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Help Drawing Fork

Old 12-31-20, 11:38 AM
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TiHabanero
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Help Drawing Fork

I do not use CAD, my drawings are by hand. The issue I am having is accurately drawing a fork to dimension. Not sure where my disconnect is coming from, but when I draw the axle to crown distance at 380, then put a store bought fork with the same dimensions to the drawing, the measurement does not match. It is about 5mm shorter on the fork than the drawing.
Anyone who draws frames and forks the old fashioned way willing to provide a step by step process they use when drawing up the fork?
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Old 12-31-20, 12:21 PM
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guy153
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There's some ambiguity about the definition. Most people measure the length along the line of the steerer rather than actually from the axle. This diagram explains it better:

https://www.bikecad.ca/taxonomy/term/52
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Old 12-31-20, 12:56 PM
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Are you measuring to the top of the where the crown race is or the bottom? They are usually around 5mm deep.
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Old 12-31-20, 02:46 PM
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TiHabanero
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When looking at the fork from the front, I am measuring straight down from the where the bearing race seats onto the fork crown to the center of the axle. I just can't figure out where I am going wrong. Suppose I will play with it a whole bunch more to figure out where the mistake is taking place.
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Old 12-31-20, 03:10 PM
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Originally Posted by TiHabanero View Post
When looking at the fork from the front, I am measuring straight down from the where the bearing race seats onto the fork crown to the center of the axle. I just can't figure out where I am going wrong. Suppose I will play with it a whole bunch more to figure out where the mistake is taking place.
Draw a line 380mm long. Turn left 90 degrees and draw one 45mm along (or whatever the fork offset is). So a big L shape. Now put the fork on the drawing, in profile, with the steerer centred on the first line and the axle should end up at the end of the second line.
​​​​
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Old 12-31-20, 05:55 PM
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my only complaint about bikecad is that it really doesn't help you design forks as much as it could, so you aren't missing anything on that front

I wouldn't trust the published specs of a production fork, they traditionally haven't been particularly careful about length.
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Old 12-31-20, 06:41 PM
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Another source of wrong dimensions WRT forks is the difference between the axle to crown measurement as done to the front edge of the crown race seat VS the steerer's axis/crown seat point. About 2+mm of A-C difference.

The next mistake could be in how the fork is laid out on the full scale drawing. One should keep the steerer parallel to the paper's surface and use a square to extend up/off the paper the crown seat. Letting the fork blade sit against the paper can result in the steerer not being on plane WRT the drawing. perhaps another 2ish mms of change.

Not saying that the OP is making these errors but they are easy to fall into. Andy
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Old 01-01-21, 06:09 AM
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I just don't see the purpose of the drawing. I clamp the steerer in the fixture, set the dummy axle at the desired ATC length and offset, put the dropouts on the dummy axle and then connect the two with tubes.
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Old 01-01-21, 07:39 AM
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"Another source of wrong dimensions WRT forks is the difference between the axle to crown measurement as done to the front edge of the crown race seat VS the steerer's axis/crown seat point. About 2+mm of A-C difference."

This is where I went wrong. Thank you, Andy.


Dsaul, I like to draw everything out before creating the pieces as it is habit I suppose. Just helps me see things more clearly. Probably not necessary.
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Old 01-01-21, 09:57 AM
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I recently had a lathe reduction shaft made to replace the well worn one. Normally I am more of a design while cutting guy for these types of jobs but I needed to have another do the turning for me. So I made a drawing, actually 3 to get all (including the general scale) correct before I dropped off the old shaft and it's drive pully to the guy doing the work. He only gave the drawing passing attention as he had the old shaft as his guide. I did find it helpful to have made the drawing as it let me think the design out, make a mistake or two, without wasting the new piece of expensive steel. I fully agree with the value of a drawing. Andy
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