8:59 PM
My DeRosa King has some serious toe overlap due to the compact geometry. I'm short (5' 8") and my frame is pretty compact. All the more reason to test ride the frame before buying. My bike fits me great, but just have to remember about the toe overlapping the front wheel.
not on my 'Nagslonefrontranger
Sep 27, 2003 6:19 AM
I have a 50cm 'cross bike (traditional geometry) and a 46cm semi-sloping roadie, both Colnago Dreams. Neither has *any* overlap, even the 'cross bike with the fattest mud knobbies that will fit. And I have big feet for my size (5'4", size 41 Euro).

My Morgul Bismark 48.5 traditional, which was custom built for someone of similar measurements to mine, has about 1cm of overlap. It handles great in crits, though it's a little twitchier than the 'Nag (steeper angles). My old Giant Cadex 49cm stock frame had about the same overlap and was a good all-round racing bike. My old 50cm Redline 'cross bike, which by anyone's standards was too big for me, had so much overlap (nearly 2cm, depending on tires) that it became a problem in some tight handling situations on trails.

I will put up with some overlap in a roadie, because when cornering fast in a crit or RR you are not turning the bars so much as leaning, and overlap never becomes an issue. And I pedal through most corners (Speedplays) even at 30+ mph.

When overlap becomes annoying is when I'm on more technical trails than I probably should be riding with a 'cross bike, or in the parking lot screwing around at low speed, or sometimes when I try to trackstand quickly at a light and don't think about it. When it becomes dangerous, as someone on this board discovered, is when a custom framebuilder gets so caught up in measurements they forget about functionality, and the resulting frame is next to unrideable in daily use.

In otherwords, 1 cm or so of overlap is not really a problem. And Ernesto Colnago is a damn good framebuilder despite being a marketing whore.
if it's designed properly, yes it shouldrufus
Sep 27, 2003 5:59 AM
if frames kept the same head angle throughout the range of sizes, then handling will be compromised in just about every size other than the one that head angle is appropriate for. steep head angles on small bikes lend themselves to very quick, possibly twitchy steering. that is why small frames use slacker head angles, and also steeper seat angles, to position the shorter-legged rider properly over the bottom bracket.

conversely, larger frames use slacker seat angles to accomodate the taller rider's longer leg length, but because of this and the longer wheelbase, the big will handle a bit slower. so the head angle is made steeper to quicken up the steering, so the bike is more responsive.

it is these changes, along with different fork rakes that change the amount of trail, that allow for quick, responsive steering across a range of sizes, without veering over that sweet area into squirrelly handling, or falling back into sluggishness. some cheaper made bikes, and the majority of mountain bikes, use the same angles and fork rakes in all sizes, compromising handling to some extent.

toe overlap does not, in and of itself, mean poor design. in fact, some toe overlap is to be expected, but it will only present itself as a problem in extremely slow speed turns, not in most real-world riding situations.