Technical FAQ: Brakes, tubes, and Dutch bikes

Lennard Zinn addresses reader questions about disc brake calipers, patching tubes, the geometry of bikes in The Netherlands, and more.

Flat-mount and post-mount calipers

Dear Lennard,
I’ve recently been ogling the new Shimano road hydro shifter/brakes, but I’ve noticed the disc brake calipers are all flat-mount. This is a problem for me, as my ’cross bike is post-mount both front and rear. Are there post-mount brake calipers that work with the new 105/Ultegra/Dura-Ace hydro shifters?
— Mark

Dear Mark,
Yes, it’s no problem to achieve what you want. You can use a Shimano road post-mount caliper in place of the flat-mount caliper. For instance, you can mount an Ultegra 6870 series BR-RS785 or BR-R785 post-mount disc caliper onto the brake hose of any of these flat-mount brakes: a Dura-Ace R9100 series brake, in place of the flat-mount BR-R9170 caliper, an Ultegra R8000 or 6800 series brake, in place of the flat-mount BR-R8070 or BR-RS805 caliper, or a 105 R7000 or 5800 series brake, in place of the flat-mount BR-R7070 or BR-RS505 caliper. [related title=”More Tech FAQ” align=”right” tag=”Technical-FAQ”]

This is not simply an issue of retrofitting brakes onto an older post-mount frame. We have had many occasions that require a modification like this on new custom bikes we’re building, with Shimano and SRAM, because there are sometimes good reasons for going with post-mounts on either the front or the rear or both. One example is on coupled travel bikes with hydraulic disc brakes. With no way to detach the hydraulic hose easily and without losing fluid, the rear brake caliper must be removed to break the frame down in pieces for travel. Unlike with flat-mounts, with a post-mount brake, the caliper can be removed and reinstalled without need for adjustment of the caliper’s position. You simply unbolt the post-mount adaptor from the IS frame mounts without loosening the post-mount bolts that hold the caliper onto the adaptor. Since the caliper-position adjustments are on the post-mount bolts going into the adaptor, they stay fixed when the adaptor is removed and then bolted back on. By contrast, a flat-mount brake caliper must be readjusted every time it is removed. Those of you who have adjusted disc calipers know that, while sometimes it is as simple as pulling the lever and tightening the bolts while the rotor is clamped between the pads, it often takes a lot of eyeballing and tweaking with the bolts loose to eliminate pad rub. This is something you’d rather not be doing in a hotel room on your cycling vacation.

Also, during this transition period in which forks are available in both styles, sometimes the fork a customer wants — for reasons like steering-tube length, axle diameter, tire clearance, or fork offset — may have post-mounts rather than flat-mounts. The bike may then end up with a front post-mount brake, and it makes it nice and easy that a Shimano or SRAM post-mount caliper can be mounted onto the hose attached to a lever normally attached to a flat-mount caliper.
― Lennard

Shimano brake compatibility

Dear Lennard,

I have a hybrid/fitness flatbar bike and I would like to upgrade the brakes. The Shimano Ultegra R8070 calipers and matching rotors are a simple bolt-on upgrade and match the Ultegra R8000 drivetrain I recently installed. The problem I have is with the levers. The XTR/XT levers use the same hose, and getting the fittings right looks like it would be no problem. What I’m not sure of is if the master/slave cylinders are compatible. Shimano’s compatibility chart says nothing about road/mountain mixing, while a web search has turned up nothing relevant (or recent). Any thoughts would be appreciated!
— John

Dear John,
Since the pistons, fluid, and hoses are all the same, you should have no problem using XT or XTR levers with your Ultegra calipers. That said, Shimano doesn’t support mixing road/mountain brake parts, so I’m not telling you to do so. Unless your frame is flat-mount I’m not sure why you don’t just use a complete XT brake system; that would certainly be the most kosher as far as Shimano is concerned.
― Lennard

Patching tubes

Dear Lennard,
For years, I’ve patched tubes with success. Lately, I’ve found my repairs lacking and the tire flat after a few days even though I’ve checked the tube leak-tight in a bucket of water. I am starting to think I should consider a patch as an emergency rather than a permanent repair.

Am I running a fool’s errand? Should I just throw away punctured tubes rather than repair them?

Do you have any recommended best practices for repairing tubes? Could I patch a tube with rubber from another tube?
— Scott

Dear Scott,
I am a believer in patching tubes and frequently do so.

I wonder if your water test is loosening the patch. I used to do an underwater pressure test after patching and before remounting the tube in the tire as well, but I had similar problems. I decided that the best way to ensure that the patch stayed glued down was to inflate it inside of the tire, rather than running the risk of getting air under the patch by inflating it without a tire constraining it.

Best practices:
1. Don’t patch holes near the valve. That is a fool’s errand. So is patching a snake-bite (pinch flat), usually.

2. Sand well with pretty fine sandpaper.

3. Apply glue to a large enough area surrounding the hole. Make sure the glue extends well beyond the size of the patch.

4. Let the glue dry completely before applying the patch. Depending on temperature, wait perhaps 15 minutes after applying the glue.

5. Peel off only the aluminum backing from the patch, and stick it down onto the glued area without touching the orange side or the glue.

6. Burnish the patch well. I rub the top of the patch with a screwdriver handle using hard downward pressure.

7. Don’t peel off the clear plastic top cover of the patch; this accomplishes nothing, and it runs the risk of peeling up the edges of the patch.

8. Install it into the tire right away and pump it up to press the patch onto the tube.

I think the patches with the gummy orange base and edge layer are always going to stick better than a piece of cut-up inner tube.

― Lennard

Dutch bikes

Dear Lennard,
I just spent a week in Amsterdam and have a question about commute bike geometry. These classic black bikes look to have a large fork offset, combined with a very slack headtube angle, which I would think would create a lot of wheel flop at the low speeds which they are frequently ridden. Any idea why they are designed this way? I can only think that it’s to create a smoother ride by increasing the leverage on the fork relative to the headset. What am I missing? I will say that I’ve seen a few more modern-looking commute bikes that seem to have bigger tires and more aggressive-looking geometry, but these are pretty rare — I believe because theft is so rampant. Someone joked that in Amsterdam the lock is usually worth more than the bike it’s protecting.

P.S. This is a super cool bike you built. It’s the first E-bike I’ve ever seen that looks like … a bike. I am astonished at the energy you can store in that battery. To be able to go over Trail Ridge road is nothing short of amazing. I think we are going to see the number of these bikes increase quite dramatically over the years.
— Steve

Dear Steve,
Actually, the wheel flop will not necessarily increase due to the slack head angle and long fork offset. Wheel flop will increase with decreasing head angle, and it will decrease with increasing fork offset. (The amount of wheel flop is equal to the sine of the head angle (in radians) multiplied by the cosine of the head angle and by the fork trail.)

As you intimated, reduced wheel flop (up to a point) is a benefit for bikes ridden at slow speeds. Greater stability at high speed is associated with high trail and high wheel flop. At low speeds, the bike tends to weave back and forth less with a reduction in trail and in wheel flop. The long fork rake would produce better low-speed performance and reduced stability at speed, whereas the shallow head angle would have the opposite effect. On balance, those black Dutch commuter bikes may not have much different steering behavior (once adjusted for the long wheelbase) than a racing bike. And yes, a smoother ride is achieved by decreasing the head angle and increasing the fork offset, and I do imagine that is the reason for that design.
― Lennard