Basic Mountain Bike Geometry And How It Affects Your Ride | Ask GMBN Tech

Basic Mountain Bike Geometry And How It Affects Your Ride | Ask GMBN Tech


(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, [email protected] 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 [email protected] 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. As always don’t forget to
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