- So welcome to Ask GMBN Tech.
This is the section on the channel each week
where we get to answer
all your mountain biking tech-related questions.
Don't forget you can add your questions in the comments
below and, of course, you can send 'em in to
the email address on the screen right now.
First up, we got a question from Sean Jones,
and this is quite a deep one to jump straight in on.
The world of bottom brackets is a minefield.
Can you give me a bit of depth explaining
the various types and the pros and cons of each?
(blows lips)
You ready for this?
There's a lot of information here so
I'm gonna keep this pretty concise,
'cause I do think there's gonna be
quite a cool video we could do with this later on.
First up, we got old threaded.
That was a 17 millimetre steel axle,
and it had a tapered edge on the end.
The cranks would bolt onto that
and then you'd have the bottom bracket in the frame.
All's well, but if your crank got loose,
basically, you would wear the end of the crank away
because it's made of soft aluminium from the steel axle,
and it would never correctly tighten.
It would commonly happen with your left hand crank.
Also, the bearings inside weren't the best
and after a while, they used to disintegrate.
Shimano, in fact, in this era,
provided cartridge bottom brackets,
so pretty much once the bottom bracket
had self-destructed, threw it away, put a new one in.
Next up, you're going to the regular
conventional threaded bottom bracket,
which is what most mountain bikes still have these days.
That's where the 24 ml steel axle,
the axle is pressed into one side of the cranks,
depending on the manufacturer whether that's left or right,
and you have outboard bearings.
The bearings are nice and big, they're fairly well sealed,
and they tend to do the job quite well.
But as with all bearings and bottom brackets,
the issue isn't necessarily the
size or position of your bearings.
It's always about where the mud and muck can get in.
The bearings are gonna work perfectly well
to the load of what you put through them.
Soon as they're dirty, they're gonna destroy themselves
basically and you will replace those bearings.
That's why we always tell you
not to jet wash bearings directly.
Next up is BB95.
This was developed by Trek, and this was a way of
using the same size axle by having the bearings
press directly into the frame.
Doing without the threaded cups that go in.
The bearings literally push into the frame,
a really neat and tidy system but the downside to this
are you have to have an extremely well-made
frame to have that housed in there.
Very expensive to produce.
You have to be very accurate.
There's a few other downsides, as well.
If you wanted to run a chain guide,
in the era when they came out, you couldn't,
because there wasn't a cup to screw it against the frame.
Later on, we got the ISCG mounts,
International Standard Chain Guide,
and that helped get around that problem.
Following that was Pressed Foot 86 and 92.
Essentially the same system as the BB95,
but instead of having the really expensive frame
to push them in, they got around this
by having the bearings in these
little plastic composite cups and they push into the frame.
It made the frame a lot cheaper to produce.
It didn't have a threaded bottom bracket.
It didn't have the expensive bearing surfaces in there.
And it works well but installation is key.
If you don't get it in right, they will creak,
and they do develop creaks over time,
and the whole pressed foot system,
they're very love or hate.
I don't really like them myself, I prefer standard threaded.
I like the way it works.
Over time, you know what's gonna happen with it.
BB30 was the next one that came up.
It's very similar to BB95, except has a 30 millimetre axle.
It's made of aluminium.
The axle's a lot bigger, stiffer, and lighter, as well,
than the 24 millimetre steel option.
The downside is the bearings are smaller.
Again, the bearings not necessarily
the problem in this case.
It was how they were put into the frame and, of course,
the contamination that gets to them.
Now SRAM have announced their new Dub system.
This is new crank axle and bottom bracket system.
The good thing about this, it doesn't require
different frames from what you already have,
unlike all the other variations,
you have to have a specific frame
for a specific bottom bracket.
They make a system for each of the systems
out there currently, BB30, regular threaded, et cetera.
And the idea is the bearings stay in the same position
no matter what bottom bracket system you have.
Bearings are outboard, and you have the crank axle,
30 ml axle, but actually it's 28.99 ml so just under that.
SRAM says that is so it can increase
the bearing size used in them.
The thing that's really key in the whole system
isn't the fact that it uses that bigger axle,
it's the fact that the bearings are ultimately sealed.
SRAM have directly addressed the problem
of contamination getting into those bearings.
We've not ridden one yet so we can't comment on it,
but it is a brand new system and
you are gonna start seeing that soon.
Next up, Christian Rahi asks, is it normal
that the front derailleur won't shift under hard load,
like going slow on a steep hill?
I've got a SRAM X5 set up.
Yeah, pretty much.
Gear shifting shouldn't really be done under load.
You should minutely let up as you change gear,
just enough time for the chain to jump.
If you look at the rear derailleur,
you can change under pressure a lot easier,
and the reason for that is the sprocket sizes
are a lot closer so the chain doesn't have to
jump as far in either direction.
But you can still damage your chain,
and sometimes your rear mech,
by changing hard under pressure.
With the front, you gotta bear in mind how big the jump is
going from, say, the granny ring,
which is the small one, up to the middle, or vice versa.
That's a significant jump.
If you're trying to change under load,
you're actually keeping the chain on there with your torque
more than the chain is trying to jump off to the next ring.
Just bear that in mind and really try to
make your gear-shifting as smooth as possible.
Next up is a bit of a suspension-related one.
From Steven Daniels.
According to the internet, the Rockshox Recon 32 ml
doesn't have a threaded top cap
for putting volume spacers in.
If I get a threaded Rockshox 32 ml top cap,
can I use tokens?
It's the same one as on your Scout.
Do you have any tokens in yours?
The fork on the bike behind me, no, actually, I don't.
I've ridden this bike virtually as stock.
I've swapped the tyres and the grips and stuff on it.
Not even set it up too much, ran it out there in Alicante,
and I actually, I do wanna put some volume spacers in it.
I had to look at this last night.
If you look at the pictures onscreen,
you're gonna find a bit of a dilemma here.
The way that Rockshox volume spacers fit is
they normally screw onto the top cap.
As you can see by this top cap,
it doesn't have any threads in it, but worse than that,
the actual design of this fork,
it's one of their more budget forks.
There's a very different air tube system on the inside.
It's got a big air tube that goes all the way up,
and the top cap pushes into that,
so there's no room, even if you fitted a top cap
from a Revelation or something like that, to do that.
And you can't go and put some oil in the tube
to decrease the air volume either,
because the way the negative spring
works on this particular fork.
I'm gonna go and see a friend with a lathe
and machine down some nylon tubing,
and I reckon we can make our own volume spacer
to simulate the size of about three volume spacers
that will push fit into that plastic top cap.
Bear with me, I'll update on the Facebook page,
the GMBN Tech Facebook page.
I'll let you know how we get on with that.
I'm loving this question here from Simon Plaickner.
I hear that some EWS pros, that's the Enduro World Series,
use one metallic and one organic brakepad in their brakes.
In theory, this should combine all the
advantages without having a noisy brake.
Have you guys ever heard of it?
Actually, I've been meaning to try this
for a long time because you're right.
You get supreme power with the metal brakes but
they take a while to heat up, to get that ultimate power,
so the resin, or organic pads,
tend to deal with that a bit better.
You could have a quieter brake and, low key,
where I ride, I find that a lot of the descents,
I'm not braking enough for the brake pads
to get heated up, and I do end up with squeal
in wet conditions.
Typically, when it's a bit icy or
one of those morning dew rides,
and it's nothing to do with the fact
that the brakes haven't bedded in properly.
It is exactly the fact that they're not heated up.
Yes, some riders do do that.
I haven't got a list of names for you off the top of my head
but I do know that Greg Minaar did that
at Cannes at the downhill last year.
In his brakes, he had Shimano set up
with one resin pad and one metal.
It's definitely food for thought there.
I'm gonna go and get the combination brake
and try it myself.
I'd love to hear it if you do it the same.
Let us know how you get on.
Next question is more of a maintenance-related question.
It's from Andy Booth MTB.
Hi guys, what is the best gear inner cable to buy.
I'm currently looking at Jagwire and they've got
six different options, ranging from standard
to slick galvanised, slick steel, pro-polished, and elite.
Please help me, where do I go?
Even gear inner cables can be a bit of a minefield.
They all vary.
First up, the basic one that's just a steel cable,
I would avoid those because they're not coated in any way.
They're not stainless, either.
They will corrode at some point, and also,
they're not as high quality in the way they're wound
as the other cables so they will stretch a lot easier.
It does depend on the style of frame you've got.
If you've got one-piece housing
running from your mech all the way to the handlebars,
in theory, it's very well sealed.
As long as you instal it correctly in the first place,
and you put a bit of light grease or some oil in there,
it should run well.
Of course, the more expensive cables have got a
higher quality coating on them
so it does mean they're gonna run slicker for longer.
But really, where those cables are better,
is if you've got unusual cable routing on your bike.
I had a GT Force in the past.
Really nice bike to ride but it had some
crazy cable routing around the bottom bracket area,
and as a result of that, I constantly had
to change the cables until I changed the actual routing
to get it right the way I wanted it.
If your bike has got unusual cable routing,
you can definitely maximise on reducing the friction
by using a cable setup like that.
Otherwise, get a fairly regular cable.
Stainless steel is fine, as long as you
instal them correctly in the first place.
Personally, I like to buy the big workshop reels
that have got a couple of hundred of those on there,
and they'll last me for years.
In fact, I've got one I've had
probably five or six years now and I'm only running out.
That's also a worthwhile consideration.
Good question here from Florian End,
relating to the YT Jeffsy I mentioned last time.
Yes, it's on a one by 11 system with
a 9-46 cassette from e*thirteen,
but can you make a comparison between
the e*thirteen 11-speed system
and SRAM Eagle with up to 50 tooth?
How can I calculate what the difference will be?
Firstly, let's have a look.
In fact, if I reach down here, I can bring out an Eagle one
and I can bring out an e*thirteen one,
so we can have a little look at these
and show you what the difference is.
For starters, Eagle cassette, ten tooth at the bottom,
50 tooth at the top, it's one piece design.
Absolutely massive.
Bit of a masterpiece of engineering, to be honest.
And the way it instals on the bike is
it literally pushes onto the XD driver body and screws on.
Job done.
The e*thirteen is a little bit different
in the fact that it's got nine tooth at the bottom,
so this is actually so small that it almost
overhangs the bottom so you can't fit it in a regular way.
What they've done is they've got a two part design.
You got a bottom part which is made of steel.
The upper part which is made of aluminium.
The aluminium part pushes on in
the same way the SRAM cassette would,
and has it's own independent lock ring
that goes on to hold it in place.
They provide the tool for that, that secures it.
Then, using a pair of chain whips
that you would normally use to fit any cassette,
the lower part actually slots on and guides its way on.
It's a really ingenious system.
It does mean that the common ones that you wear out,
the steel ones, you can replace them
separately from the rest of the cassette.
It's a really good system but being 11-speed,
it's not gonna have quite as much
range as the 12-speed SRAM, obviously.
I'm gonna run you though this.
The way it kinda compared is the e*thirteen says it's 9/46.
That's a nine, 10, 12, 14, 17, 20, 24, 28, 33, 39, and 46.
And then on the SRAM, with the 10/50,
you've got 10, 12, 14, 16, 18, 21, 24, 28, 32, 36, 42, 50.
Other than the jump with 42 to 50,
the SRAM is a lot smoother so the gear change
in between each one is a bit more of a seamless transition.
Bit more of a jump with the e*thirteen.
However, one of the cool things about the e*thirteen
having the nine tooth on the bottom
and the slightly smaller one on the top,
means you can use a smaller chain ring
to get a fairly similar range of gears.
The advantage of that is if you're bike
has got a long wheel base or it's got a low bottom bracket,
it gives you a bit more clearance.
You notice that a lot of riders running the SRAM Eagle
are opting for bigger chain rings because of that
huge fifty tooth dinner plate outer sprocket.
Just to make some calculations
so you can roughly compare the e*thirteen
with a 32 tooth chain ring compared to the SRAM with a 34.
This is based on a 27 and a half inch wheel on both sizes,
with a 2.3 tyre and a 170 millimetre cranks,
and you're doing 80 RPM cadence.
Fairly average for going on a flat cranking.
In the nine tooth sprocket on the E-13,
you'll be doing 23.8 miles an hour,
and that gives you 100.2 inch gear.
That means, for one revolution of the cranks,
you will move 100.2 inches.
In the 46 tooth sprocket, it's 4.7 miles an hour,
and that's a 19.6 inch gear.
With the SRAM, in the 10 tooth sprocket,
you're doing 22.8 miles an hour,
and it's a 95.8 inch gear, and in the 52 sprocket,
it's 4.6 miles an hour and 19.2 inch gear.
You see they're fairly similar.
Of course, you can chop and change your chain ring size,
and it's gonna have different effects
with different wheel sizes and cranks links.
If you wanna actually do this yourself,
gonna throw you to the Sheldon Brown gear calculator online.
This guy is a bike tech wizard.
There's barely anything this guy hasn't
figured out and got online on his page.
You can input all that information for yourself,
to suit your preferences and your bike,
and you'll get a bit more of an
accurate representation of what you're looking for.
Check it out, it's really cool.
A couple of shorter questions now.
Hernan Valverde.
If I'm on the trail using a 29 inch wheel
with a 29 inch inner tube, can I use a spare tube
from a 27 and a half inch wheel?
If so, what is the recommended tyre pressure?
You can use a 26 inch tube in a 29 if you really have to.
You can stretch it in.
Gotta bear in mind, they're made of rubber.
They will stretch and they will fit.
I recommend people, despite what wheel size they have,
to carry a 27 and a half inch wheel tube.
That way, it'll definitely fit the 27 and a half,
and you can downsize it to 26, upsize 29.
It's the middle ground.
It'll fit all of them comfortably.
Make sure you get that tube with a Presta valve, as well,
because if you've gotta lend that tube
to someone who's got a different rim, it'll always fit.
Presta valve will fit a Presta rim drilling
and a Schrader rim drilling.
If you have a Schrader valve,
it's only gonna fit the fastest Schrader drilling.
As far as pressure goes, it makes no difference.
It's a rubber tube, it will fit.
Ideally, long-term, you don't wanna
be running a 26-inch tube in a 29,
but a 27 and a half will fit all of them.
Hopefully that's helpful for ya.
And Jo Miller ask is it really worth fitting
a dropper post to a hardtail?
Hundred percent.
Obviously, it does depend on your budget,
but a dropper post significantly changes
the way a bike rides.
I've said it before, I'd go as far as saying
that a dropper post has more effect
on the way you ride a mountain bike than
having a front suspension fork if it's a hardtail.
I genuinely think you're in more control
if you're lowering your centre of gravity.
It's a safer and a better way to ride.
Not to mention, it's a lot more fun.
There's none of those moments where you come into a section
and you're a bit out of control,
your body weight is far too high and forwards.
You can preempt stuff, you can drop it on the fly,
just means your whole ride is a bit more fun.
I would totally recommend anyone to use a dropper post.
Finally, another good tech question
here from Jesus Zambrano.
I've read in several forums that
changing the travel of a fork by 10 millimetres
equals one degree on the head angle.
Is that correct?
For example, I have a Giant Reign with a 65 degree head
angle, according to the original geometry, with a 160 fork.
I'm using a Rockshox Lyrik with 170, that means the head
angle is slacker but how can I measure it?
Is it really 64 degrees?
That's a guideline, yeah.
It does roughly change it 'bout one degree,
and the best way to measure it out,
first, you're gonna need some sort of method to measure it.
I choose to use one of these.
It's a digital angle gauge and you, basically,
you centre this on whatever surface the bike is on.
You zero it on that surface.
Whatever angle you turn this centre to from there,
you'll get a very accurate reading.
Don't forget my phone here,
there's various apps available on the app store
for doing this same sort of thing.
It's not quite as accurate on a phone as
a proper digital angle gauge but
you will definitely be able to get a reading.
Knowing what your bike is to start with,
you can certainly work out if it's
a degree slack or two using one of those apps.
The way to measure it, some people would measure
on the stanchion tube or the fork.
That might be accurate on something
like this Rockshox Recon, where the dropout is
forward of the fork but if the dropout is in line,
it's quite often that the fork is
actually raked out a tiny bit so it's not that accurate.
The simple most accurate way to do it is to remove your stem
and measure on the steerer tube itself.
That is the most accurate way to find out
when you're steering, where your head angle is.
That is the end of Ask GMBN Tech for this week.
Hopefully there's been some good questions in there.
Please, don't forget to fire your questions in.
The email address is on the screen right now,
and of course, add 'em in the comments below.
I can already see 'em firin' in.
If you want some more great tech-related videos,
click over here for my Instagram selfies video.
It's all about taking a really good
photograph of you and you bike.
Pretty helpful video.
Some people love it, some people hate it.
Let us know if you do.
If you wanna find out about over-hoarding your headset,
that one's for the home DIY mechanics,
click right down here.
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