(dramatic music)
- Welcome to ask GMBM Tech, this is our weekly Q&A session.
If you've got any questions,
let us know in the comments below
but please use the hashtag ASKGMBNTECH
if you're asking the question
cause makes it super easy for us to find them
and add you on next week's show.
If you'd rather send us an email,
the email address is right there
at the bottom of the screen, hellotech@gmbn.com.
Get crackin.
Okay, so this one's from Gabriel Alexander,
Can you give a basic bike geometry introduction?
I have no idea how a shorter chainstay or a longer wheelbase
or anything like that, affects how a bike rides.
Yeah of course we can do that.
This is huge topic and I think it is one that deserves
it's own video, but for now, I'm going to go old school
and get a pen and a bit of paper
and do a very bad sketch of a bike
and just show you a few things in here.
I'm gonna tell you exactly what they are
and roughly what they do
and what it means to you when you're riding.
Okay, so, firstly let's look at the basic
geometry of a bike.
So, we'll do bottom bracket height first.
Now if I was gonna draw a horizontal line
it would go in between the wheel axles.
And you would measure, there's two ways of measuring this,
you get bottom bracket drop, which is from that line
down to the bottom bracket axle, itself.
And then there's bottom bracket height,
which is the height of the bottom bracket axle
to the floor, on that axis.
So that is your height.
Now you'll see bottom bracket height
referred to in manuals and spec sheets or websites.
The lower the bottom bracket is,
the more stable your bike will feel.
The higher it is, the more agile it will feel.
Pretty simple.
Now the chainstay length, that is the measurement
from your rear wheel axle to your bottom bracket,
that is this measurement here.
Or should I say, this measurement here.
Now the shorter the bottom bracket is,
the easier the front end lifts,
the more traction you directly get, however,
if it's too short, the bike can feel unbalanced.
If they have a nice long chainstay,
i.e.your axle's further out,
you're gonna get more weight on the front wheel
so you'll get more front wheel grip.
But there's a trade off to that.
It's gonna be harder to lift the front wheel,
so you end up having to have a higher handlebar.
Okay so that's getting a bit complex,
so I'm just gonna move this aside
so we've got another one I've already drawn here.
Now, so to go along with your chainstay length
you also have wheelbase,
so that is calculated from the rear wheel axle.
So let's draw a line here and the front wheel axle,
that is your wheelbase.
Now you might also hear front center used.
That is the measurement from your front axle
to you bottom bracket.
So you have front center, you have chainstay,
and you have total wheelbase.
The longer a bike is, the more stable it is.
The shorter it is, the more agile it is.
But also, it can feel very nervous when it's short,
likewise, it can feel real lazy when it's long.
Now the next two things I'll talk about are angles.
Now you hear about the seat angle and the head angle.
They're both very important to how a bike rides.
The seat angle is representative of,
imagine a line going from this axle here,
which is your fulcrum of the bike,
up to where your saddle is based.
Now that angle is calculated from the flat ground
that you're on.
So you get a steeper angle or a slacker angle.
Now a slacker angle, traditionally, you woulda seen
on road bikes and other bikes for comfort.
The slacker the angle is, the less that shocks are
transmitted from the ground to the rider.
However on a mountain bike this is not important.
What is important is having a steeper angle
because when you're climbing and you're going up
steep terrain, it means your saddle stays closer
over your crank for good weight distribution
onto the front wheel and a good climbing position,
which means you can really use your legs
to maximum advantage.
So the steeper the seat angle is, the better a bike
will climb off road.
Now the last one I wanna talk about is the head angle.
So this is affectively the angle from here to here,
running through here.
And it's measured in exactly the same way.
The slacker the head angle is, the more relaxed
a bike will feel, more stable it feels at speed.
The steeper it is, the more agile it will feel
and the more responsive it will feel.
But also when you're riding fast or steep terrain,
it can feel a bit nervous and a bit twitchy.
Now to get all of these elements correct together
can be very hard.
You can't just make a bike with a nice long chain state,
a steep seat angle and a good head angle because
you mess with the whole geometry of the bike.
Now the final thing I wanna talk about is not an angle,
it's a measurement called reach.
Now this is something that's commonly misunderstood
but it's actually very important.
And it refers to basically the length of a bike.
Now you get various different ways to measure
the length of a bike but the reach simply is
a vertical line that goes through the bottom bracket axle,
this is supposed to be a vertical line,
if you excuse my drawing here,
and it's the horizontal measurement that goes to
the center of the frame so between here and here,
give or take.
That is the reach and it's a bit more of a universal
measurement to work out the distance between here and here
which is more accurate than just measuring the top tube
because that only really counts when you're sat on the bike
because of the fact the seat angle varies.
This measurement will always be the same.
That's why the reach is a good one to understand
what works for you.
A 500mm reach is about perfect for me
and it's a good idea to sort of get an idea on what works
for you because then you can start comparing bikes
and try to understand what's gonna work for you.
Okay and one more just to summarize all these points.
You have your wheelbase, which is calculated from center
of the front wheel axle to center of the rear wheel axle.
You have your front center, that is a measurement from
the bottom bracket axle to the front wheel axle.
You have your chainstay length, which is your bottom
bracket axle to the rear wheel axle.
The seat angle, quite simply, is an imaginary line
between the bottom bracket axle and the top
of the frame there.
Now the misleading thing on some bikes is if you have
a frame design unlike this one that has an interrupted
or a curved tube.
Don't be drawn into looking at the tube too much,
you need to just basically do the plumb line to get that.
And then there's the head angle, which is the
steering axis up front.
And they all have big effects on how a bike handles.
But there's much more we can cover in a lot more detail
because there's further things to talk about like offset,
stack, trail, all those sort of things and we'll get
into that on a detailed video soon.
Byron Farrington.
Can you have a SRAM mech but a Shimano shifter?
Sometimes it is possible but it's not always that simple
so it does depend on a particular sort of speed transmission
you have because they're gonna pull different
amounts of cable.
So a rear derailleur on a 10 speed.
Shimano pulls 3.4mm of cable, SRAM pulls 3.1mm of cable
and for 11 speed it's 3.6 and 3.48 so as you can see
there is difference between the two.
Now the cassettes themselves have the same pitch
so you can use a SRAM cassette on a Shimano system
and vice versa, which is why you see brands like YT,
for example, specking E13 cassettes in place of
a SRAM cassette.
It's not about the brand you pick, it's about the speeds
that you pick.
However if you are really interested in mix and matching
your componentry like your SRAM and your Shimano,
with, in particular, with the shifter and mech combinations
like you're talking about, there's a really
cool little invention called the Jtek Shiftmate,
this is it on the screen here.
Now there's various different amounts and it basically
has a cam in it and it compensates for different
amounts of cable pulled so you could use a certain
SRAM shifter with a certain Shimano rear deraileur
so this is a really cool little bit of kit.
Now on screen now is just a screen grab from Arts Cyclery.
I'm gonna put a link to that in the description below
this video, you should definitely go over to their site
and check it out.
It's a really, really good resource for looking at all
the stuff like cable pull between different deraileurs,
different shifters, the pitch basically you get between
different cassette sprockets, spacings, all of that
sort of stuff so you can actually fine tune and work
out the exact ones that you wanna try yourself.
And let us know, actually, what you get on with.
If you end up using one of those Jtek Shiftmates.
I've never used one, I just know that they exist
so I'd be dead keen to find out if it works
and it works out well for you.
Good luck.
Next up's from Live Your Dreams.
Is it possible to use a tapered fork on a
straight tube, as in a straight frame.
If it's possible, is it safe?
Well it totally depends on the head tube size you have
on your particular bike.
Yeah you can use a tapered in a straight head tube
if you've got a 44 more head tube you can do that,
if you've got a 1.5 you can do that.
But it completely depends on your particular frame
and what headset options there are available.
I'm gonna throw a link in the description below
to the Cane Creek headset finder and it's a really,
really useful tool I always talk about and it means
you can input your frame and you can put your fork in there
and it will find a headset combination to see if that
is compatible in there.
But if your headset tube is inch and an eighth and your
steering tube is inch and an eighth straight,
you're not gonna find a way of getting a tapered
one in there.
And also one thing to note.
Never try and use ex tunnel headset cups on an
internal style frame.
I've heard some horror stories about people pressing
those in and they either crack or they crack and
damage the frame.
Not safe either.
Mm, this is a puzzling one from Kimmo P and I'm not sure
I know the correct answer, actually for this.
Dry or wet lube on chains when temperatures
are sub-freezing.
Well for starters I think there's a couple of schools
of thought on this.
So a wet lube, in theory, shouldn't freeze because
it's got oil in it anyway and also in theory the wet lube
itself would drive away any moisture that could freeze,
i.e rain or mist or anything like that.
So that school of thought would suggest that using a wet
lube should be fine in sub-zero temperatures.
But at the same time, a dry lube, really the only thing
that's a liquid in it at all is the solvent that gets
those lubricating particles into place.
That will evaporate or dry into a film and that
shouldn't freeze either.
However if you're riding in sub-zero temperatures
and that gets washed away, the moisture from the water
or anything that washes it away is likely to freeze.
So technically I think you should go for wet and I would
say wet lubricant is the best for that but I'm actually
really keen to find out from anyone that does ride
in sub-zero temperatures a lot because it's not something
we get a lot over here.
We get a few cold days of snow, perhaps two or three a year,
and it's enough to turn the whole country to a standstill,
which is pretty amusing considering half the rest of
the world has got insane levels of snow and everything
functions correctly.
So I'm really keen to find out what people use in
genuine sub-zero temperatures.
Maybe you're a phat biker from somewhere extreme
like Alaska, I don't know.
Give me some feedback guys I'd love to know from all
you people that ride in super cold temperatures.
Tell me what else is affected as well.
Is your tire pressure affected?
Does your suspension react differently?
Anything that reacts differently, I'd love to know
a bit more about it.
Alternatively drop me an email and hellotech@gmbn.com.
I'd love to hear from you.
There you go, there's another weekly Ask GMBN Tech
in the bag
if you've got any questions, let us know in those comments.
Don't forget to use the hashtag ASKGMBNTECH if you've
got any questions specifically for this show to go
on next week's show and if you wanna see a couple of
great videos, click down here for my sort of
Pandora's box of cool kit that I like to have in
addition to my tool kit, and click down here if you
wanna see Blake's bike that the crazy nutter raced
the Valparaiso Urban Downhill on.
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