Update October 1 2019

Seb of Luna is upgrading his front rotor to a 225mm from the OEM 203mm. He asked me about the steel spacers I used so I went to the hardware store to get him two. Yep he’s such a nice guy I wanted to do this for him. So I took photos of the bin where they reside for those who may want to forgo spending money on a fancy extender. These are $1.39 USD each.

The 1/4 X 3/8 X 1/2 inch spacers are what I use and work perfectly.

Original Post July 1 2019

I currently use the Sur Ron OEM brake calipers and have been very happy with them. I know MANY OTHER owners have purchased other third party brake calipers from Shimano, SRAM, Magura and others. I’ve found that the modulation with the OEM calipers suits my taste in braking AS LONG AS THE SYSTEM IS BLED PROPERLY AND AFTERMARKET PADS ARE USED AND BEDDED PROPERLY. My protocol is NOT to automatically replace things unless I’ve done all I can to get what comes on a vehicle to work as well as possible.

I opted to change my front wheel to a 21″ 1.6″ wide rim. I prefer that size since I grew up racing motocross and desert. The tire selection is endless and I’m able to run very low tire pressures without the fear of pinch flats. The downside is I found that I needed to upgrade my rotor in the front from a 203mm to a 225mm.

I recently found out that Hope out of the UK manufactures a floating disc in the 225mm size. So since I’m a real bitch about brakes and suspension I decided to swap out my 224mm solid disc which I had purchased. I prefer a floating rotor because of its superior heat dissipation qualities. Be aware that I needed to purchase two steel spacers with a 6mm hole and 12.62mm long. Doing so places the pads in the optimum spot on the disc.

Hope 225 floating rotor.

My floating Galfer 203mm rotor on top of my previous 224mm solid rotor.

Sur Ron Rear Rotor

Sur Ron opted to manufacture their own rear hub in order to place a larger bearing into the hub to handle the bike’s torque. Most of the components on the bike are bicycle parts, but a bike hub would not have worked well on this bike. Some have mentioned that Sur Ron ‘should have’ used a bicycle chain, but that makes no sense for this bike.

Because of the hub bearing size the pattern of the rotor is proprietary to Sur Ron. No known bicycle rotor will fit the rear of this bike. I’m working with an engineer who is fabricating a floating rotor for my bike.

Rear rotor on the right, front on the left. It’s evident how much larger the rear’s center hole is and how the bolt pattern is wider. Both are 203mm in diameter.
Front rotor laid over the rear hub/rotor for comparison.

I wanted to link to an explanation that explains the effect of a larger rotor. The short answer: it’s just like using more leverage with a longer wrench, more torque:

“Simply put a bigger rotor provided better braking, and a four pot caliper provides better braking – better meaning more, and more control (Everything else being equal).

For the same force between the disc and pads, a bigger rotor generates more torque on the wheel – i.e. more stopping force. It is running though the pads faster, generating more friction for the same pressure, more stopping power, and as it has bigger surface area, is dissipating more of the generated heat, meaning the disc and pads run colder. So, for the same caliper and brake pad, a bigger rotor generates more braking for the same pressure or the same amount of braking for less pressure.

The disadvantage of a bigger rotor is weight and as its larger, more prone to warping and distortion.

This applies to all disc brakes from bicycles to aircraft……

For a bicycle, there comes a time where you have “enough”, and quickly get to “too much” – with high quality equipment its a surprisingly small disk for most people. Big disks a more prone to warping and damage, which is a good reason not install them unless you need to.

Heat buildup is not normally a problem but low end pads can suffer fad. Small rotors don’t typically over heat on bikes (Tandems and loaded tourers might manage it). The main advantage is more breaking for less pressure – so you get more control and feel, and your braking hand tires less (If you have ever done a 1000ft vertical descent in technical tracks on crap brakes you know the feeling)

A 4 pot caliper provides advantages of more and more even pressure, meaning better braking. Also as there are usually larger pads – more heat dissipation can be achieved.

Note: A 29’er would usually need a bigger disc than a 26″ wheel for the same stopping power.

Don’t forget the pad and disk material – these can make a bigger difference than size, but its not as visible. Do not go to big discs and 4 pots unless you are already at the higher end of the quality spectrum.”

It’s very important to keep in mind that the brakes we use are bicycle brakes. An average weight for a downhill fully suspended mountain bike is around 45 pounds. The stock Sur Ron weighs 110 pounds, more than double a mountain bike. Plus the speed that the bike can achieve is much higher than most pedal bikes attain. Just food for thought….


  1. I recently did a deep dive into brake upgrades and wanted to share a vintage option I found from Hope called the Moto V2. It’s got a floating rotor design with three layers for supposedly better heat dissipation. The master cylinders also look to be a bit larger and the calipers have the bleeding nipple pre-attached and capped, similar to traditional moto brakes for potentially easier servicing too.
    They’re a bit tough to find in decent shape these days, but I managed to piece together a few NOS sets by scouring eBay UK.

    As you mentioned, I’ll need to figure out a custom fabricated solution for the rear inner part of the rotor since there doesn’t seem to be enough material left to try to cut out the Sur Ron bolt/hub pattern…d’oh.

    If your Bay Area shop is willing to take on additional fabrication work for another Sur Ron owner (referral discount for you??) feel free to shoot me their info via email! I wouldn’t mind trying out that rear shock extender bit either 🙂

    • Hi Soumik, my fabricator isn’t really interested in making more stuff. It was really just a one off thing for me. A guy in the UK, Stag Motorcycles is making a floating rotor for the rear! You may want to contact him.

  2. 255 is too costy 🙁
    220 is great price. and the spacer should not be a problem

  3. mmmh…SRAM centerline discs are not exactly the bees knee….they are noisy!

  4. Hi Mark,
    I just purchased a new Sur-Ron from Luna, (I have not received it yet) I have on order a custom built 21″ front wheel from a wheel builder on Ebay and I now need to order a new larger front brake disk, do you know if there is a part number for the 225 mm disk from Hope or will any disk they make in this size fit the front wheel of the Sur-Ron? Also, are there no other 225 mm disk manufactures in USA?

    Look forward to your reply,

    • Bill I would get this rotor it’s 224mm. It’s made by Hayes which is a USA company located in Milwaukee. You will need to add a spacer to your caliper. You can find what I used listed under Brake Rotor and Caliper.

  5. So where did you buy the 225mm rotor?

  6. Hi Mark,

    I just ordered a 225mm Hope floating rotor …. question, what pads did you find best for the rotor? Are you using hope pads, the finned Shimanos? Sintered? Resin? Thanks for all your posts, they are great. Tim

    • Hi Tim, I believe you will find the larger 225 greatly improves your stopping power especially if you are running the OEM 19 or 17 inch rims. I cannot recall the brand I’m running in brake pads, but they are NOT stock. I prefer organic pads which don’t last as long as semi metallic, but I love the feel and modulation. Any of the ones you sighted should work really well for you.

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