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Tektro Mira Mechanical Disc Brake - why is it so weak?

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Tektro Mira Mechanical Disc Brake - why is it so weak?

Old 07-20-18, 08:02 PM
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alias5000
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Tektro Mira Mechanical Disc Brake - why is it so weak?

Hello everyone,
a friend of mine bought a brand new road bike a few months ago (2017 Specialized Diverge A1 Sport), which came with Tektro Mira mechanical disc brakes. The stopping power has always been an issue to the point where it affects her choice of speed significantly, especially on downhills.
I did the bedding in process for front and rear myself and I did notice the improvement of braking power from very bad to so-so (with my larger and stronger hands, though). The brakes do not feel spongy and cable movement is good (i.e., rules out housing related issues). I have readjusted the front caliper with the best method known to me (it worked perfectly for my Avid BB5s which are of similar design) and I am sure that contamination is not an issue. This basically covers almost all the commonly given advise on mechanical disc brakes.
The only thing left on the list is to swap the pads (and I'm currently between gifting her new koolstop/swissstop pads, or just BB7s right away - which I can at least test beforehand as I have a set myself).

Now, what makes these Tektro (Mira) brakes so bad? Is it really just the pads? The caliper itself appears to be very similar to Avid BB5 brakes just with larger pads - BB5's can be made work very decently with good care. While the actual solution might be BB7s (or risking new pads without knowing potential upfront), I dislike the idea that brand new calipers are unable to deliver a good performance and might just end up becoming unused junk. And I fail to understand what the technical differences would be between, say BB5s and Tektro Miras. Their both pretty big pieces of metal in a similar design.

==> Does anyone have any experience with these particular brakes? Or any technical insight beyond you-get-what-you-pay-for logic?

Thanks!
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Old 07-20-18, 08:41 PM
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I looked them up, and for the life of me, I see no difference compared to the Tektro Aries brakes on my Diamondback. Given the price of my bike, I'd be shocked if the brakes are anything but entry level. Yet they work extremely well. So I doubt there's a get-what-you-pay-for issue here. Something must be wrong. On these brakes, it looks like they need some initial adjustment by moving them around with the mounting screws loose, which I didn't do, since the bike came from a shop. So it might be worth looking at that part of it, and satisfying yourself that it's set up correctly.

Also, this should be a warranty issue if the bike came from a shop. I'd be willing to generalize and say that on any new bike, the brakes should stop the bike, no compromises.
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Old 07-21-18, 12:05 AM
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Some of it may be that they have a poor cable pull ratio in conjunction with the levers. Recent Shimano brakes more cable per degree of lever arc than older levers and other manufacturers, so when used with brakes designed for those levers, they have low mechanical advantage, hence a firm lever feel and poor power.
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Old 07-21-18, 06:04 AM
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Originally Posted by alias5000 View Post
... I am sure that contamination is not an issue.
While you may be sure, some cleaning with an automotive brake cleaner such as BRAKLEEN will do no harm and may be helpful.
In 50+ years of troubleshooting I have found that very often the thing you immediately dismiss winds up being the problem.
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Old 07-21-18, 09:09 AM
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I liked aftermarket pads better than what was stock, on, in my case .. Avid BB7 M.

I cannot help your set up issues from here ...

I'd not call them junk, just a lower cost parts pick, By product mgr,
to get the bike selling price down, to what you were willing to pay..







....

Last edited by fietsbob; 07-21-18 at 09:13 AM.
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Old 07-22-18, 10:31 AM
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Do you have super spongy brake cable housing? Your brakes rely on stiff housing to make them work well.
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Old 07-22-18, 11:24 AM
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^^^ This.

Unless it's already on there, switch the brake housing to Jagwire KEB-SL. Compression-less housing makes the biggest difference. Beyond that, people seldom have nice things to say about stock Tektro pads. I've had better luck with no-name Chinese pads off of Amazon.
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Old 07-22-18, 03:48 PM
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Hey guys,
I feel like I had an urge to rant, rather than introduce a fully structured conversation. Yesterday, I had the opportunity to borrow the bike for an hour and try a few things. This is the outcome:
  • Baseline: Braking distance with existing system, full squeeze front brake from hood: about 6m from 23 km/h (all estimations!). Hands hurt after doing this a few times. Cannot get close to a rear tire lift-off. (I can using the drops, not sure about my friend)
  • Install road BB7 from our tandem (with standard pads) on front, initially forgot to bed in: Friend's comment: 'It's getting there', i.e. it's not enough for her to feel as confident as usual regarding braking power. It sounded like, it's a "70%".
    Benchmark test: maybe 4.5-5m. I cannot lift off the rear tire from the hood. Noticeable improvement.
  • Do the bedding in with the BB7s: further slight improvement of braking power, maybe we're now "80-85%" there? Braking power from the drops is very strong, but that isn't the primary goal. Time was up, I had to revert back to the original brakes.
  • Sanded down the existing Tektro brake pads (Tektro A10.11 organic). No effect.
From a technical stand-point, this route would like end up here: BB7 road on both wheels, Kool stops on the front. If it's not enough from the hoods, get her to always use both brakes simultaneously. The way she uses her bike makes the braking power from the hoods important (lots of city riding, still feels more comfortable/confident on hoods, rather drops. Does not ride for competition, rather recreation, does not speed down hills at 70 km/h anyways).
I am unsure as to how much improvement just the replacement of pads on her existing Tektros would bring. The price difference between new pads and BB7s is there, but on the Canadian markets, it's maybe somewhere in the 2:1 range. We ordered BB7s. I am hoping that BB7+koolstop on front+using both brakes will be a happy solution for her.
One observation I made, was that the Tektro spring seems to be stronger than the BB7 one. This would translate in more force needed on the lever.

The more I think about this, the more this is an issue about the minimal leverage you get on the hoods when you have smaller hands. Normal MTB-style levers are much better in that case. I am sure that she would have very good braking power from the drops with BB7s, but it just doesn't fit the use case in most situations.

Now, to reply to your comments:

Originally Posted by cpach View Post
Some of it may be that they have a poor cable pull ratio in conjunction with the levers. Recent Shimano brakes more cable per degree of lever arc than older levers and other manufacturers, so when used with brakes designed for those levers, they have low mechanical advantage, hence a firm lever feel and poor power.
It certainly is a firm lever feel. There is no way to push the levers all the way to the handlebar. I tried to google around regarding pull ratios, but these numbers seem so hard to come by. Tektro Mira mechanical road disc brakes, and current Shimano Sora brifters.

Originally Posted by dsbrantjr View Post
While you may be sure, some cleaning with an automotive brake cleaner such as BRAKLEEN will do no harm and may be helpful.
In 50+ years of troubleshooting I have found that very often the thing you immediately dismiss winds up being the problem.
I will try this (as well as sanding the rotors) the next time I can get a hand on this bike for this purpose.

Originally Posted by fietsbob View Post
I liked aftermarket pads better than what was stock, on, in my case .. Avid BB7 M.
I'd generally agree, though there are some hit and misses. And it's hard to predict how much improvement will be there. Since pad shapes are so individual, you can't swap them around if they're too weak by themselves.

Originally Posted by DrIsotope View Post
Beyond that, people seldom have nice things to say about stock Tektro pads. I've had better luck with no-name Chinese pads off of Amazon.
Thank for your comment! I'm short in time (and I ordered BB7s before seeing your comment), so it'll stand untested for now. Maybe I'll order some Chinese pads and keep the Tektro brakes for a later experiment.

Originally Posted by n0+4c|u3 View Post
Do you have super spongy brake cable housing? Your brakes rely on stiff housing to make them work well.
No spongy feeling, housing looks and feels good to me. Housing ends look good, too.


Thanks guys for your comments. In a way, my question is probably done with this for now, let's see how things look once BB7s are on the bike.
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Old 07-22-18, 08:48 PM
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What about the Brake levers?

Maybe SD7 could make the desired difference? Remember, those can be dialed in. On my Mariner 7 the brakes (V) weren't so great, so they were replaced by SD7 Brake levers and the SD7 brakes with koolstop.

Just a thought.

And yes, the discussed brakes in question are mechanical. So, SD7 are a viable solution.
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Old 07-23-18, 06:36 AM
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..This is a drop bar bike with brifters.
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Old 07-23-18, 11:51 AM
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Try two BR-R317 for about $30 each they're designed explicitly for Shimano brake pull ratios (new Super SLR, which is what those recent Sora levers use) with compressionles housing. Clean the rotors before install. I bet it'll work very well. TRP Spyres are also a very good choice and have the advantage of moving both pads, making adjustment for pad wear much easier.
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