"Fork Length" or "Fork Length Minus Sag"

[Tom Gagliardi]

I'm building mostly hardtail mountain bikes with front suspension. I'm working on my 10th frame and up until this point I have been designing the geometry using Fork Length in my calculations. I was wondering if this is correct. Should I be using Fork Length Minus Sag? I'm designing my frames using BikeCAD. Thanks in advance for any comments.

[edoz]

Although I don't actually use the "fork length minus sag" setting in BikeCad, I do account for the sag when I input fork length. You want to design the frame around the fork being in the sagged position, as that's where the neutral point will be when you're riding the bike.

[afwalker]

Maybe different forks have different sag, say a 100mm fork sags 20-25% so an 80mm would be different. The frame I just built at Metal Guru with Carl was a 100mm hardtail and we worked the bikecad to sag 20mm. Lessons in welded steel frame building « Metal Guru
cheers
andy walker

[Jake Rosenfeld]

I use 20% as a rule, for an 80mm that's 16mm, for a 100mm that's 20mm.
if its an expert rider or for myself I work closer to 30%, not that i'm an expert rider but i'm front heavy and I stand to climb about 95% of the time.

[steve garro]

You can draw then either way as long as you stick to your way & take BB drop into consideration.
- Garro.

[Tom Gagliardi]

Ok. Although I'm getting away from it, I'm still at the point where I sometimes look at the geometries posted by bike manufacturers to get input in my designing. Would it be a safe assumption to assume that when a bike company posts their specs, lets say a mtb and they are showing the geometry based upon a 100mm suspension fork, that they are taking into consideration the fork length less sag? I'm not heading in the direction of copying bike geometries, I'd much rather ride my bikes and adjust to my liking, but just trying to verify this...in case I have a need to try and duplicate a frame spec.

[dsaul]

There are many here with more knowledge about this subject than me, but I don't think it would make much sense to design a frame without factoring in sag. In reality, production frame geometry numbers are based around an imaginary fork and are subject to change depending on which fork and headset the end user decides to put on the bike. 100mm forks vary in axle to crown length somewhere between 470 and 480mm even among different models from the same manufacturer. That 10mm difference is roughly a .5 degree head angle change all by itself. If you factor in the differing stack heights from various headsets, you might be looking at a full degree of head angle change.

If you need to duplicate a frame spec, build your frame to the published numbers taking into account the actual fork and headset that will be used on the frame you are building with sag factored in. This way the frame will have the correct angles with the rider on the bike, where it matters.

[fortyfour]

Here's how I do it: Get the spec from the fork manufacturer for Axle to Crown Measurement (A2C) and fork offset for that given fork you're using (these will also show you clearances for the crowns adjustment knobs too - a fine detail for down tube clearance). Most times these numbers will come with a tolerance of +/- 5mm from the manufacturer. I use 20% sag as a rule unless the client states otherwise (some prefer their setups a bit stiffer, others like them a bit more plush). Taking the sag percentage into account, I then calculate what the forks A2C length will be and use this as my number for the drawing. Everything (Head angle, Seat Angle, BB Drop, etc) is based around that A2C with Sag.

The reason for this is if you don't account for sag, you're building that bike with "parking lot numbers". So on trail, "Just riding along", for example, your head angle and bottom bracket height are going to be completely different from what you intended them to be as a starting point. That creates a huge problem especially if you're attempting to fine tune how your bike handles and get real time feedback to improve how your ride experience is thus translated to future builds. Naturally, on a mountain bike, the head angle (for example) spec'd on the drawing is rarely constant given the terrain is ever changing, body positioning is ever changing etc. However, if you have a known starting point that includes sag, and you're feeling something on trail, you can base decisions a bit more on an educated guess rather than shooting for something completely unknown.

I will also add that how the bike handles on trail is the sum of many numbers, angles and measurements. Not just head angle. Not just bb drop or bb height. Not just chain stay length. Not just trail. It's all of those and much more.

[steve garro]

Use more % sag as the amount of travel increases.
Many off road raceing machines run 50%
it's not just that it goes up & down but also sticking to the ground.
Suspention is not about absorbing bumps as much as hugging the ground.
Shit gets real when you need travel limiting straps............!
- Garro.

[blasdelf]

There are many here with more knowledge about this subject than me, but I don't think it would make much sense to design a frame without factoring in sag. In reality, production frame geometry numbers are based around an imaginary fork and are subject to change depending on which fork and headset the end user decides to put on the bike. 100mm forks vary in axle to crown length somewhere between 470 and 480mm even among different models from the same manufacturer. That 10mm difference is roughly a .5 degree head angle change all by itself. If you factor in the differing stack heights from various headsets, you might be looking at a full degree of head angle change.

This calculator is great for answering such questions: http://bikegeo.muha.cc/