A version of this article was originally
published in Triathlete Magazine

Ian,

I read an earlier Tech Support article (September 2006) comparing dimensions and fit on an Elite and Cervélo. This was helpful, but made me think a bit more about some of the other dimensions listed in frame geometry charts. I keep seeing “Bottom Bracket Drop” and “Fork Offset” listed. I understand where these dimensions are taken on a frame, but how do they relate to the fit and ride of the bike?

Jim, via the internet

Dear Jim,

If you like numbers and how things relate, you will probably like this article. If you do not, get the aspirin out now – this might give you a headache.

While the fundamentals of how a bike fits are found in seat tube angle, top tube length and head tube height as discussed in the September article, the details of fit and ride are found in dimensions like bottom bracket drop and fork offset.

Bottom Bracket Drop: This dimension is the height difference between the center of the bottom bracket/crank and the center of the dropouts (where the wheel’s axle connects to the frame/fork). Some manufacturers replace the drop dimension with “Bottom Bracket Rise”, which conversely measures from the ground to the center of the bottom bracket. Both dimensions refer to the same thing in regards to fit and handling.

The aforementioned Elite and Cervélo models serve as excellent examples of the spectrum currently offered in the market in regards to bottom bracket placement. Cervélo’s 700c aero bikes have 6cm of bottom bracket drop (28cm of rise), while many of Elite’s tri bikes have 8cm of drop (26cm of rise). 2cm is a lot of difference in frame geometry and here is what it affects:

1) Effective head tube length – Effective head tube length matters because it determines how high or low a handlebar can safely be placed on a bike. Because the rider is lower in relation to the top of the headset’s upper steering bearings, a frame with a lower bottom bracket height will have a taller effective head tube length than a bike with a higher bottom bracket. From a fitting perspective, this means that with a given head tube height and style (integrated or non-integrated), a frame with 8cm of bottom bracket drop allows for up to 2cm more handlebar height than a frame with 6cm of drop. Likewise, a frame with 6cm of drop allows for up to 2cm lower handlebar height than a 8cm drop frame.

2) Handling and stability – Bottom bracket drop is one variable among many that impacts a frame’s handling and stability characteristics. This being said, in general, bikes with taller bottom brackets have a higher center of gravity, more ground clearance and tend to be more reactive/sensitive to rider input; bikes with lower bottom brackets have a lower center of gravity and tend to be on the more stable and predictable side.

Fork Offset: Also known as “Rake”, fork offset has to be considered in conjunction with the head tube angle of the bike for its implications to be understood.

Head Tube Angle + Fork Offset = Trail. Trail is a significant contributor to how a bike handles while also having some fitting implications. Fork offset is the distance from the center of the front wheel axle directly back to the center of the bike’s steering axis (invisible line tracing the frame’s head tube angle to the ground). Currently, on most aero forks, this means an offset (rake) between 40mm and 45mm.

“Trail” is a relationship between head tube angle and fork offset and is determined by plotting a line straight down the fork’s steering axis (head tube angle) to the ground and plotting another line straight down to the ground from the center of the front axle. Trail is the distance between where these two lines contact the ground.

So, what does trail affect? In general, bikes with more trail have greater castor effect, meaning the wheel tends to self-center more readily and thus the bike feels more stable, while bikes with less trail tend to steer quicker, but can also require more rider input to keep them going in a straight line. While some manufacturers tend to keep trail consistent between frame sizes, others vary trail size-to-size. Because of this, and the fact that there are many variables beyond trail involved in handling, it is difficult to recommend an average trail number. This being said, for an average sized rider looking for an aero bike that self-centers predictably and offers reasonably quick steering, between 6.2cm and 6.5cm of trail are usually reliable dimensions.

A strong argument can be made that aero forks would benefit from having more fork offset options than what is currently available and that in conjunction many aero frames would benefit from slacker head tube angles. As a frame’s head tube angle becomes more slack, a fork needs more offset in order to maintain the same trail dimension. A frame with a 73 degree head tube angle and 40mm offset fork and a frame with a 71 degree head tube angle and a 52mm offset fork have the exact same 6.21cm of trail. However, what differs is that the 71 degree/52mm combination will have a front-to-center (front wheel axle to center of bottom bracket) and wheelbase measurement over 3cm longer than the 73 degree/40mm rake combination. Three centimeters is a huge number in bike dimensions. In many cases, especially with a properly fit triathlon bike, the additional front-to-center length of the slacker head angle and greater fork rake counteracts the additional weight on the front of the bike and can help make the bike more stable and predictable.

To further add credence to the argument above, from a fitting perspective, forks with greater offset and frames with slacker head tube angles offer more toe clearance between the rider’s foot and the tire during sharp cornering at lower speeds. Especially with 700c wheels, this is a notable benefit as many riders do experience toe overlap on small to medium sized stock frames.

While top tube length, seat tube angle and head tube angle are the most important dimensions on a frame when it comes to fit, dimensions like bottom bracket drop and fork offset not only affect fit, but also handling as well. While hyper analyzing each detail of a frame’s dimensions may not be required, it does pay to know what matters in regards to your body and riding position and that is where a knowledgeable bike fitter or technician can be of great value.

Train hard and train smart!

Ian

Originally published December 2006/Copyright © 2006