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Fork length "L" parameter...

Hmmm... not sure how people haven't noticed this, but it would seem the library forks (I'm personally referencing the "Fox-like" 29er forks), use the axle-to-crown (AC) length as the L length, which is incorrect since:

AC^2 = L^2 + R^2 (basic trig...)

So with a rake (R) in the range of 44mm to 51mm, that makes the "virtual" fork in BikeCad ~3-4mm longer than it should be... which is enough to throw angles out by about 1/8 degree. Or if angles are matched, then linear dimensions then become out of whack. :)

Easy workaround is obviously:

L = (AC^2 - R^2)^0.5

... though BikeCad itself should obviously have a "switch" to define L using AC and R, or AC using L and R.

Fork length

Contrary to what you're claiming, BikeCAD does list fork lengths that are in agreement with the diagram. I think your confusion stems from the fact that suspension forks in BikeCAD are compressed (shortened) to account for the specified amount of sag.

I sanity checked before I

I sanity checked before I posted... and these forums don't allow image uploads... so I'll explain it to recreate the bug. ;)

1) Started a clean install of BikeCAD 7.11
2) Opened the default MTB bike template
3) Open Dimensions dialogue and tick all 4 fork length ones and the one for fork rake
4) Move numbers so they can all be seen clearly.

The MTB template is a 26er with a suspension fork where L is defined as 500.8mm, travel is 90mm and sag is 13mm. It has a rake of 44mm.

The length L can be see in the model view as the length of the fork along the steering axis, to the virtual point where it intersects a perpendicular line from the axle.

L-sag is a simple linear dimension (with suspension forks being telescopic!) and is simply 500.8mm (L) - 13mm (sag)... and is confirmed in the model as 487.8mm.

The AC fork length, on the model is shown as 502.73mm (L^2 + R^2)^0.5... which is, in fact (500.8^2 + 44^2)^0.5 = (250800.64 + 1936)^0.5 = 252736.64^0.5 = 502.729191... 502.73mm!

AC-sag length is on the model as 489.78mm ((L-sag)^2 + R^2)^0.5... which is, in fact (487.8^2 + 44^2)^0.5 = (237948.84 + 1936)^0.5 = 239884.84^0.5 = 489.780399... 489.78mm! :)

So "L" in the fork perimeters dialogue box is in no way the same as the AC dimension. BikeCAD itself says so... ;)

Fork length

Again, this is not a bug! The diagram in the fork dialog box does not suggest that the "L" dimension is the same as the axle to crown length! This is explained here.
Below is an image displaying the dimensions you reference in your comment above. In one state, the dimensions are shown as they are with 13mm of sag and in the other state, the dimensions are shown as they are with 0mm of sag. These dimensions are shown exactly as they were intended. If this is still causing you confusion, perhaps we should have an e-mail discussion about this.

Sample fork dimensions

I know the "L" parameter isn

I know the "L" parameter isn't the AC length and indeed can also see the diagram never said so. :) I have however seen in more than a few places having Fox forks (in this case the 80mm travel F29) having the AC length listed as 500.8mm.

So I actually trawlled around the Fox website and found this...


I can now see that Fox actually lists the fork length as the "L" parameter (ie: along steering axis) and it seems dealers are incorrectly listing the linear length as the AC length (they should do some trig! ;)), which in turn made me think it was a library component bug (never said there was a bug with the software itself :)).

In any case, I also found the specs for the 2012 34 F29 forks:


So I'm a happy bunny and actually see schematic confirmation that said forks (what I actually have) are indeed 51mm offset... though I'm surprised Fox's tolerence for this is +/-2.5mm! Also nice to learn that the "L" parameter for said Fox 34's is 552.8mm rather than a scaled 560.8mm (so I wasn't imagining things when I measured my fork)! :)

Disaster averted! ;)

Fox Forks

Thanks for the clarification! Yes, we all have to be careful when discussing fork lengths as not everyone uses the same terminology.

That's one of the great things about BikeCAD. You can display any dimension you need in the context of a CAD drawing and eliminate the ambiguity that would exist in a verbal discussion.

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