Mountain Bike Reviews Forum banner
1 - 13 of 13 Posts

·
Your killing my Strava
Joined
·
628 Posts
Discussion Starter · #1 ·
I am starting to compair more and more different frames. But I have no idea on what the frame geometry means from bike to bike. For example. the angle of the seat post? What does that affect. The angle of the head tube? Bottom bracket height, wheel base. Steering steam length.All these different measurements and I have no clue what any of them mean or what they affect. Any help?
 

·
It's the axle
Joined
·
1,765 Posts
I thought I saw a discussion the other day on where to find the answers. But I can't find it.

I googled, and found a couple of places, one of which I know is a good place to get answers, if not a bunch of great info. Moulton's site.

Dave Moulton's Bike Blog: Bottom Bracket Height

Then this place has at least the bottom bracket height info, and perhaps more.

Black Mountain Cycles: Slightly lower bottom bracket height...

Sorry I can't give more at the moment. I have a friend who knows it all.

Head tube angle, wheelbase, chain stay length. Those are pretty critical.
 

·
Registered
Joined
·
681 Posts
Here are some quick descriptions and gross generalizations about what I feel is the primary impact for each measurement. Just remember that no dimension exists in a vacuum. There are a lot of different things that interplay resulting in a bike's total handling. Here are some basics though.

Seat angle: This is as simple as that. It dictates where your seat will be in relation to the cranks.

Head tube angle is a little more tricky. Two of the primary measurements this impacts are wheel base (how far between the wheel hubs), and trail. Trail is measured by extending the headset axis of rotation all the way to the ground and measuring how far it is in front of the wheel's contact patch. Do a Google search for this. There are tons of good explanations of trail particularly related to motorcycles. A headtube angle lower in degrees ("slacker") will result in a longer wheelbase and more trail. A longer of either of those numbers will make a more stable yet slower steering bike.

Bottom bracket height (and its inverse, BB drop) seems to be a little more controversial for some reason. Bikes with lower bottom brackets are better for turning and yield a very nice handling bike where you feel more down in the bike. Higher bottom brackets make it easier to manual and eliminate hitting your pedals on stuff.

Chainstays: Longer increases your wheelbase. It also makes your bike less likely to manual when you're climbing up a hill. The downside it is harder to manual.

Sorry, that isn't a ton of detail but hopefully it can get you started.
 

·
Registered
Joined
·
2,459 Posts
When thinking about geometry ask yourself what kinds of riding you do. How do you want the bike to handle? Do you need lots of ground clearance. Do you need stability at speed? Or, do you like quick steering in single track?

Once you've answered these questions geometry begins to make sense. First and foremost you need to find a frame that fits correctly. A very big frame will almost always have a longer wheelbase than a small frame, simply because it is longer. It will often have slightly different head and seat tube angles to account for the larger frame size.

Don't worry too much about seat tube angle as your choice of seatpost and where you place your saddle allows for a huge choice of positioning on order to dial in the correct relationship between a$$ and cranks.

Headtube angle is more critical with regard to handling, and affects wheelbase too.

Chainstay length on 26er XC bikes is pretty much fixed in stone at 16.75". I don't know what the #s are for 29ers or fullies.

As mentioned BB height is a trade off. Both my bikes have low BBs and I love the handling that results at the expense of lots of pedal strikes and big ring damage.

Trail is an important # but one you have very little control over aside from picking a frame with a particular head angle. Pretty much all of the fork manufacturers use the same offset on their forks and this have very similar trail #s. The exception to this are custom made rigid forks and the G2 specific forks made for Gary Fisher G2 geometry frames.

Stem length is important in dialing in your correct fit and also affects handling. How much weight you have over the front wheel is one of the chief considerations here, but the type of riding you do is also crucial. An XC type position sucks for downhill for example.

You've got a lot to think about don't you?

Drew
 

·
Your killing my Strava
Joined
·
628 Posts
Discussion Starter · #5 ·
I ask because I am coming off a 07 Stumpjumper FSR and i just bought a 2011 Specialized Enduro...OMG what a difference how the bike handles. like night and day. I wanted to look and both frames measurements and see the differences and know what I am looking at...If that makes sense
 

·
Registered
Joined
·
681 Posts
I haven't ridden the latest generation, but I have owned some enduros in the past and they have always been sweet handling bike. What you are primarily noticing is probably the effect of the long wheelbase and low bottom bracket. It makes it extremely confidence inspiring and rail like nobody's business.
 

·
Your killing my Strava
Joined
·
628 Posts
Discussion Starter · #7 ·
My main problems are loose downhill flat (no berms) turns, It is like the bike doesnt want to turn and when I start to get it to turn the front feel like it wants to wash. Other that that it is one amazing handling bike. Also climbs really well
 

·
Picture Unrelated
Joined
·
5,123 Posts
My main problems are loose downhill flat (no berms) turns, It is like the bike doesnt want to turn and when I start to get it to turn the front feel like it wants to wash. Other that that it is one amazing handling bike. Also climbs really well
You just need to get used to the new bike. It takes time to find the right balance on a completely different bike. You'll be railing turns in no time. Have you seen the AM skills video with Fabien Barel? Good cornering tips there.
 

·
Registered
Joined
·
98 Posts
My main problems are loose downhill flat (no berms) turns, It is like the bike doesnt want to turn and when I start to get it to turn the front feel like it wants to wash. Other that that it is one amazing handling bike. Also climbs really well
Not on the same level as you, but I went from an '08 Trek 6000 to an '11 Specialized Camber Expert and had the same feeling. It felt like I was riding a huge, ridiculous machine that didn't want to turn and felt moderately unstable at the same time. After about 100 miles, I get on the Trek and it feels cramped and tiny, and really obnoxiously twitchy; and my Camber feels right at home.

It's really odd the difference geometry makes. I went and rode a Trek Fuel EX 7 while I was test-riding and the bike felt a lot like my 6000 even though it was a full-suspension like my Camber. Just a completely different feel between the two manufacturers.

I'm still a bit of a newbie, but for me it boiled down to just putting a bunch of miles on it until I got used to it. Now I love it!

-Eric
 

·
Registered
Joined
·
681 Posts
My main problems are loose downhill flat (no berms) turns, It is like the bike doesnt want to turn and when I start to get it to turn the front feel like it wants to wash. Other that that it is one amazing handling bike. Also climbs really well
Hmm, are you getting enough weight over the front? For an XC bike you naturally have more weight on the front. The Enduro geometry is more toward the DH end of the spectrum, so you will need to make a more conscious effort to weight the front wheel to get it to bite. If you're not used to it, it will feel like a much more committed (read: scary) position than you're used to, but you'll be able to rail turns.

I have no idea of your skill level. If you want to learn more, take a look at Lee's website. He has a lot of stuff on cornering.

http://www.leelikesbikes.com/cornering-drilling-the-basics.html

Also consider picking up his book. Its fairly easy to understand.
 

·
Your killing my Strava
Joined
·
628 Posts
Discussion Starter · #11 ·
Hmm, are you getting enough weight over the front? For an XC bike you naturally have more weight on the front. The Enduro geometry is more toward the DH end of the spectrum, so you will need to make a more conscious effort to weight the front wheel to get it to bite. If you're not used to it, it will feel like a much more committed (read: scary) position than you're used to, but you'll be able to rail turns.

I have no idea of your skill level. If you want to learn more, take a look at Lee's website. He has a lot of stuff on cornering.

http://www.leelikesbikes.com/cornering-drilling-the-basics.html

Also consider picking up his book. Its fairly easy to understand.
See thats the problem. I am not getting enough weight on the front tire for the fear of washing out and hitting face first. I come from a dirt bike background so the body movements are much more radical on a MTB than a dirtbike,
 

·
Picture Unrelated
Joined
·
5,123 Posts
See thats the problem. I am not getting enough weight on the front tire for the fear of washing out and hitting face first. I come from a dirt bike background so the body movements are much more radical on a MTB than a dirtbike,
Well there ya go, you can't expect a tire with no weight on it to bite. Keep practicing and you'll get the hang of it.
 

·
Registered
Joined
·
681 Posts
See thats the problem. I am not getting enough weight on the front tire for the fear of washing out and hitting face first. I come from a dirt bike background so the body movements are much more radical on a MTB than a dirtbike,
That's understandable. It is much easier said than done. If you're not on the front then it will not inspire the confidence needed to move forward and trust it. Its chicken or egg. On a motorcycle you use the controls to transfer weight. On a mountain bike you do much more with your body. Don't be afraid to really throw your weight around. I used to own an older Enduro and an SX trail and both of them responded well to being manhandled. This isn't a dirtbike, so you're the bigger one now. Get out there and smack it around. Show it who's the alpha dog.
 
1 - 13 of 13 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top