icemilkcoffee 

I bought my son a used MTB to ride to school with. It came with mechanical disc brakes. The front one was really stiff and the stopping power unimpressive. I looked at the caliper carefully and noticed that only one piston and brake pad moves. The other brake pad is stationary. The brake works by bending the disc towards the stationary pad! So a lot of effort is spent bending the disc rather than braking. What a dumb system. Who on earth designed these things? If they wanted the disc to move around, why didn't they adopt the floating discs routinely used on motorcycles?

Anyways, I moved the stationary side as close to the disc as possible to minimize the need to bend the disc. But it took a lot of finagling, and I imagine with brake wear I'd have to do this repeatedly. Is it worthwhile to upgrade to double piston mech calipers? Is there a noticeable difference? Or would the two springs increase effort to the point that it negates whatever theoretical advantage that two moving pistons bring?

Litespud

The one moving/one stationary pad design of many mechanical calipers doesn't automatically make them bad. For example, the Avid BB-7 is a pretty decent mech. system with one moving piston - I had them on my MTB for years and they were great. If your son's brakes are bad, look for any of the many possible reasons for this beyond simply "one piston bad". How are the pads? Worn? Maybe a better pad compound? Rotors are clean? Compressionless, properly-routed cable housing (I found the Avids very sensitive to cable friction)? Good housing will give you much better braking. Pads correctly adjusted (the Avids have very easy-to-adjust dials on both sides - I had the stationary pad very close to the rotor, so that "rotor bending" was minimal before both pads engaged. I checked adjustment every time I removed/reinstalled a wheel, and I treated the rotors with kid gloves to ensure that they never took a hit that might warp them). Brake levers appropriate for the brakes?
What bike/brakes are these? With no disrespect, but a cheap bike with cheap brakes means that the brakes may never work particularly well. It could well be that there's nothing left but to replace the brakes, in which case dual-piston would be the way to go.

Rolla

The overwhelming majority of cable-activated disc brakes are single-piston designs, including the exorbitantly expensive Paul Klampers. On most of them, you have to manually adjust for pad wear; it's not a big deal, but I find the whole design kind of finicky. TRP Spyke brakes are cable-activated but have two pistons; they're pretty great. Alternately, you can get hybrid brakes that use a cable to activate hydraulics in the caliper (like Yokozuna or TRP HY/RD); the pistons accommodate for pad wear. They can be expensive, but still less than some full hydro brakes.

GamblerGORD53

Yup. One piston was always a stupid design. For sure, get a cable TRP Spyre caliper. Mine is on my Rohloff14 tour bike, 120 lbs loaded.
ZERO squeaks and minimal fiddling. I have the gap bigger because I have track dropouts and it doesn't seem to matter the gap anyway.
I have really nice SA levers set to long pull. Stops on a dime any time. And besides, the TRP is 14 mm less chunky than a horribly UGLY BB7.
With my SA dyno drum brake in front, I have 100% all weather reliability.

Germany_chris

The internet love Spyres but they didn't work for me, but my single piston Klampers do..

I'd try good brake pads before I bought new brakes.

icemilkcoffee 

It's a Trek 3500. The more I look at this bike the more I hate it. The front fork has a conventional downwards opening, so hard braking could force the hub downwards. The brake levers have no reach adjustment- why would they omit this on a kid/youth bike? The headset uses caged ball bearings instead of catridge bearings. Good job saving $3, Mr. accountant. The handlebars is steel. Must have saved another $2 there.

c_m_shooter

The 3500 is an entry level bike. You get what you pay for. Single piston mechanical brakes are fine, been running them for years.

Iride01 

You might be surprised that a lot of disc brakes on cars only have one piston.

While your cable pulled single piston brakes might not stop as well as others, I bet they stop well enough. Or perhaps you simply need to get them adjusted. But don't expect them to stop as well as better brakes.

hevysrf

On a modern car, as the piston pushes the pad to the disc, the caliper floats moving the non piston side pad towards it also.

I don't know if that is the case on the OP's bike.

cxwrench

No, it's not a stupid design. It works fine when set up properly, which many, many people don't know how to do this correctly. I see that issue all the time.
As others have said it's a low priced bike. It's absolutely fine for what it is. Don't be that guy.

Iride01 

On a bike, the rotors are flexible enough that they can be pushed into the other pad. No need for a caliper sliders like in cars with a very inflexible rotor.

cyccommute 

It’s not that stupid a design. It works well for most mechanicals as long as they are set up properly. Issues with braking have more to do with the set up than with the caliper design. That said, the TRP Spyke/Spyre have their own problems. The biggest is the 3mm hex head socket on the pad adjuster. The pad adjuster is an aluminum part and the 3mm socket is very easily damaged. You can get replacements but getting the stripped pad adjuster out is difficult and can end up damaging the caliper.

Additionally, the pad adjuster can be too tight from the factory which means you are more likely to strip it on install. Nothing like having a shiny new set of brakes installed and ready to go but you can’t go use it because you are waiting for a replacement part.

icemilkcoffee 

that’s how it is in all car brakes with angle pistons - the calipers float. You will never find one that bends the brake disc to meet the other brake shoe.

Rick

icemilkcoffee: Wow I am impressed! You looked over your kids bicycle. Ok after adjusting the calipers and making sure that the brake housing has no burs in it and there are no kinks in the cable. You get on the bicycle and see how well the brakes work. If you are not happy with the braking, then it may be a good idea to upgrade to a better stopper. The Avid bb7 has been around for some time and gone through several Improvements. BB7 MOUNTAIN S and BB7 MOUNTAIN. The S has stainless steel hardware and black anodizing.

dedhed

At least it works that way on the ones that the sliders are greased. Pulled plenty of sliding calipers that didn't slide here in the rust belt.

Kai Winters

It's an inexpensive method for stopping a bicycle...that is it.
Want a good brake? Spend the money and buy a double piston disk brake caliper.
I use the TRP Spyre carbon model on my '22 Aethos...they work great, easy to set up and adjust...and perhaps most important they stop the bike quite nicely.
A lot of complaining for a relatively easy fix at a modest expense.

wolfchild

Single piston isn`t the problem here. The problem is cheap bottom of the barrel brakes. Try putting in new braked pad and adjusting the brakes

csport

Exactly! Paul's video on his Klampers (starting at 1:10):
So it is not just me. TRP Spyres were my first disc brakes. Stripped one adjuster on install. Had to wait for a replacement. Another problem was adjusters vibrating loose after you turn them to adjust for some pad wear. I have read that the purple loctite can rebind, applying it kind of fixed this problem.

cyccommute 

I’ve actually stripped out two of them. I dislike the design so much that I went back to the BB7 (can’t justify another set of Klamper…yet!)

cyccommute 

You also won’t find a car disc rotor that weighs 120g (0.25 lb). I don’t think you want to carry around a 9500g (21 lb) rotor and all the associated engineering need for that caliper to float. Cars also don’t have to worry about brake pad rub as the car engine has enough power to overcome that little amount of friction. The underpowered motor of a bicycle can’t stand even a little bit of friction so there is an air gap between the rotor and the pad.

base2 

Between worthless single piston mechanicals & rim brakes...Rim brakes for the win. Why pay the "disc brake premium" in the sticker price for something that works worse, & requires constant maintenance or adjustment?

That equation changes for dual-piston mechanicals or hydraulics, of course. But unless someone has plans to install a parts bin upgrade, from parts already in hand the better deal is often the bike with the good brakes or the cheaper rim brake option.

Single piston mechanicals is what got that little girl killed on the Rad bike from a few days ago. Single piston OEM mechanicals is what nearly got me killed in 2016 trying (& failing) to stop for a traffic signal on a simple speed limit descent.

Single piston mechanicals are garbage & should be banned by the CPSC.

cyccommute 

Single piston mechanicals have never been something that need constant maintenance nor constant adjustment. Nor have I found them to be worthless. I’m not saying that they are vastly better than rim but they aren’t any worse.

I’ve not been impressed with hydraulics…I find them far too touchy and the very opposite of “well modulated”…and dual piston mechanicals don’t perform any better than any other mechanical. I’ve had both dual piston and single piston. Neither is notably different from the other. They both stop well…but then so do most any brake.

Are you talking about this case? That was a year ago. The parents claim the brakes were defective but the claim isn’t that they didn’t work but that they induced a wobble to the wheel when applied. Strangely, they said nothing about letting a 12 year old ride a bike that specifically says not to be used by anyone under the age of 18. These are extremely heavy bikes and a 12 year old may not have the strength nor motor control to keep one upright at speed with any kind of brake.

As to your experience, it’s not one that I’ve shared. I’m not a small guy and I carry relatively heavy loads on a mountain bike on steep rugged downhills in the Colorado mountains where I can reach speeds of 30 to 40 mph. I’ve never had any kind of mechanical brake that failed to stop the bike. That includes both dual and single piston mechanicals. For that matter, I’ve never had a instance with any kind of brake where I thought the brakes would fail me. I’m not shy about letting gravity have its way with me.

Single piston mechanicals are safe, effective, and simple if properly adjusted and properly used. Most all issues I’ve run across (other people’s bikes, not mine) have been issues with improper adjustment…even on low end mechanicals. Properly tuned, even cheap mechanical discs are effective.

icemilkcoffee 

My other son's MTB has V-brake rim brakes, and I have to agree that it stops better than the single piston discs. I purposely looked for a disc brake bike this time around thinking it's progress and progress has got to be good, right? Wrong. Double piston hydraulic brakes are progress. Single sided mechanical disc brakes are invented to scam consumers who just knew they want to buy a disc brake bike.
Come to think of it, they should make a single sided hydraulic caliper. That way you can scam consumers who are sophisticated enough to buy hydraulic, but neglect to read the fine print about single sided vs double sided pistons. Hey bike manufacturers, here's another $3.50 savings for you! Go for it!

jgwilliams

When I got my new mountain bike I was rather surprised to find that the brakes are 4 piston. Have to say that they're very good brakes, but I do wonder whether those extra pistons are really necessary.

kwb377 

You weren't scammed...you bought a bike with bottom-tier components. Just like buying a vehicle...you can't buy a stripped fleet model work truck, then complain they didn't put full leather interior, navigation, dual-zone climate control, 14 speaker audio system, etc. in it.