Frame Geometry

The most frequent questions (at least 15 a day) we receive through e-mail and over the phone, are:

1- I have a frame with such stretch, such rake. How long should my tubes be?
2- I lowered the rear of my bike 2 inches. How much should I rake to keep the bike horizontal?
3- I saw a bike in a magazine with a 10-inch over. I want the same look. How much should I rake my bike?
4- Etc…

The answer is: it depends!

Not all frames are born equal. A 5-inch backbone stretch doesn’t mean the same length on the top tube from one brand to the other. (Don’t ask me why.) The neck of different frames doesn’t stand at the same distance from the ground. Front leg stretch, lowered shocks or adjustable air shocks on a Softail are examples of other parameters that affect the geometry of a frame, its distance from the ground, and the overall look of the bike. When people require a 10-inch over front end, it means 10 inches over stock. But stock Harley bikes, depending on the model, have different front end lengths!

So, here’s how I set up the geometry of Expresso. I like my bikes to be a compromise between look and function. It means high enough to be rideable, and low enough to look good. I like them when they sit at 4 3/4 inches from the ground (meaning from the ground to the lower portion of the lower tube of the frame). It’s the distance I want when the bike carries all its components, with a biker of average weight seated on it. When the bike I customize is a dresser with floorboards, I increase this distance to 5 inches minimum. Same when it’s a two-seater.

On this project, I use Progressive Suspension shocks. I attach the swingarm to the frame and bolt the shocks to the frame and swingarm. I install the 18-by-9 rear wheel equipped with an Avon 250-by-40-by-18. (At this stage I am not preoccupied by the fact that the wheel is or is not perfectly centered in the frame) We mock up the bike with cases (our engine is not ready yet), a transmission, a starter, an inner primary and all the main components available in the shop to be close to the weight that the bike will have to carry. I take measurements and I realize that the Kool Kat frame with stock height shocks sits 5 inches from the ground. I know because the lower tube of the frame is parallel to the lift table when we put two pieces of wood, 4-by-4 plus 4-by-1, on the front and back of the frame.

Now we have to figure out the tube length to keep the bike parallel to the ground. First, Sean installs a triple tree. Then we lock a 21-inch wheel in the vise of the lift (with a piece of wood covered with carpet on each side of the tire to avoid scratching the rim). We run one neon tube through one of the holes of the triple tree, and move the frame on the table until the tube crosses the center of the axle of the front wheel (the opposite can be done, not moving the frame, by moving the front wheel back and forth, but it’s difficult to keep the front wheel from moving while taking measurements). Now I measure the total front end required. It’s the distance from the top of the triple tree to the center of the front axle. It’s exactly 34 inches. Now it’s easy to figure out the tube length I need. Because tubes sit about 7 ¾ inches from the bottom of the lower legs, I need tubes 26 1/4 inches long (Front end 34 inches - 7 3/4 inches).

At the shop, we always keep tubes in different lengths. All have internals (springs, seals, etc., pre-installed so that we don’t lose time when mocking up a bike). For information, 26 ¼ inch-long tubes are 2 inches over FXDWG stock tubes (24 ¼ inches).

I install the tubes in the lower legs, slide the front end in the triple tree and lock it. We tie down the bike in front (not under heavy pressure because we need to lift the bike to remove the pieces of wood under the frame). Our last exercise of the day is to jump on the bike to unfreeze the swingarm, and then to “ride” the bike on the lift to check again the distance from the ground. Sean is 195 pounds, quite a few pounds heavier than me. So, he “test rides” the bike while I take measurements. I am all smiles when I realize that the bike sits perfectly at 4.75 inches from the table, front and rear.

It’s 7 p.m. Sean proposes to celebrate with a cup of coffee at Starbucks, not far away from the shop. I know why. It’s this time of the day when pretty secretaries must do last minute photocopies at Kinko’s. And Kinko’s are always located next door to Starbucks. Or the opposite. Whatever. Sean is single again. Let’s make Sean happy. I’m gonna drink my eighth cup of coffee of the day, and probably go back to the shop to read my e-mails. Any questions?

On to Part 4........

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