Welcome to the first edition of MBUK TV's mountain bike tech Q&A, where we answer all your burning tech questions.
For this inaugural edition, we received questions on how to deal with a sticky piston to ensure optimal braking, how to install rim tape to avoid air from leaking and how to make sure the gears work when you replace a rear wheel when running SRAM AXS Transmission.
MBUK TV presenter Tom Marvin joins workshop manager Will Soffe to answer your questions.
Watch the video below or read on for our answers. If you have a question you want us to answer, leave a comment below this article or on the YouTube video, or contact us via our social media channels.
Dealing with a sticky piston
Woduk asks…
How do I make sure all the pistons on my four-pot brakes come out smoothly when I’m servicing my brakes?
Our answer…
To ensure perfect braking, Will explains all four of the pistons should push on the pads and retract back into the caliper evenly.
If one of the pistons is sticky, it will need cleaning, while making sure the other three are kept intact.
To get the one sticky piston to come out, Will outlines how different manufacturers offer various blocks, or you can use a pad spreader or tyre lever. Tom cautions against using something metal to avoid damaging the pistons.
Wedge a block in between the pistons that aren’t sticky and then use something to press gently on the final non-affected piston.
Squeeze the lever to encourage the sticky piston to come out and give it a clean, being careful not to push the piston out too far. Once clean, push the piston back in and repeat the process until all four pistons come out smoothly.
Rim-tape application
Johnny asks…
How do I make sure my tubeless tape is properly sealed so air doesn’t leak?
Our answer…
Will explains the aim of tubeless tape is to make an airtight seal for the air in your tyre to remain in there, so sticking it down on your rim is more of an art than a science. Firstly, make sure your rim is clean and free of contaminants or old sealant. Will says the best way to clean a rim is with disc-brake cleaner (or isopropyl alcohol).
Now, make sure you select the right width of tape for your rim. It should be wide enough that it covers the whole bed of the rim, including the well. It’s crucial you apply the right amount of tension when you’re installing it too – Will says this is the biggest takeaway tip.
To achieve this, Will advises bracing the wheel so it’s against your chest or the floor. You can then pull the tape nice and tight. Keep working it around the rim. Will says you want to finish up overlapping the valve stem, so you’ve got double the thickness there. Use a pick or something sharp to press through the tape for the valve – don’t use your valve to do this because it might mangle the tape (Tom admits he's done this in the past!).
If you’ve still got air coming out, the way to deduce if it’s a tyre or rim-tape issue is if it’s coming out of the spoke holes, it’s most likely the tape, according to Will. To solve this, you’ll need to remove the tape and start again.
When you’re installing a tyre, be careful if you’re using a tyre lever (especially a metal one) to make sure you don’t accidentally press on the tape, which will then push it away from the rim – you’ll be in a whole world of pain then.
Wheel replacement with SRAM AXS Transmission
Chloe asks…
I’ve got a bike with SRAM AXS Transmission. How do I make sure the gears still work whenever I replace the rear wheel?
Our answer…
Tom's tested plenty of bikes with SRAM AXS Transmission, so is well qualified to answer this question.
He explains when the rear wheel is off the bike, the rear derailleur is free to move forwards and backwards. When you’re re-installing the wheel and tightening the axle back up, you need to ensure the derailleur is in the right place. Push the derailleur forward and make sure the two lines on the Full Mount are lined up.
If they’re not lined up, the position of the jockey wheels will be different, which will affect your shifting.
Fork fettling
Sidney asks…
When I see myself riding on film, I can see that my RockShox Zeb fork is always in the middle of its stroke. Do I need to adjust the compression or the rebound damping on my fork?
MBUK's answer…
Will says there isn’t a simple answer without seeing you riding the bike, but there are some things you could try. Firstly, speeding up the rebound will help the fork get to that sag point quicker, and adding more compression damping will mean you’re adding resistance to the fork as it goes into the stroke.
But there is a compounding factor here, which is your volume spacers. If you’re running too many, it could mean the fork is sitting high in the travel, or if you’re running too few, the fork might be sagging.
Will and Tom recommend starting from the very beginning – set your sag, set your damping, go for a run and see how it goes.
Air-spring removal
Alex asks…
How do I get into my Fox fork’s air spring? The spanner flats are pretty shallow.
MBUK's answer…
Will explains the top caps are indeed shallow and soft, and they're anodised on the latest Fox forks, so you definitely don't want to scratch them.
He says the top caps are easy to scratch because many sockets have chamfers on them – you need to buy a socket with flat faces. Brands such as Abbey Bike Tools offer options with this.
Before these solutions existed, suspension service centres overcame this problem by grinding the chamfer off for a flush fit, but Will admits that's not always the easiest thing to do if you’re at home.
If you’re in a pinch, he recommends using some bench pliers, such as the Knipex Pliers Wrench. But remember, always let the air out of your air spring first. Then, when you’re replacing the cap, start by tightening it by hand to avoid cross-threading it, and then nip it up.
