December 14, 2007

A full suspension build

As some of you know, I’m nice and sick (here’s a little tip, kids – when you KNOW you’re starting to feel sick, and your wife calls to see if you want to go to an all-you-can-drink-and-eat beer and cheese fest at Avery, just hang up the phone). In any case, work is proceeding VERY slowly, because it’s about 25 degrees in the shop and my interest in contracting pneumonia is fairly limited. But huzzah! My loss is your gain, net-surfing cubicle slaves – I’ve got plenty of motivation for a long and potentially even educational blog post. Prepare to be dazzled as I explain…

How I build a full suspension frame

I do all my XC type full suspension frames using rear triangles from Ventana mountain bikes in California (as some of you may remember, I narrowly beat out Sherwood for “29er man of the year” way back in 2005). Sherwood very nicely sells me the rear triangles (along with some helpful hardware, of which I only use a little bit – more on that later) and I build custom front ends for them which have gone over pretty well with the half-dozen or so folks who have purchased one.

The first step, obviously, is to do some design work. Ventana *wants* you to use 465mm chainstays and very close to zero bottom bracket drop. While this is great for someone 6’5″ who rides in really rocky terrain where pedal strikes are a huge concern, it’s not so great for the other 95% of riders. Hence I usually throw the Ventana specs out the window and start from scratch. In this case, I’m doing the following basic geometry (note that the design assumes NO sag, and that this bike will have 4″ travel front and rear):
Head tube: 4″ at 72 degrees
Toptube: 24″ effective (1 1/4″ diameter), severely sloping to meet seat tube at 14″
Seat tube: Offset 10mm forward, 74 degrees (effective 73), 18.5″ to top
Chainstays: 450mm (17.7″) at 4.5degree drop
BB: 35mm drop (for ~13.2″ BB height without sag)

So, on to building. First I’ve gotta miter a seat tube, but if I want to offset 10mm forward (to provide clearance for the suspension) I’ll need to do something to make sure the seat tube miter all makes contact with the BB shell. In the industry parlance, we call this “ovalizing”, but it’s basically a process of controlled smashing using the vise and a few blocks of wood. I ovalize the bottom of the seat tube until the narrower portion is about 25mm, then miter it on the mill, along with a miter (done at the same time to make sure they’re aligned) for the main pivot.

Here’s a picture of the ovalized, mitered seat tube cleaned up and ready to attach.

Next up, I tack in the seat tube, and then use the Ventana rear end (in my Anvil fixture) to align the main pivot to tack as well. Here’s a shot of the main pivot in position – tacking with the rear triangle in the way is a pain, but it’s the easiest way to make sure everything stays in perfect alignment during the construction.

For those who worry about these things, the tube that comprises the pivot mount sucks up enough heat that there’s no concern about overheating and damaging the (aluminum) shaft that holds the pivot together. I add the pivot before finish welding the seat tube to the BB because, since the miters have to be done simultaneously to keep them aligned, I would be unable to effectively backpurge the seat tube/BB welding if the pivot mount weren’t in place (since there would be a gaping hole right near the weld to allow nasty non-inert gases in)

Here’s a shot of the tacked joint.

Finally, the last shot of the day: the finish welded seat tube/bb area. There will be some additional bracing added to connect the pivot directly to the BB as well down the road, but this is the progress I made last night, so this is where the post stops for the day. Assuming I get healthy, look forward to another installment next week!