compact vs traditional frames

You will get a lot of opinions on this topic. There is no perfect geometry. Everyone is different, and the best frame for one person is not going to be the best frame for another person. I have even seen arguments against the compact frames based only on the fact that they look silly or that there is too much seat post showing. I do not care what it looks like as long as it is comfortable and efficient.

Agressive geometry is one that has the rider's back in a more horizontal, and theoretically more aerodynamic, position. A relaxed geometry is one that is more upright, which is more comfortable, but less aerodynamic.

Handling usually has more to do with top tube angle, rake, and stem length than it does with whether it is a traditional or compact geometry. The Specialized Roubaix and Trek Pilot are both compact geometry, but are among the best in terms of comfort and handling. The Giants are also compact geometry and are comfortable, but tend to be among the worst in terms of handling.

Speed has more to do with optimizing aerodynamics without causing so much discomfort that one becomes inefficient. The more aerodynamic, the faster until you get to the point that you cannot transfer power from your legs to the pedals. The point where incresed aerodynamics is offset by decreased power transfer is different for everyone.
Accelleration has a lot to do with wheel weight and power transfer. While the latter is affected by frame stiffness, it is not really a function of frame geometry. There are stiff compact frames, and there are flexy compact frames. There are flexy traditional frames, and there are stiff traditional frames. The stiff frame will accellerate faster, but it will not be as comfortable. Again, another trade-off. What good is accelleration if speed cannot be sustained. On the other hand, what good is comfort if you can never get to speed. It is all about finding the right balance, and the balance will be different depending on the ride duration and conditions.

Frame geometry is only one of many factors in determing which is the best bike for an individual rider. Even for the same rider, the best bike will be different for different rides. I would not select a bike or reject a bike solely based on wether it is a compact or traditional frame. My ride of choice, though, happens to be a Specialized Roubaix Elite.

the5h4rk said:

what do people mean when they talk about 'aggressive geometry' and 'relaxed geometry'? ... and what geometry is best for handling, speed, acceleration, comfort, stiffness, strength/weight?...and what do you prefer

Here's an old school take on your questions:

Aggressive or relaxed geometry used to refer to the angles of the the seat tube and the head tube. Steeper angles (higher numbers) translates to a quicker steering more responsive and twitchier bike. These were often coupled with higher bottom bracket heights making them well suited for criteriums where handling and the ability to pedal deeper into corners was a plus. Bikes with steeper angles, especially head tube angles(for a given fork) also require more attention to ride straight and aren't as well suited for longer rides especially for non racers.

Another big variable here is fork rake or how far the hub is held in front of an imaginary straight line from the steering tube down the fork blades. It used to be that all forks had an elegant forward curve placing the hub in front of this reference line. This forward distance is called rake and effects steering quickness vs. stability. With the advent of mountain bike shocks the rake is often implemented with just an extended dropout instead of a gentle curve in the fork itself.

Typical road bikes built for all around racing use have head and seat tube angles of around 73 degrees sometimes varying with frame size. Touring bikes often have gentler angles and crit specific or track bikes often have steeper angles. Back in the late '70s to early '80s it seemed crit bikes were the vogue and it took some searching to find a fast road bike with 73 degree angles. I rode a Basso for a while that was something like 74 degrees in the seat tube and 74.5 in the head tube. The thing was awful twitchy and the high bottom bracket made me feel like I was a mile up in the air. It was a fast bike, but not for me. I went through a sucession of Italian frames before settling on an Davidson bike with angles near 73 degrees a moderate bottom bracket height, a slightly long top tube (as opposed to what we used to call square geometry bikes that matched top tube to seat tube lengths which means a real long stem for me). It's plenty fast, corners great and I've ridden double centuries as well as crits on it. My track frame is steeper and twitchier, but then that feels just right on the track.

Anyway I've watched from a distance as new concepts like the compact geometry came along and some of the terminology has either slipped away or evolved but I'm still plenty happy with a traditional geometry as long as the angles, tube lengths and bottom bracket height work for me but everybody is different and there's plenty of options to choose from these days.

There are some real good books out there on frame design that you'd really appreciate as an engineering student. It's all about angles, lever arms and feedback systems melded with tradition and art. Check out sites like this one: littlefishbicycles.comindex.htmlOpen ↗ and here's a page with some frame building reference books listed: phred.orgdesign.htmlOpen ↗

Hope that helps,
-Dave

cheers thanks dave
they are some great links
all very interesting

the5h4rk said:

cheers thanks dave
they are some great links
all very interesting

No problem....

A little more food for thought:

- A relaxed geometry often means longer chainstays and a resulting longer wheelbase. Long wheelbase bikes track straight but are sluggish in tight turns. Think touring bikes here.

- Rake in a traditional curved fork bike serves two purposes, one to position the wheel in relationship to the steering tube and the second to act as a leaf spring to suck up some road shock. The second is optional and many racing bikes now have straight forks but they're connected to the steerer at an angle that still creates rake. However you get there, rake leads to a concept called trail which is a virtual lever arm that directly impacts handling. I don't know where you are in your engineering studies, but read up on trail and then model it as a linear feedback system - classic control system stuff - it'll give you some insight into why small changes in design can have so much impact on a bike's handling. I always found engineering school a lot more interesting when I could apply it to stuff I actually cared about 🙂

-Dave