Changing fork lengths, does it matter.

There are many people in this world that fuss over every minute detail on their bicycles. They care about this angle or that angle. They care about whether or not the wheel is 1 mm out of true or 1 mm out of round. Are the tires filled precisely to 110 psi front 115 psi rear.

I admit, sometimes I fuss over the same things too. But does it really matter? Will there be a significant change if it is not measured so precisely? Well this question came up when I decided to change the fork on our Cannondale Tandem.

I sought out others who claim some expertise on tandems and most told me that there would be a significant change in steering geometry and that I should try to match the fork length precisely or else I simply wouldn’t know what to expect.

With that piece of knowledge in hand I set out to find a carbon fork that would match the original Cannondale fork in length. Problem is, there simply aren’t any available. So now I was stuck with a dilemma, I just had to know how much changing fork length would make a difference. Some high school trigonometry and a few cells in an excel spreadsheet answered this question for me.

So does changing fork lengths really make a significant difference in steering geometry? The simple answer is NO!

Here is the more complex answer. In order to calculate any effect this might have on head tube angle and hence trail measurements we need to look at the following formula:

Difference in HTA = arcsin((old fork length-new fork length)/wheelbase)

So in the case of our tandem the original fork was 410 mm from dropout to crown. The new carbon fork was 395 mm. That is a significant difference, surely it would effect head tube angle significantly. The wheel base on the tandem is 1717 mm. Plugging those values into the formula above you end up with a difference of .400°, yes that is correct 4 tenths of a degree. That much difference is close to the manufacturing tolerances usually given for head tube angle measurements anyway. So for all intents and purposes on the tandem it simply can be ignored.

On a single bike, with a much shorter wheelbase, surely the difference is larger. So let’s do the math one more time. From memory the biggest difference in available aftermarket forks is Easton, all their forks are 365 mm, and Alpha Q, most of their forks are 371 mm. That is a 6 mm difference in a much shorter wheelbase of a single bike.

Most bikes my size (54 - 56 cm), wheelbase averages about 980 mm. So plug in the numbers and here is what you get. On our average size bike changing the fork between these two changes the head tube angle .350°, repeated just one third of a degree.

For those of you that care, stop! No frame in the world is built to this precision nor will be, so there will be no change in performance or steering geometry with a change this small.

The key element in changing steering geometry is fork offset. So if you are ever in a situation where you need to replace a fork, don’t worry so much if the new fork is a few millimeters longer or shorter, be more concerned if the offset is a few millimeters different.

In the end, just go out and ride your bike.