Papa Tom

I just replaced a set of old Shimano CT90 brake arms with a set of the supposedly-improved CT91 model and there is a bit of forward and backward "play" in each arm. They appear to be exactly the same size as the old ones, which did not have this play.

Assuming this is just the quality of the crap parts that are out there, can I get away with a small amount of jiggling back and forth, or should I put the old arms back on and assume I've wasted the money?

Andrew R Stewart 

The usual reply is "does the play hinder the function or performance" The usual answer is no.

But I have shimmed canti posts with brass shim stock (it comes in tiny amounts of thickness starting with 0.0015" in my supply) to tighten up the clearances. Andy

Papa Tom

They certainly don't have the braking power of even the cracked, worn-out CT-90's they replaced, but no, I don't think the "play" is the problem. Where can I get this "brass shim stock" you're talking about?

dsbrantjr

McMaster-Carr has shim stock sets and many hardware and homeowner's stores have a K&S Metals display which has different shim stock thicknesses. Probably also auto parts stores.

dedhed

https://www.ksmetals.com/deals

Andrew R Stewart 

Tom- Exactly what I use. Good source for a lot of industrial stuff. I order from them a few times a year. I have a some brass shim pack I bought years ago and about 2/3 of it is gone by now. If one knew what thickness was needed I would see if I had a bit. The stuff would mail in a common envelope in the amount cantis would use. Again the challenge is how thick. If you are interested PM me and we'll talk. Andy.
https://www.mcmaster.com/shim-stock/...aterial~brass/

dsbrantjr

An automotive "feeler gauge" set should get you the answer. https://smile.amazon.com/s?k=feeler+gauge+set or an auto parts store.

Papa Tom

This is a lot of work for an $8 set of brake arms. If I go to Home depot when/if this stuff is all over, will they have multiple thicknesses I can try? I don't want to start buying gauges and trying to read flea-sized measurements. Been there and done that with my recent bottom bracket debacle!

Andrew R Stewart 

Tom- It's not about the parts it's, IMO, about the experience of using the brakes. Your choice. If I were you I would try local options and decide after that. I'll still be here later.

BTW one factor in canti "power" it the straddle cable VS arm angles. Too low to the tire and the brake is real mushy but leverage builds up. too high a cable and the leverage drops with lever pull but the lever feel firm on pad contact. With the link wire this adjustability requires a different wire or the pads get run in and out on their mounting shafts. I prefer bare straddle wires with a pull up yoke clip for max adjustability. Andy

Papa Tom

Andy: What you said about the straddle cable height hit a chord for me because I feel like the straddle cable is too low and the brakes are definitely mushy for the first time ever. I wish I had a better grip on the terminology you are using, but I am a hobbyist bike mechanic, at best (got the tools and know how to use them but don't really understand how everything works). This sentence, in particular, completely baffled me: "With the link wire this adjustability requires a different wire or the pads get run in and out on their mounting shafts. I prefer bare straddle wires with a pull up yoke clip for max adjustability." Can you explain how I can raise the straddle?
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Andrew R Stewart 

Sure I'll translate my poor phrasing As the straddle cable lowers to the tire the lever will travel further for the same amount of pad movement. Asd the straddle cable is lengthened to be higher off the tire the pads will move more and thus the lever will feel less mushy. Somewhere between lowest and highest straddle above tire set up there will (generally) be the most leverage, and whatever lever feel, solid or mushy will be what it is. So by changing the straddle cable's length or the pads in and out position WRT the arms you change the leverage changing rate. When Shimano introduced the link wires with a fixed straddle length they were trying to make the new bike's set up faster for less assembly cost and also insure a fairly consistent leverage at the same time. However some of the geometry of the system is determined by rim width and post to post width, both not it total control by Shimano. So Shimano offered as options other link wire lengths. Shops have scads of different link wires on their shelves (or they should if they care about this stuff). By replacing the link wire with straddle cable and it's pull up yoke one can play with the geometry with no other parts, and do so while test riding the bike.

I never liked the CT 90/91 "Low Porfile" cantis because of this adjustment limitation, the link wires and that the arms are nearly vertical. The more vertical the arms get the less solid a feel the lever will have although the rate of leverage will ramp up during the lever's pull. I tried to find my diagram of the two canti designs side by side but don't find it. So here's the classic "wide profile (in this case my favorite Shimano AT series arms) canti showing the geometry and what is really at play in terms of leverage, pad/pivot/arm shapes. Note that it's the dotted lines that are the actual leverage force vectors (fancy term). There are other images on line for your search, I'm sure.

There is a debate (or was prior to disk brakes becoming The Way) about which side of the leverage fence makes the better brake set up. Is it the mushy but increasing leverage rate design like the CTs or is it the dropping leverage rate of the CT series, Added to all this (besides the rim and post widths) is the tire size. The straddle or link wires HAVE TO clear the tire, with increasing tire width (MtBs) came the need to lengthen the canti arm lengths too.

I hope this helps you understand the various factors that go into a brake system and it's various set ups. As mentioned I like the ability to explore the range of set up to end up with the lever feel and leverage (Power) that a set of cantis can have. I prefer a more solid (less mushy) lever feel. With my mechanic's hands I don't have much issue with applying enough strength to stop well with a solid but less leveraged set up. It seems that you might be the same. Andy

Andrew R Stewart 

Tom- In my search for an image of the Low Profile cantis I rediscovered where I stole the classic style's image I posted. It's from Sheldon Brown's site. here's the link. perhaps it's text will be of help. https://sheldonbrown.com/cantilever-geometry.html Andy

JohnDThompson 

Local hardware stores may carry it. Otherwise, industrial supply places like McMaster-Carr, or even eBay and Amazon.

N.B. I find the rolls better than the sheets. Wear gloves when handling the stuff; it's thin and sharp and can easily cut you.

Papa Tom

So, based on Sheldon's article, it seems the mushiness I am feeling with these new arms is actually indicative of better braking power. However, I feel like my brakes were rock solid before I installed the CT-91's, and I was able to actually hear the pads contact the rims.

By the way, I only replaced the original CT-90's because (like ALL CT-90s, eventually) the plastic spacer covering the springs had cracked. I almost want to put them back on and just ride them...till they fail...on a steep hill...at a busy intersection. OK, never mind.

So back to your very detailed reply, Andy. Are you saying there are straddle cables of various sizes? I actually re-used the one I've been using all along, but perhaps I will check the one that came with the CT-91 and see if it is any different. To raise the straddle further away from the tire, would I want a smaller straddle cable, or a larger one?