I’m having some difficulty understanding brake mount placements. There seem to be some inconsistencies in Sram’s Fit Specs unless I’m missing something.
Front
Flat Mount = 1.7mm Dropout face to hole pattern centerline
Post Mount = 0.94mm Dropout face to hole pattern centerline Variance = 0.76mm
Rear
Flat Mount = 7.05mm Dropout face to hole pattern centerline
Post Mount = 9.2mm Dropout face to hole pattern centerline Variance = 2.15
I am pretty sure the rear is based on tge 135 old axles which mormally sit flush wiyh the surface of the dropout. Where as the 142 and 148 old axles sit in a recess. Thats the only place I can account for the differences.
I don’t think that’s it Sean.
There’s a “C” value for 135mm hub spacing in one of the tables in the screenshots. Quoted dimesions all refer to the recessed 142/148 dropout surface.
I wrote a lengthy reply that did not really explain why, but it verifies the differences. Can’t really wrap my head around why there would be a difference.
Unfortunately my reply is stuck in moderation. Not sure why. Must have used a banned keyword or something…
There is 0.5mm difference between the base dimensions front to rear. The tolerance stack for both brake mounts and hub mounts could be up to three times more than this (1.3 + 0.2). It really is a non issue. The slots in the mounts have enough range to allow for the 0.5mm to be absorbed.
There is NO significant variance in the Shimano dimensions.
PM Fork .94mm INboard
FM Fork 1.7 OUTboard
Difference 2.64mm
PM Frame 5.7mm INboard
FM Frame 3.05mm INboard
Difference 2.65mm
In theory the Shimano FM Frame dimension could be from 3.05mm to 3.8mm. That means the difference could range from 2.65mm to 1.9mm. The nominal dimension that is most commonly used is the 3.05mm dimension, resulting in an insignificant variance.
I am using the standard 100mm QR and 135mm QR drawings from the Shimano 2020-2021 Products Technical Information.
Ooh, good question!
My brain broke a bit trying to visualise it all without the aid of a sketch so I modeled it up using caliper models straight from SRAM’s official channels, and yes indeed there’s a variance. I don’t know why…
The following screenshot shows a front and rear flat mount (I used a standard front configuration, not the thru-bolt fork spec).
The slot in a SRAM front adapter plate is about 8.2mm wide, so you get a 3.3mm adjustment range. The thru-bolt spec (front & rear) calls for a 2.2-3.7mm adjustment range (7.2-8.7mm wide slot).
The G2 Post Mount caliper model I have has a slot that’s 10.5mm wide, which means you get a 4.5mm adjustment range.
On top of this, different hub manufacturers will place the rotor at slightly different distances from the dropout face too. And DT Swiss seems to have a variance in rotor spacing between their MTB and road hubs which further complicates things.
Looking at the Shimano spec, I can see that they have noted a 1.6mm tolerance for the dropout face to hole pattern centerline for Post Mount.
Front: 0.94 +0.3/-1.3mm - So anywhere from 0.36mm to the outside of the dropout face to 1.24mm to the inside.
Rear: 9.2 +0.3/-1.3 - So anywhere from 7.9mm to 9.5mm inside the dropout face.
For Flat Mount Shimano has a 0.8mm tolerance range (+/- 0.4mm).
They also don’t use the slot centerline to specify the slot location for the rear. Instead specify the distance from the dropout face to the center of the start of the slot, so the numbers stated on the spec sheet are a bit different.
Front: 1.7 +/-0.4 (1.3-2.1mm)
Rear: 5.55 +/-0.4mm (5.15-5.95mm)
Slot adjustment range: 2 +1.5/-0 (2-3.5mm) // Converted to slot width, we get the same as for SRAM (7.2-8.7mm), and the slot centerline can thus be anywhere from 6.55-7.3mm from the dropout face.
So assuming we’re maxing out the slot width, the center of the slot when comparing the Shimano and SRAM spec does not end up in the same spot - Shimano places the center of the slot closer to the centerline of the bike.
In the Shimano docs I have access to (2023-2024 version), they do not have a spec for thru-bolt for forks.
Sorry, the post contained a string that looked like a phone number. I had to set up that filter to avoid those “I have things for sale; here is my contact.” scams.
Thanks so much for making that drawing! Fully explains what I was missing. The FM Front Mount is OUTBOARD of the axle where as all others are Inboard. I completely missed that. I agree 0.5 is in significant but it is still odd.
Looks like we found the culprit of the .5mm variance. Sram’s Spec states 3.55mm for 135 where Shimano specs 3.05mm. Do we think this is an oversight on sram’s part or deliberate for some reason?
My hunch is that is was deliberate to allow for clearance of BB Style Centerlock Lockrings. I know of a couple production frames that won’t fit them and am willing to bet they were made to the Shimano spec.
Thank you SO MUCH for the detailed reply! It’s gonna take me several reads to get my head around everything you have here but I really appreciate you taking the time.
3.55mm(SRAM) inboard is CLOSER to the rotor than 3.05mm inboard(Shimano). Shimano has a range of slot length than allows the slot to be a minimum of 2.0mm long, and a maximum of 3.5mm long. Perhaps some manufacturers are using the 3.5mm length, and using too much material around the slot.
