Jim’s Tech Talk
By Jim Langley
The subject this week is a fussy detail related to increasing the typical 140mm rotors on disc brake road bikes to 160mm. The brakes are Shimano Ultegra R8070 flat-mounts.
Why would you care? Because if you have 140mm rotors on a Shimano Ultegra disc bike you might someday want to go to larger rotors. They provide more braking power, which is why mountain bikes usually have a larger one in front. And, also, because you might buy a bike in a box “kit.”
That’s how I ran into this issue. My buddy Rocko bought a sweet Specialized Allez Sprint Disc Frameset, plus a Shimano Ultegra R8070 Di2 group and carbon tubeless-ready wheels. He went with Specialized’s S-Works Aerofly II Carbon Handlebars and Power Expert saddle. Nice stuff.
He brought everything to me in the frame box with all the components packed in around and on top of the frame. The wheels were separate. Even though it’s his bike, it was like Christmas for me opening and laying everything out on the workbench.
Rocko had provided the 160mm rotors, too. He said in the box I would find the adapters to raise the disc calipers to work with the larger rotors. I like to make sure I have all the right parts before commencing a full build. That way there are no surprises or delays.
So I installed the fork on the frame quickly – without cutting the steerer. I then put the 160mm rotors on and put the wheels in the frame. At that point, I was able to hold the adapters between the brake calipers and the frame, and I test fit the front and then the rear brake to the 160mm rotors.
Obviously, there needs to be clearance above the rotors or the brake calipers can’t be attached to the frame. They will bottom out on the rotors. Which is exactly what happened when I test fit both brakes even while holding the adapters in place.
Since Rocko thought he had the correct adapters, I tried to figure the problem out looking at them. They were identical. Which didn’t make sense since the fork and frame mounts are so different.
Hunting for the Right Adapters
Hoping to find a clear and simple adapter chart for flat-mount Shimano brakes, I Googled it. But, I had soon followed so many links without finding a sure answer that a couple of hours flew by. Frustrating. Not too long ago I built a Pinarello in a box which had SRAM HRD discs. It too needed adapters that turned out to be very hard to figure out and find.
The thing that makes it so complicated is that there have been at least three different disc mounting standards. Plus, different brake brands and models have their own specific adapters. There are also adapters for matching brakes to mounts they weren’t made for.
Try as I might, I couldn’t determine from the internet which adapter was correct for Rocko’s brakes. Thinking back to the Pinarello, I ended up purchasing three different types of adapters and through trial and error, finally found the right ones. I didn’t want to spend that much time and money again.
About then, it occurred to me that I was going about solving the problem the wrong way. What I needed wasn’t so much a disc adapter expert, but a Specialized expert. Since who would know better than someone who sells and builds Specializeds 24/7?! So, I called the nearest dealer – and also the shop that sponsors our masters racing team – Spokesman Bicycles.
They explained that the adapters Ross had were both fronts. And, that the reason they hadn’t worked for me is that they’re directional. Put on one way they’re for 140mm rotors. Flip them and they work for 160mm rotors. Looking at them it didn’t seem possible that they’d raise the brake enough by just flipping them over, but they did. Woo-hoo!
They also told me which Shimano adapter I needed for the rear brake. Better still, they had it in stock.
Remember your Bike Shop
To me, this entire episode was a great reminder that as helpful and convenient as the internet is, I should never forget the local bike shop. It’s a source for both hard-to-find knowledge and parts. That latter is super valuable today when so many components and parts are completely sold out online.
In case you decide to go to 160mm rotors on your Shimano Ultegra bike, or build bikes for your friends, the photos show the front and rear adapters that worked. The Shimano part number is written on the rear adapter, but not on the front. So I put the label in that photo. It has the numbers on it.
An easy way to show your shop these adapters if you go to buy them is to open this story on your cell phone and show them the photos. They probably won’t need the numbers that way. They’ll know by looking what you need. Similarly, if you have a different rotor, brake compatibility challenge, you could bring the bike in and show them what you’re working with.
Ride total: 9,765
My 8070s came with mounts that fit either 140 or 160. Out of the box they were setup for 140 and I just had to flip then over.
Not sure why you had an issue.
Bike fitness coaching says
Please see my article building a titanium bike later in the newsletter. Exactly what was needed.
Jim Langley says
On this bike the front brake came with the correct adapter mounted to the brake for 140mm rotors, Lamar. So I was good there. But the rear brake did not come with an adapter. These brakes were new in the box as sold. The fact that the front came with the adapter and the rear did not suggests that Shimano believes roadies are only sizing up their front rotors, not their rears.
Thanks for the comment. I’m glad you had the correct adapters to start with..
Road Bike Rider says
I recently bought a new disc brake bike in the last couple of months, and the guy who was building it up recommended the smaller rotor in back because it still gives you plenty of braking power and also makes the brakes perform similarly to a rim brake bike so there are no surprises when you jam on the brakes.
Even the worst rim brake can lock up a rear wheel, so a large rotor in back isn’t necessary, is it?
I’m curious what everyone else does. Does it make sense in areas with long descents?