Modern Mountain Bike Geometry Defined - Transition Explains Effective Top Tube Versus

Ivan

Ivan

Eats Squid
http://www.vitalmtb.com/features/Mo...-Explains-Effective-Top-Tube-Versus-Reach,841

I though this was worth a cross-post

This short discussion by Transition about geometry, specifically ETT and Reach measurements, is really good and is worth a read by anyone who buys a bike without the chance to test ride it first, or is just interested in this stuff like I am.

a small summary below:
"
One of the most common problems we see is over-emphasis on Effective Top Tube length. As a whole, bikes tend to be getting longer front ends these days. But that can be done with a longer ETT, a longer Reach, or a combination of both. Riders who look only at Effective Top Tube without considering Reach may be selling themselves short, literally. That's because seat angle has a huge effect on ETT, but zero effect on Reach. Watch what happens when we make the seat angle slacker"

"With a longer Reach dimension, riders can run a shorter stem while still having enough room in the cockpit, whether seated or standing. Short stems provide stability, which is a good thing when you're hanging on for dear life down a high-speed singletrack descent or pointing it through a technical section of trail. When climbing, your tire is less likely to get knocked off line by a root or ledge in the trail. We're going to make a blanket statement here that's intended to offend plenty of people... anyone who doesn't consider themselves a purely XC rider shouldn't be running a stem over 60mm on a mountain bike. That's being generous too. Really we think that max stem length should be closer to 55mm, maybe even 50mm, but we'll leave some wiggle room in there.
We see people defying this rule regularly - probably because their bike's Reach is too short so they put on a longer stem to provide enough room to move around. Or because their bike came spec'ed with a 90mm stem out of the box and they never changed it."
 
Just as telling: "There are no right answers here. We're opinionated about this stuff"

They talk about varying seat angles like it makes a big difference, most bikes I know have a seat angle between 72 and 75 degrees. 3 degrees, what's that gonna do to the length of your top tube?

Then they say: "Let's assume that you take a frame and increase the Reach by 25mm. The distance from the BB to the front axle will grow as a result and you now have a bike with a longer wheelbase. You can now shorten the bike's chainstay length to get the wheelbase back to where we started."

Oh sure, we'll just magically make the chainstay shorter, never mind the wheel diameter and the suspension links and the fat tyre. Bike designers have been striving to shorten chainstays by a quarter of an inch since Tom Ritchey and Gary Fisher rode down a hill on clunkers. Transition just do it, just like that. Wow. Except they dont, look at their geometry charts, their 26" bike has a 16.9" chainstay.

For every brainiac who says "everyone should have their bike set up like this" there is a person who is unusually tall, has unusually short arms, has uneven leg lengths or just rides somewhere different from you, who sets up their bike the way it works for them. You can only say it's wrong if you can prove that the only reason they can't whip your arse is the fact their stem is 15mm longer than yours.

Transition wanna steepen their seat angles? Great. If I ever ride one I'll just slide my saddle back on an offset seatpost so I can sit behind the bottom bracket where I want to be.
 
Slippy,

you claim 3 degree's in seat angle is no big deal, and then state as a conclusion that if you do have to ride a bike with a steep seat angle you'll go to the trouble of running an offset post and sliding your seat all the way back. So either it does make a difference or it doesn't. pick one.

You've successfully missed the entire point of the article which is about too much attention being placed on effective top tube, without regard for Reach.

FWIW, I have calculated the difference in effective top tube length is about 33mm between bikes with the same reach, same wheelbase, but seat angle at 72 or 75 degrees (for a stack of 600mm, which is about a medium sized bike). That is significant.
 
Last edited:
I agree, it is all how I want a bike set up, I am a pretty fat bloke and if I am on a bike with a slack seat angle and short reach all it wants to do is lift the front on climbs. If I was 20kg lighter it wouldn't be a problem as the centre of mass is lower and it takes less effort to accelerate a normal weight human, sure I could lose weight but I'd still be riding a shit bike.

People seem to think you need a long stem to put weight on the front wheel but really you just need a properly designed frame, something like a Transition/Mondraker/NS(and a few more) that has decent reach and a steeper seat angle/shorter ETT, that way you still sit in a comfy position and can corner seated without the front washing out.
 
My point was that the difference in seat angle can be acheived by simply altering your saddle and post set up. It's not a revolution. It's a minor adjustment. Everything in cycling is a trade off. Steepening your seat tube may improve your reach but it also puts you in a suboptimal seated climbing position relative to the bottom bracket. It depends on your priorities.

I just don't buy the argument there's a "right" and "wrong" geometry, it's horses for courses and individual riders. Transition's own marketing spiel said "there's no right answers here" and then proceeded to argue the complete opposite. I'm 6'2", I like a 75mm stem. How is that wrong?
 
I surprised that geometry doesn't change slightly from XS to XL frames. particularly around seated position, just making frames larger is not good enough.

As a long legged tall bugger a slack seat tube angle is horrendus for climbing, particularly if there is 300 of seat pole exposed, this is magnified for effective STA, which is becoming very common due to chainstays becoming shorter and clearnace issues.
 
The other thing that is implicit in their article is that ETT s are part of what someone might demand for fit, but because it's attached to the seatube it changes on a dually and between different amounts of suspension travel, so using ETT is very simplistic and only helpful when you are comparing like with like
 
I agree with 50mm stem max. If you need longer for climbing, it's probably because you have to slack a head angle, so wrong tool for the job. But right tool for everything else :nod:
 
Whilst ETT reasonable for road bike, reach and stack are the relevant numbers for mountain bikes. Hell some bikes give you effective seat tube angles - where seat tube is offset from bottom bracket and they work out the effective angle at some point. Problem with this is if you are at either end of the scale you aren't anywhere near the effective seat tube angle :frusty:
 
MARKL said:
Whilst ETT reasonable for road bike, reach and stack are the relevant numbers for mountain bikes. Hell some bikes give you effective seat tube angles - where seat tube is offset from bottom bracket and they work out the effective angle at some point. Problem with this is if you are at either end of the scale you aren't anywhere near the effective seat tube angle :frusty:
Click to expand...

Dead on Mark. Yet, the pages here and elsewhere are full of people after. Xx mm of ETT etc.

I know on mtbr heaps of people are always posting up their inside leg measurements and saying they have long or short torsos etc and therefore have to do xyz to the bike , BUT, when I've searched through medical literature and forensic pathology tables, there is really very little difference indeed between long bones of people who are 6'1" etc. The conclusion after trolling through the info is that if you have short femurs you are of short stature, it's amazing how little difference there is - height is a really accurate predictor of your long bones and vice versa.

However there is definately a big error margin in the measuring of things like inside leg.
 
pharmaboy said:
Dead on Mark. Yet, the pages here and elsewhere are full of people after. Xx mm of ETT etc.

I know on mtbr heaps of people are always posting up their inside leg measurements and saying they have long or short torsos etc and therefore have to do xyz to the bike , BUT, when I've searched through medical literature and forensic pathology tables, there is really very little difference indeed between long bones of people who are 6'1" etc. The conclusion after trolling through the info is that if you have short femurs you are of short stature, it's amazing how little difference there is - height is a really accurate predictor of your long bones and vice versa.

However there is definately a big error margin in the measuring of things like inside leg.
Click to expand...
My ex was a good inch or two shorter than me, and her legs were a good inch or two longer.
 
No Skid Marks said:
My ex was a good inch or two shorter than me, and her legs were a good inch or two longer.
Click to expand...

One of, or both are freaks ;)

Women do have longer femurs as a % of height - it's about 2cm longer I think. I was thinking within the sexes though (and races for that matter - nearly all data is within a racial genotype )

Each cm of femur predicts height +-1.5cm for a 180cm tall Caucasian female - accuracy is slightly less for male - that's 2 confidence intervals, so 96% of the population or thereabouts.

Anyway, when 2 people of the same sex and height provide inseam lengths that differ by 2 inches - it's extremely likely that the difference is a measuring error and not a true indication. ( muscles, joints and fat make measuring outside the body difficult)
 
Probably No skid marks + I found out about female femur length the same way - The doggie position.
This conversation was getting too serious .
where is Moorey ?
 
rowdyflat said:
Probably No skid marks + I found out about female femur length the same way - The doggie position.
This conversation was getting too serious .
where is Moorey ?
Click to expand...
That and not all females can get their knees behind heir ears.
 
Last edited:
slippy said:
They talk about varying seat angles like it makes a big difference, most bikes I know have a seat angle between 72 and 75 degress.

Transition wanna steepen their seat angles? Great. If I ever ride one I'll just slide my saddle back on an offset seatpost so I can sit behind the bottom bracket where I want to be.
Click to expand...

I would say the biggest argument to run a steeper seat angle on a dually is to compensate for how slack the seat angle becomes once you are in the sag.

If you start at 72° and add 25% sag you end up way behind the bottom bracket which can compromise climbing ability. On the other handif you start at 75/76° and run the manufacturer s recommended sag you will probably end up at 72/73° where you are right where you should be for technical climbing.

My opinion of course.
 
Last edited:
I very much like this article, and the discussion it causes. I'm looking forward to the next part(s)!

To me it makes a lot of sense to seat the rider just about in the middle of the bike for times when sitting down is required (steep seat tube angle), sharpen up the steering for slacker head angles (short stems), and give a lot of nut clearance (low standover).

I had a bike with a slack seat angle, I didn't like being pushed further back as the seat went up. I now have the opposite, and like it a lot - it puts me exactly where I always tried hard to get to on the old bike.

Of course, horses for courses, everyone is different, yadda yadda - but at least thinking about reach and stack is based on actual mountain biking, not just a crossover from road riding.
 
Just shows the traps we all fall into in making a narrative to suit our ideals.... I find it interesting how they make a whole discussion/argument founding in some sort of technical data or facts (great stuff, that's their profession)... But then they turn around and just throw out a complete bullshit opinion about stem length with no facts to back it up what so ever and we're supposed to just swallow that whole because they are the geo gurus..... (Argument from authority???? for any skeptics out there).....Haha.... Or maybe that's what part two will be......

How about just like reach/ETT when it comes to stem length we all widen (pun intended) our thinking and consider stem length, bar width and sweep to gain an idea of what I will call Effective Hand Position (EHP) relative to the steer tube or fork axle.....

Super short stem + super wide bars = hands behind the steer tube and further away from the axle....... How does that effect handling??...... Faster, slower, more stable, less stable, deader, more lively, wallowy, twitchy ??....... Hmmmmm....
 
Bluman is right its not that black + white.
Post about 2008 with 29ers Short stem wide bars v pre 2008 long stem short bars.Its the hand position v the pivot point which is the fork steerer which counts as the leverage ratio.
And then there is leaning involved in cornering .
Stating that no one should have > 50mm stem is just dogma.
 
I'm happy to go along with the whole 50mm as a max stem length, having your bars going in a big arc feels as stupid now as it did in the 90's.

I bought my current frame not really knowing a huge amount about reach numbers so I went off ETT and STA, sadly the ETT is 20mm shorter than advertised on the site. I could fit a 70mm stem but I'd prefer sell the frame and buy one with decent geo, 440+ reach, 73+ STA and a ~66deg HA, luckily these numbers are getting more common with hardtail frames.
 
You must log in or register to reply here.
Back
Top