Understanding Geometry

Jamieg285

I'm looking around for a new bike, probably a hard tail, but maybe full-suss.

I've been reading loads of magazine and online reviews and there's lots of talk about a bikes geometry- long this, short that, steep or shallow angles.

I've not yet been able to find anything that explains these, and what differences would be felt by going one way or another.

Does anyone know of a decent guide, or feels able to explain some of the basics?

Cheers,

Jamie

Anonymous

Right, basics...

Head angle. This, in it's simplest form is the angle of the forks, measured from the ground. So a 90 degree head angle would be vertical.
The steeper (closer to vertical) the head angle, the faster the bike will want to steer, but this can lead to making it feel twitchy at speed, over rough ground. It will also lose some tracking stability, since the front wheel will be more likely of trying to steer itself instead of holding a line.
The slacker the angle (furthest from vertical), then the more stable the bike becomes at speed, but low speed turning is reduced. This can compromise the bike's ability to navigate twisty singletrack and tight switchbacks.

Top tube length.
A long top tube length will give the rider more room to stretch. A more stretched position results in ultimately greater efficiency, and a more racy position. It can also place a rider's weight firmly between the wheels, making it quite stable.
A shorter top tube allows the rider to sit more upright, and to move around more on the bike to shift his/her weight effectively. However, it compromises the efficiency, and can make epic all day rides a bit of a chore. It does however, lend itself well to a bike that is intended to be ridden quite aggressively and thrown about.

Bottom bracket height
The higher it is, the more likely it is of clearing obstacles, but, it also raises the centre of gravity which makes it harder to change direction. Especially noticeable if you want to quickly turn left/right/left for example.
A Lower bottom bracket makes changing directions quicker, since the centre of gravity is lower, but increases the chances of the bottom bracket, cranks, pedals or chainrings of colliding with obstacles, or the floor.

Chainstay (or effective chainstay) length.
Shorter chainstays reduce the bike's wheelbase, but more importantly, allow the rider to shift his/her weight much further over the rear wheel. It can make a bike more responsive, and generally means a stiffer back end due to the shorter spans of material required. This can give the bike a "snappy" feel when pedalling - where it feels as if the pedals are directly connected to the rear wheel.
A longer chainstay increases the wheelbase, and causes the bike to feel more stable, particularly at very high speed. It also puts the rider's weight towards the centre of the wheelbase.


That's the important bits.
Anything else?

Anonymous

Oh, one more thing....

Seat tube angle.
It is common on short top tube bikes, to have a seat tube that is laid back, quite far from vertical.
The reason for this becomes clear when you raise the saddle to an efficient riding position - it increases the distance between the bars and saddle, making for a roomier, more efficient cockpit.
But, with the saddle down low, it returns to being a short cockpit bike, great for throwing around and mucking about on.

Jamieg285

That's some great info - thanks.

The next thing I'd want to know is what is considered long/short, steep/shallow, high/low for each of those?

Anonymous

There's no general hard-and-fast rules, as bike shapes change yer over year. Pick a bike for the kind of riding you want to do, and you can pretty much ignore the figures.

Generally, racy XC bikes have low bottom brackets, short chainstays, steep head angles and a long cockpit.

Bikes for a bit of everything have shorter cockpits, and slacker head angles.

DH bikes have exceedingly slack head angles, and long wheelbases, with long chainstays generally, but are still short tob tubes.

Like I aid, pick a bike that suits the kind of riding you want to do, and don't get hung up on numbers.

Ryan Jones

Don't be afraid of trying out a different cockpit setup before you buy, my cannondale felt rather long and racy in the head down, a*se up kind of way which would be fine for racing, however I'm a bit more aggressive and just by reducing the stem length and putting wider handlebars on, it completely transformed the character of the bike and I love it now !

I was inspired to do this by the reviews on here, so yes while they're not gospel, hints about cockpit setup may well be worth noting while weighing up potential purchases and more influential than half a degree/inch here or there

supersonic

Asso search the BikeRadar articles on bike fitting - myself and Steve Worland have wrote extensively on this topic.

Northwind

Ooh, hark at 'im.

One thing is, you can't really predict how a bike will ride from the numbers, not without an enormous amount of experience anyway. Bikes are the sum of their parts so sometimes a bike that looks steep and twitchy on paper won't ride that way. Riders like to nerd out a bit and there's nothing better for nerding out than a big chart of numbers but it doesn't always mean that much on the trails IMO. (some magazines totally obsess over numbers because it makes for easy articles too)

There's a load of perceived wisdom that doesn't always work out, for instance, you hear people say that slack bikes climb badly but I slackened my full suss off by 2 degress last year (headset cups) and it ended up climbing slightly better. No clue why, weight redistribution maybe or the slightly lower centre of gravity or somesuch.

Shaggy_Dog

yeehaamcgee wrote:

Oh, one more thing....

Seat tube angle.
It is common on short top tube bikes, to have a seat tube that is laid back, quite far from vertical.
The reason for this becomes clear when you raise the saddle to an efficient riding position - it increases the distance between the bars and saddle, making for a roomier, more efficient cockpit.
But, with the saddle down low, it returns to being a short cockpit bike, great for throwing around and mucking about on.

The downside to an overly slack seat angle is that it puts your weight quite far back when seated, making the front wheel lift when climbing steep sections. I had this problem on my 2006 Specialized Enduro until I fitted adjustable travel forks, but that's another story altogether.

mcduff

Hi....thought I would post in this thread as I'm seeking similar advice.
I bought a trail bike couple months ago to get back into mountain biking after 16 years away from the sport. The bike rides well going down rough terrain and trails. The problem I have is that it is so slooooow going up hills!
It feels like I have no power in the pedalling and it takes me forever to climb.
I find it hard to keep up with other riders on climbs.
I'm thinking about selling it and getting a faster more xc orientated bike.
My bike has a slack head tube angle 67 deg. which is great for going down rough the stuff but is this what's giving me poor pedalling effiencey?
Would a longer stem make any difference?

Advice is much appreciated thanks.

supersonic

What is the bike?

Northwind

Head angles have absolutely nothing to do with pedal efficiency, so don't worry about that. But, a bike with a slack HA is probably designed more with descending in mind than with XC mile munching.

What bike did you get?

mcduff

It's a Kona Buffer. Got a pretty good deal on it.
My bike from the 90s is really quick, it feels like the pedals are connected directly to the wheel when pedalling. It really takes off. Riding position is very racey on it though.
My new bike feels like I'm pedalling and I'm getting nowhere.
here are the specs:-
http://cog.konaworld.com/wp-content/uploads/2010/05/Buffer.pdf

Anonymous

mcduff wrote:

Hi....thought I would post in this thread as I'm seeking similar advice.
I bought a trail bike couple months ago ... after 16 years away from the sport.

The problem I have is that it is so slooooow going up hills!

It feels like I have no power in the pedalling and it takes me forever to climb.

I find it hard to keep up with other riders on climbs.

I'm thinking about selling it and getting a faster more xc orientated bike.

Advice is much appreciated thanks.

Honest advice - it's not the bike it's you. If you've not ridden MTB for 16 years, then of course you won't be keeping up with others, and your fitness will be holding you back a hell of a lot more than bike design.

Northwind

That's a nice bike. Not as light as a full-on racy XC bike but then it's more sensible allround.

Yeehaa is spot on. Thoooough, I would take a look at your tyres, if they're both 2.35 Nevegals... See what compound they are. Nevegal is not a fast tyre at best but if they're both 2.35 stick-e's that'll be like towing a fat lass around.

mcduff

I know my fitness isn't greatest but I have taken my old bike out on occasions and the difference is noticeable compared to my new one.
And being away from mountain biking doesn't mean I haven't done other sports or go gym.

Anonymous

mcduff wrote:

And being away from mountain biking doesn't mean I haven't done other sports or go gym.

Doing other sports, or going to the gym doesn't neccesarily translate well to being a powerful rider though.

supersonic

What is the old bike? Any weight difference?

mcduff

I'm not saying I'm a powerful rider, I never was. I just wanted to know why I can climb faster on my old bike compared to my new one.....anyways.
The old bike is a Marin and weighs about 1kg less, Supersonic. It has 2.1 tyres on it.
I will try changing the Nevegals and see if that makes any difference.

Thanks.

Northwind

Try just swapping your tyres from the other bike, see what happens. But if it does have a stick-e nevegal on the back, that's insane and something to sort out immediately.

mcduff

Not sure if they are stick-e nevegals. I'll have to check.
My fitness has improved since getting the bike but I'm still not getting up hills much quicker.
I just feel less tired after going up them.
Regardless of my fitness......would a racier position give me better pedalling efficiency?
And what tweaks can I do to achieve this?

Thanks.

tom_howard

Having experienced the stick-E nevegals, ill say its a fairly strong chance that its them thats slowing you down, they make the bike feel like you are riding through treacle!

What tyre pressures are you running on both bikes?

As for a racier position, long seat post, long stem, narrow bars and try barends. though this will affect the amount of control you have, and imo not for the good!

And if you want to get really racy, ride in heels and suspenders :twisted:

mcduff

Thanks Tom, I've tried the suspenders but they chafe too much when pedalling.

I took the nevegals off and put my old tyres on today. It has made a slight difference. Running 50psi.
My old bike is still a lot faster. It rolls faster and accelerates faster too. The difference is night and day. I suspect it's the weight difference between the bikes.

Bobhellen

50 psi is insane you should be looking at some were between 20 and 35psi off road and would only go to that psi with slicks for road use. it must feel like you are riding with solid rubber tyres