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Technical FAQ: Cassette spiders, sealant, and double brakes

This week, Lennard Zinn addresses questions pertaining to cassette spiders, the amount of sealant needed, and assembling double brakes.

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Cogs and spiders

Dear Lennard,
I was wondering if having more cassette cogs mounted on a spider increases shifting performance. For example, an 11-speed Ultegra cassette has five cogs on two spiders, whereas an 11-speed 105 cassette has only three cogs on one spider.
— Chris

Dear Chris,
Maybe there is a shifting performance improvement, but I doubt it’s noticeable. The main benefit of the cogs mounted on spiders is in dropping grams; having more of the cogs on aluminum spiders decreases weight by replacing the majority of the cog’s steel (or titanium) mass with aluminum spider arms.

I would guess that the thick aluminum spider, to which the outer steel toothed ring is riveted (and it is only a thin ring with teeth on it, rather than an entire, stand-alone cog), is probably stiffer than separate single, flat, steel cogs, especially as the cog sizes increase. I’ve never measured the stiffness difference and don’t know of any independent tests of this, so I have no scientific evidence on which to base this opinion.
― Lennard

Sealant minimums

Dear Lennard,
While I understand that instructions from manufacturers call for half a bottle of sealant to be used on tubeless tires, I’m wondering in a weight geek kind of way what’s the minimum amount that one could get away with. It looks like something like a teaspoon would be needed to plug a given leak, but of course that teaspoon would have to be in the right place at the right time. So, what’s the minimum?
— Andrew

Dear Andrew,
How much sealant is required completely depends on the tire’s porousness and its fit to the rim, as well as the size of the tire.

If it’s a tubeless ready tire as opposed to a UST tire, tire standards do not require it to have airtight sidewalls (which UST tires must have), and it can take a lot more sealant to fill all of its pinholes. And if air can get around the rim/tire interface, then it will take considerable sealant to fill these leaks, as they are not on the top of the tire, where the sealant is naturally thrown to. And obviously, a 29×2.4-inch MTB tire is going to require more sealant than will a 700x25C one of similar air permeability.

From a weight-weenie perspective, if you also have an interest in reducing the amount of time devoted to screwing around with getting your tires to seal, I think it makes a lot more sense to put in the recommended amount of sealant, seal it up, and then remove the excess. You can take out the valve core and push out most of the sealant once it has completely sealed up. That should still leave a little bit sloshing around in there to fill thorn punctures.
― Lennard

Double brakes

Dear Lennard,

I’ve met a Paralympic cyclist who can use only his left hand. He still has a right hand, but as far as I understand, his right-hand fingers can only pull very weakly, so he cannot shift or brake with them. He joined some races using a standard road bike, operating the rear brakes and front derailleur using a left-hand shifter, effectively serving as a 2-speed bike.

We’re planning to solve the shifting problem using Synchro Shift, assigning the rear shifting buttons to the left and having the system operate the front derailleur automatically. However, we still don’t know how to operate both brakes using one hand. I’m aware of dual lever setups on one side, but the Paralympic Committee disallows his participation using a flat bar bike for his class of disability. Therefore, we concentrated our thoughts on operating two brakes with one hand. His left hand is not particularly strong, so we think hydraulic brakes are the way to go.

The No. 1 problem that comes to my mind will be the increase in lever stroke, as we’ll need to fill two calipers instead of one with just one lever. After the brakes are full, the total force on the bike would be the same until rear wheel lockup, at which point the bike would be quite close to flipping over anyway. Do you think the lever stroke will be enough to operate both calipers, or will it be hitting the handlebars? How can we mitigate the free stroke? For example, would slightly overfilling the system work? Or maybe using the brake calipers of another brand that has smaller pistons?

 Another problem is that in order to actuate the brakes using one lever, there are series and parallel applications for different setups. Which one is better in your opinion in terms of reliability and ease of brake balance adjustment?

Lastly, can you suggest any other option for adjusting the brake balance? We’d like to have it easily adjusted without having to swap parts or re-bleed the system, but that may prove difficult.

— Efe

Dear Efe,
No, I think the lever will hit the handlebar before stopping the bike. It will help to turn the adjuster screw on the lever to eliminate the free stroke and then do a super good bleed, but I think it will still be iffy. And overfilling won’t do it; you need a bigger piston and master cylinder to provide the additional volume of fluid you seek.

I don’t know what you can do about balance between the two. They both should move the same, so if you’re OK with having the same braking force front and rear, then that much balance is probably what you’ll get. I don’t know how you could get one brake to be more powerful than the other one, besides using two different rotor sizes.
― Lennard