Bike Frame Geometry – what happens when you ride a Time Trial Bike.

The design of a time trial bike is quite different to the design of a road bike. The time trial bike is specifically designed to help you get through the challenges of a triathlon.

Triathlons within themselves place unique stresses on the body.  Swimming places strain on both upper and lower body, cycling works the lower body in a seated position, whilse running works mainly the lower body but in a standing position.

Over the last number of years the concept of a bike designed specifically for the sport of triathlon has evolved. The two major changes on the bike are the steeper seat tube angle and tri-bars.

The purpose of this article is to look at what a steeper seat tube does to the riding experience.

A little bit about seat tube angles:

A typical road bike has a seat tube angle of between 70 – 76 .  You sit in a similar position to sitting on a chair with your feet a bit forward from your hips. A Time Trial bike has a steeper seat angle normally greater than 76 .

Some observations with regard to the different effects caused by the seat tube angles.

1. Your cardio and respiratory loads are slightly lower on a Time Trial Bike thus making a TT bike a more efficient bike to ride.
2. Road bikes with a less steep seat angle are easier to handle in typical road riding situations.  You have a better weight ratio between front and back wheel on a road bike – that makes the bike easier to handle.
3. Road bikes are safer, in that you generally have quicker access to your brakes.
4. A Time Trial bike allows the hips to be extended, the spine to be more flattened and the rider to create less drag than on a road bike.
5. You are able to run faster after climbing off  a Time Trial bike.
   Garside and Doran (2000) found that cyclists were able to run the first 5km of a 10km run significantly faster after a 40km time trial, when using a bike with a seat angle of 81  as opposed to a bike with a seat angle of 73 .
   They also found that stride length was greater after transitioning from a TT bike indicating that the hamstring group shorten in the traditional bike position.
6. Under intensive testing (Richard et al – 2005) analysed Electromyography (EMG)  results of various leg muscles. EMG is a technique for evaluating and recording the electrical activity produced by skeletal muscles. The most profound result of the test was that the Biceps Femoris and Semimenbranous muscles worked a lot less in when placed in a bike with a steeper seat tube. The fresher the hamstrings, the better your run afterwards.
7. Tight Biceps Femoris muscles explain why some triathlon runners appear to run in a squat like position after climbing off the bike. One of the benefits of a TT bike is that the Biceps Femoris muscle is forced into a stretched position thus enabling the runner to run in a more efficient position.

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Regards
Mike