Technical FAQ: Hydraulic brakes, Van Avermaet’s fork
SRAM and Shimano hydraulic brake compatibility
I am curious if one can use SRAM’s road hydro brake lines with Shimano road hydro brakes and levers. Just the brake lines, I know the brands use two different fluids, (DOT vs Mineral oil) and are not compatible. I believe the SRAM hydro lines are compressionless, whereas the Shimano, I think, are not. The SRAM road brake feel is amazing and the Shimano road brake feel isn’t. My hope was that a hydro line swap might bring brake feel closer together.
I have no idea if they are compatible or not, and I’m quite sure if you asked either company, they would not go on record saying their hoses work with their competitor’s brakes. That said, I don’t think it’s your hoses that are making the difference; I think you have some air in your Shimano system.
I have experienced that difference in performance between SRAM and Shimano hydraulic road disc brakes on two of my own bikes. What I finally discovered is that I was not bleeding the Shimano brakes properly. Specifically, I was not loosening the stem clamp (mark the handlebar position first relative to the stem, and loosen the clamp enough that you don’t scratch your handlebar) and rotating the handlebar up and then down while pumping the lever with the caliper bleed closed and a bleed block between the pistons. There is a corner in the top of the Shimano road hydraulic lever that traps an air bubble, and its precise location varies depending on brake model, hence the rotating up and then down of the lever to catch all possibilities. I was rotating the entire bicycle in the bike stand and was not getting enough of an angle to get at that bubble.
I believe Shimano has instructions for each brake lever, whether you rotate it up to 50 degrees or 30 degrees or down to 30 degrees, or whatever, to get to it. If you rotate up to 50 degrees and down to 50 degrees (the bleed cup obviously can’t be full or it will spill, but it must be full enough that there is always fluid covering the hole in the bottom of the cup) you’ll cover all of the bases. And, obviously, if you see a bubble trying to come up out of the lever and not quite able to make it, keep tapping at the hose with a screwdriver handle while squeezing the lever until it finally bobs up into the bleed cup.
I think you’ll find that you can get pretty similar lever feel and performance with your Shimano brakes this way.
Van Avermaet’s Paris-Roubaix fork
For the Paris-Roubaix race, I noticed that Greg Van Avermaet was using a different fork on his BMC GranFondo RBX bike. Normally the fork of that bike has a significant kink down there, just before the drop out. And that fork, they say (and it looks like), has a rake of 50mm. But the fork Greg used was another one. It was either a prototype or the fork from the standard BMC TeamMachine SLR bike.
Anyway, it is clear that the fork Greg was using had less rake. I’m thinking that he probably did not like the feeling of the front wheel being too twitchy (because big rake makes for small trail) so he asked for a more stable front wheel, thus a fork with less rake (making a bigger trail) — which made the wheelbase a few millimeters smaller, which they say makes for a little less stability over the cobbles (a claim I believe is exaggerated).
Indeed, Greg was using a standard SLR01 Fork for Paris-Roubaix.
1. He prefers the steering behavior with the SLR01 Fork. The SLR01 fork increases the trail slightly and therefore changes the steering behavior. Greg used this bike for three years and invested a lot of time in fine-tuning the setup — this is what he liked the most and gave him the best performance on the cobbles. Maybe it was this fine-tuning and confidence that paid off and made him a winner!
2. The SLR01 Fork is 50 to 60 grams lighter. Even if the weight doesn’t matter in this flat race, it also influences the bike handling behavior.
— Stefan Christ
Head of R&D BMC Switzerland
Feedback on tire stress
I’ve been reading with interest the discussion about the effect of tire width on rim stress. I especially liked the finite element analysis in the last installment of Tech FAQ. I couldn’t top that, so I performed a physical test demonstrating that yes, a wide tire does place more stress on a rim than a narrow tire. I was able to measure rim deflections of ~.015 to .030″ while pressuring up various tires to 100 psi.
I made a 5-minute video documenting my results.
I also wrote up the results on my blog.
— Mark “Killa” Barrilleaux
I love so many of your articles and enjoyed the one on tire stress and rim width. In fact, I now run Vittoria Hyper 35mm tires as a low rolling resistance comfy tire. You probably have seen this Mavic engineering article, but if not I think you will find it interesting.
Feedback on fork twist
Just out of curiosity, I checked my 2016 Specialized Roubaix yesterday to look for any disc brake twist. Pushing hard, I couldn’t see any visible twisting. If it’s there, with my 12mm thru-axles, it’s sure not much. That said, it’s also clear to me that there is one price I pay for having disc brakes — compliance. The Roubaix is good on the big hits, but on the little stuff like heavy chip seal there is a buzz through the bars I never got with my very old Trek 5200. I can’t see that there is really any way for the bike designers to get around this. Do you have any ideas, other than going to a real suspension fork? My solution was to drop my front tire pressure to 70 psi, which seems to work well with no real downside. What I feel would be ideal is something like the Specialized future shock, but much softer and with just a few millimeters of travel. I was hoping to get this by upgrading to carbon bars, but that didn’t pan out as I hoped. I would still love to find some shallow drop, very flexible carbon bars.
No, I can’t see a way around it. I notice the difference in compliance between the forks on my disc-brake cyclocross bikes and my cantilever-brake bikes. A disc-brake fork simply has to be stiff at the bottom to oppose the braking forces, and you as the rider pay the price for that on high-frequency small bumps.
Yes, a single-sided brake does create unequal loading on the fork legs. Our chief suspension engineer (Manitou) has studied this issue in development while working with a major motorcycle company. Their unique brake system was a single-side brake in a sport bike application. The unequal loading caused differences in handling under trail braking conditions. So while braking in a turn in one direction, increased brake pressure would cause understeer and in the other direction it would cause oversteer. Changes in the relative stiffness of the fork legs can counteract this phenomenon, and increasing overall stiffness makes the braking effects less noticeable overall. The studies that our engineer learned in his previous employment has carried over to bicycle fork designs.
— Scott Boyd
Product Manager — SUNringlé (Hayes, Manitou, Answer, Wheelsmith)