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Technical FAQ: Hydraulic brakes, high-speed shimmy, and more

  • By Lennard Zinn
  • Published Dec. 10, 2013
SRAM hydraulic brakes use DOT fluid and their Shimano counterparts use mineral oil, so the two are not interchangeable. Photo: Lennard Zinn | VeloNews.com

Inter-brand combos for hydraulic brakes

Dear Lennard,
With hydraulic road brakes becoming more mainstream, what do we know about compatibility and possible inter-brand combinations?

I’m thinking specifically at the moment of the feasibility of mating the new Shimano ST-R785 DI2 levers with SRAM Hydro rim calipers, since Shimano doesn’t offer a rim caliper yet. It’s a common practice in motorcycle racing to mix and match different brand master/slave cylinders, but the lever/caliper piston areas, and thus relative pressure ratios, need to be in sync to give proper feel and power for the application.

My local dealer says the SRAM hydro levers are the same for the disc or the rim calipers, and the RED disc caliper piston sizes are listed as 19mm/18mm, front and rear, respectively. So if that is the target caliper piston size, — it seems the questions to answer would be: What are the Shimano BR-R785 road disc piston sizes? Are the Shimano lines compatible size-wise with the SRAM calipers? Are the two fluid-compatible?

Time to put your mad scientist hat on and cobble together some Frankenstein hydro brakes!
— Steve

Dear Steve,
Don’t do it!!!!!

Even if the threaded parts were compatible, the issue is the fluid (i.e., the answer to your third question is no, thus negating any discussion of the first two questions). Shimano brakes use mineral oil, while SRAM brakes use DOT fluid. The two are not compatible and will cause failure, cracking, or swelling of seals intended for the other brand’s fluid.

There is a rumor that SRAM is considering offering some mineral-oil brakes, since some customers, presumably ones who are used to Shimano and Magura brakes, are requesting it.
― Lennard

Installing disc brake pads

Dear Lennard,
Like Brian, I had great difficulty putting new pads in my Elixirs. I felt like I needed a crowbar to open up the pistons. I finally ended up doing what was suggested in the Art’s Cyclery video on installing disc brake pads, which is to wind out the pad adjuster and open up the bleed port. With the Elixirs I always do a lever bleed after putting in new pads anyhow, so there is no penalty if I lose a little fluid, but more often than not I just end up bleeding out a little air.

I really had no luck spreading the calipers on my Elixirs for the past four years, so I would always end up opening the bleed port to put in new pads. But when I was changing a rotor and pads this past weekend, I decided to give it another go (your post inspired me) — just spreading the calipers. I used a pretty good size flat blade screwdriver and levered against the old pads.

To my surprise, this time it worked perfectly. I couldn’t believe it when the pads just moved right out and stayed there. I wonder if it’s because it has been four months since my last bleed, so there was enough air in the lever to accommodate the extra fluid. Whatever the reason, I can no longer say it’s never worked for me to just spread the calipers.
— Steve

Follow-up to countersteering

Dear Lennard,
Regarding Pierre’s doubt as to the reality of countersteering, there’s an easy way to prove to someone what’s happening. Just take the front wheel off the bike, give it a spin, and try to point it to the right, then watch the person’s face in amazement as the wheel leans left!
— Eric

Dear Eric,
Thanks. I should have mentioned that. I have loved this demonstration ever since seeing it in an angular-momentum demo in freshman physics class!
― Lennard

More on high-speed shimmy

Dear Lennard,
Years ago I failed at finding any article that helped explain the phenomenon of high-speed shimmy. Truly drove me nuts. And although your most recent article is fascinating (even for average roadie enthusiast), I still didn’t find information that would help advise me of how to prevent it. Of course that wasn’t the purpose of the article, but just the same … anyone who has experienced it definitely doesn’t want to repeat it. And while they experience it, they immediately want to know how to get out of it.

After reading the article it basically sounded like frames have a shimmy threshold based on rigidity. Which is great, let’s get them labeled as such (kidding), but I truly believe a correct bike fit is the missing parameter in the analysis of what makes the bike shake, or what prevents it from shaking on the first place.

I first experienced the shimmy at 42 mph while seated upright on my carbon/aluminum frame. Braking only seemed to make it worse, so somehow I managed to ride it out to a lesser speed at the bottom of the hill, and survived intact, though mentally I was a mess.

Even after upgrading to an all-carbon bike, I still had shimmy (albeit less of it) and would apply all the tricks I gleaned (keep one pedal low, weight on handlebar drops, one knee pressed on top tube). I had some upper-end bike fits done, but the bike still shimmied.

Until I found one guy that really knew what he was doing. He adjusted everything. And sitting there in my new cockpit I felt for the first time what it was like to have weight on my front handlebars naturally, as opposed to forcing my weight on them. The feeling of being on a platform with my legs as pistons was unreal, and that was just in the fitting room.

I tested my new fit on the zippy, crosswind-riddled route down from the Golden Gate Bridge — a personal shimmy nemesis of mine. And to my surprise, my bike provided a stable platform throughout the entire descent.

So although a stiff frame definitely helps, the correct fit in my experience is also vital to get rid of the shimmy for good.

Anyways, just thought I’d share in hopes of helping any other enthusiasts out there who continue to struggle with the shimmy.
— Mo

Dear Lennard,
Reading professor Bollt’s explanation of speed wobble brought me back to this past July. I crossed the threshold of my frame/wheel stability. I thought it was a combination of using different rims (bladed spokes versus rounded), wind buffeting from a steady stream of vehicles on my left, and diagonal cracks in the pavement. I had felt a shimmy once or twice before on that bike and had the wheels checked for true because I thought that might have caused it. This time I did feel the “weave” and oscillations beginning at about 31 mph, and then by 35 mph I knew I had to slow down or wreck. Then I hit one of those cracks (or a chunk of broken pavement) and got launched.

After my lab experiment, his explanation of speed shimmy makes sense. I also disproved my theory that I could stop the shimmy with my knees gripping the top tube and that the shimmy would stabilize at some point (like breaking the sound barrier). I will keep that bike under 30 mph to be safe.
— Brian

Dear Lennard,
In an attempt to end this once and for all, I bought a Ridley Excalibur with its huge rigid fork and a 1.5-inch lower steerer bearing.

In my case, the bike was not the problem. I don’t do that much high-speed descending anymore and have gotten rusty. Going downhill where I need a lot of lean, I tend to get nervous, not lean over far enough, and then my arms start to shake, out of fear. Then the bike starts to shake. And it keeps shaking — one memorable time even as I slowed down to a very low speed.

I have been experimenting with this by taking a particular curve at a speed that is just barely above my comfort zone, so that it is easy to recover.

The bike and rider system is much more complex than a bridge or an airplane wing because of how the human body responds to fear. However, I am not saying that what I experienced is the only thing that could cause shimmy.

Countersteering really helps to keep things under control on fast downhills. Control the line by controlling the lean. Countersteering has nothing to do with the front wheel pointing opposite the direction of motion. That is a misunderstanding of the concept.

Oddly enough, my old Cannondale ST-500 seems to be a much more stable and forgiving dive bomber than the Ridley.
— Richard

Dear Lennard,
I just read your VeloNews letter from the math professor about high-speed shimmy.

Almost everyone who attempts to analyze this subject fails to take into account that bike wheels are grossly out of balance. Yes, just like a car or motorcycle wheel, bike wheels, with their protruding valve stems and other issues, are often wildly out of balance. Spin any wheel and there will be a “heavy” point that, after the wheel is done spinning (off the ground), will end up at the bottom.

Please take into account that most cyclists are completely unaware of how much their bicycle wheels are out of balance, and that this is a major factor in high-speed shimmy. Bike wheels need to be balanced just as with any other wheel! Whether taped-on weights or otherwise, they need to run in balance.
— Pete

Dear Pete,
Thanks for mentioning that. I actually went into this years ago and recommended stacking up threaded valve collars on the valve stem until the wheel had no preferred place where it stopped its rotation. And I remember highlighting this feature in an article I did on visiting ADA wheels in Holland. ADA founder Cees Beers built in a weight with an embedded microchip into the wheels to not only record production information but also to balance the wheel.
― Lennard

Dear Lennard,
Have been an avid fan of your thoughts and advice for a very long time … the most recent VeloNews bit on tank slappers was excellent. Now in my 50s, I’ve been a bike racer since my teens and also successfully road raced motorcycles … and have encountered the dreaded speed wobble/tank slapper a few times. The whole discussion reminded me of a terrific video that I hope you’ll find entertaining as well as informative on the subject. I hope you enjoy it.
— Parker

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Lennard Zinn

Lennard Zinn

Our longtime technical writer joined VeloNews in 1987. He is also a framebuilder, a former U.S. National Team rider, and author of many bicycle books, including Zinn and the Art of Mountain Bike Maintenance and Zinn and the Art of Road Bike Maintenance, as well as Zinn and the Art of Triathlon Bikes and Zinn's Cycling Primer: Maintenance Tips and Skill Building for Cyclists. He holds a Bachelor’s degree in physics from Colorado College. Readers can send brief technical questions to Ask LZ.

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