A little Rockshox Axle to Crown Trivia

[davidd]

This may or not be news but I was surprised....

When I was researching my Venture build I was trying to calculate where I would have to run my stem to maintain my position on the bike.  I thought I had better compare Axle to Crown measurements for the forks I was currently using (26 "Reba 100's) and my intended forks (27.5" SIDs or Reba 100's).  I had always assumed that as they are supposed to use the same basic construction that they would be the same but this is what I found..

                    26"      27.5"    29"
Reba 100           493      507      526
SID   100     473      487      506
 
20mm is quite a lot, equating IIRC to about 1 degree change in head tube angle.  I would imagine this would have a significant impact if head angle was already steep or slack. 

When a frame is measured what axle to crown measurement, I wonder, are the angles based on?  Is there a standard?

[cmh]

When a frame is measured what axle to crown measurement, I wonder, are the angles based on?  Is there a standard?

No standard - if you're *LUCKY* then the manufacturer lists the A-to-C in the frame geometry, otherwise you have to figure it out from the stock spec, like what fork they put on a normal build. Sometimes you've got to take a guess.

[JohnnyNT]

There is no standard, "good" frame manufacturers include default A2C in the drawing. Then you can easilly work out the change of angles caused by a different fork by some geometry I am really surprised by the SID-REBA difference though.

[JohnnyNT]

Ok, I was a little uneasy about these differences so I dug up SRAM tech file, it seems that 29 Reba has 506 A2C, same as SID.
https://sram-cdn-pull-zone-gsdesign.netdna-ssl.com/cdn/farfuture/u999KSmwseMzhrpiNBs-7l8It4acSOXONevV3QEXI6g/mtime:1372788169/sites/default/files/techdocs/user_spec_reba_29_15_mm_tapered.pdf

[davidd]

Hi JohnnyNt,
Yes, that would make more sense.  I didn't find what I was looking for on the SRAM site and went looking for specs on store sites. The layout below is copied from bikeman.com and was repeated on other sites in the same format eg http://smartbikeparts.com/search_details.php?itm=FK7710.  Who knows where it originates.  If I had found the SRAM stuff to begin with I'd have been happier...

PRODUCT SPECS
Manufacturer: RockShox
Mfr Part #: 00.4018.789.010
UPC: 710845750953
Model: RockShox Reba
Size: Tapered
Type: 15mm Thru-axle
Style: 29er
Color: Black
Weight: 1664
Brake Usage F/R: Front
Fork Rake: 51
Axle to Crown Length: 526
Crown Race: 39.7
Steerer Tube Length: 265
Brake Type: Disc
Wheel Size: 29er
Travel: 100
Front Hub Spacing: 100
Front Axle Type: 15mm Thru
Spring: Solo Air
Wheel Mount Center Offset: 0
Lockout: Crown Mounted
Defined Color: Black
S.H.I.S. Clamp Diameter: 28.6
Item Name: Reba RL
Steerer Type: Tapered