[Bicycle] Retrofit Post Mount Disc Brake Calipers to IS Mount Fork and Frame

This article is kind’a out of place, but as a tinker/gear head/wrench I also fix 20~40 year old bicycles for neighbors who can’t afford a brand new $3000 war rig. I just wanted to make a quick note about IS mounting points for brake calipers – but it became a full page article.

Last week, we found a Porsche Bike FS covered in dust in a warehouse. Sold in 1998 and originally priced at $4500MSRP (valued at $270 currently on Bike BB), this bike has a Votec frame, Votec GS front fork and Riesse rear suspension, Formula hydraulic disk brakes, SACHS grip shifters, and SACHS top/high-normal derailleurs. There was virtually no rust but everything with grease or oil in it was seized. Rather than take apart the mechanics and replace all the O-rings, I decided to bring everything to 21st century standards for easier future maintenance.

The BIGGEST HEADACHE was MOUNTING THE BRAKE CALIPERS. During the 90s when hydraulic disc brakes were becoming popular, bike manufacturers came together and agreed on a sizing standard they called “International Standard” or “IS”. Mind you, it is not an ISO standard, so traditionally ISO countries like Japan (read Shimano) wouldn’t stick to this standard for too long.

The measurement standard for IS mount calipers were simple but impractical. The caliper mount holes were situated perpendicular to the rotation of the wheel. It was a standard decided on a table rather than the road. You see, wheels and discs rotate, flex, and bend. The IS mounts did not allow for on-the-go adjustments for lateral shifts in bike structure.

In comes the Post Mount standard. The caliper mount holes were set parallel to the rotation of the wheel AND the caliper base had oblong holes for lateral adjustment. This way, if the wheel or rotor deforms on the road, the rider can simply recalibrate the caliper position with one allen wrench. No adding or subtracting of washers needed.

So how do we put a Post Mount caliper onto a IS frame? Use a 90 degree adapter of some kind? YES. Shimano, SRAM, etc. makes (made) all kinds of caliper adapters for all kinds of mounts… BUT, as less and less older bikes remain on the road, they are making less or even stopped making some of these parts. So I had to go hunting for these adapters and their spec sheets.

The important thing to remember when using an IS to Post adapter is that: At its lowest setting (i.e. no adapter, no washers) IS was made for 160mm front rotors and 140mm rear rotors.

The Porsche Bike FS came with 185mm front and 160mm rear rotors. The old Formula calipers were already mounted on custom machined aluminum IS mounts and had no other IS standard mounting holes on the fork/frame. That meant short of making custom aluminum parts, I had to stick with 185/160 or go bigger. I decided to stick with the rotor sizes.

An easy mistake to make here would be to get the mounting adapters for 185 front and 160 rear – but remember, the IS standard is 160/140. This bike ALREADY HAS +25/+20 mounting holes. So the correct part to get is a ±0 IS to Post adapter. I decided to go with Shimano calipers so I got the SM-MA-F160-P/S adapter for both front and rear (SM-MA-R140-P/S does not exist and would be exactly the same as F160). By the way, SRAM is a little more scientific about this and labels their adapters by the height it adds.

A caveat to this article is that 185mm is an old rotor standard and is hard to find. New rotors are 180mm. I have yet to see anybody sell a -5mm or any minus mounting adapter. So if this bike needed a new brake rotor in the future, then it might be time to buy a new bike… or shave the adapter down a little?