Understanding geometric impact of different frame designs

[thimble]

Used to be that all cycles had a horizontal top tube. Now there are some with a top tube that goes from the handlebar stem to the rear axle, almost in a straight line. Like this Specialized Crossroads hybrid:
Specialized Bicycle Components : Crossroads Sport

What's the practical impact of such a design? I tried one of these in a bike shops parking lot and liked it a lot, but want to understand more about the effect of the frame design and how it works. Can anyone help out? Does this make for a stiffer ride that's responsive, how does it affect nimbility (being nimble?) etc.

 

[tonyzackery]

Practical impact is manufacturers can produce less sizes of the bike (s, m, l, xl vs. 48cm, 50, 52, 54, 56, 58, 60, 62) with the diagonal top tube (called "compact") and still reasonably fit 95+% of the bike-buying public with simple adjustments (among others) to seatpost length and handlebar height (adjustable stem and/or spacers). Manfacturing less different sizes ultimately translates to higher profit margins...my $0.02CAN worth...

 

[thimble]

According to this (Bike Geometry: Frame Angles - What You Should Know About Frame Angles - Epinions.com) with the 69 degree head tube angle there'll be "quicker and more responsive the steering" than with the more common 75-90 degree angle.

Darned if I can understand why.... could that be true?

 

[alienator]

Used to be that all cycles had a horizontal top tube. Now there are some with a top tube that goes from the handlebar stem to the rear axle, almost in a straight line.

What's the practical impact of such a design? I tried one of these in a bike shops parking lot and liked it a lot, but want to understand more about the effect of the frame design and how it works. Can anyone help out? Does this make for a stiffer ride that's responsive, how does it affect nimbility (being nimble?) etc.

What Tony said is a big factor in the popularity and proliferation of "compact" frames, that is frames whose top tube slopes. What practical difference such designs have for the rider depends and may be none. For riders of atypical proportions, compact frames can offer the benefit of having more stand-over clearance while offering longer reach to the cockpit (a benefit for people with comparatively short legs and long torsos). There may be no real performance benefits for compact frames over traditional frames (with level top tubes), so absolute statements about things like stiffness, "agility", and etc. can't be made. Such things are unique to each design.

.....with the 69 degree head tube angle there'll be "quicker and more responsive the steering" than with the more common 75-90 degree angle.

Darned if I can understand why.... could that be true?

I don't think the article says that. If all other frame geometry metrics are equal, steeper head tube angles (i.e., bigger angles) create quicker steering, more nervous bikes. This is because steeper head tube angles reduce trail (the distance from the center of the tire's contact patch to the point where the axis of the head tube intersects the ground) and because the steeper HT angle causes a larger deflection of the tire at the contact patch than that caused by a more shallow HT angle. Less trail means that there is a lower re-centering force for the wheel which creates the more "nervous" behavior. Alas, all other things aren't equal as steering behavior is also modified by fork offset (the horizontal distance from the axle axis to the head tube axis). All other things being equal, shorter fork offset yields a slower steering and more stable feeling than does a larger fork offset. Wheelbase is another factor. A longer wheelbase will tend to be a bit more stable than a shorter wheelbase and will also tend to steer a bit more slowly (whether the rider can tell the difference or not is an entirely different matter.).

 

[thimble]

So what you're saying is that it's all important but in the end it's just up to the user to decide...

 

[alienator]

So what you're saying is that it's all important but in the end it's just up to the user to decide...

Pretty much.

 

[thimble]

Understood. Thanks for lightening the mental load of what should be sheer enjoyment.