DH bike sizing

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DH bike sizing
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flag JRadM
Posted: Jan 8, 2015 at 6:33 Quote
I want to buy a used downhill bike, but I have a question about what size I should be looking for. I'm ~6'1". Would I fit medium-sized bikes or should I only look at larges?

After searching in local classifieds for a couple months I have yet to find any good deals on large size bikes. Yet I've seen a few medium sized bikes come up.

Most recently, I saw a medium Brodie Diablo for $650 that looks pretty good. When I look at the specs online, the top tube length of the M is 585 vs 602 for an L.

Is that 17mm likely to feel significant?

EDIT:

Some additional thoughts. I was thinking the seat tube length wouldn't be critical on a DH bike when I first posted, but after looking at the picture I'm second-guessing that assumption. Since the seat tube slopes backwards, I'm thinking it would impact the cockpit room as well.

Unsecure image, only https images allowed: http://www.brodiebikes.com/bike_archive/04/pics/diablo.jpg
The seat tube is 480mm vs 520mm for M/L. So maybe the difference is bigger than I assumed?
flag Loki87
Posted: Jan 8, 2015 at 6:58 Quote
You should definitely go for an L.
If you are looking at older bikes, that´s even more reason to look at the larger size because those were smaller compared to todays standards.

You´re right about seatangle affecting toptube measurements. Those aren´t really applicable to dh-bikes as you normals don´t sit on them.
Google for "Reach" and "Stack" measurements. Those tell you much more accurate informations about the front center of the bicycle.
flag JRadM
Posted: Jan 8, 2015 at 8:15 Quote
Thanks Loki! Do you happen to know what kind of reach and stack numbers I should look for at my height? Or where I can find that out?
flag Loki87
Posted: Jan 8, 2015 at 8:50 Quote
There´s really no numer for that.
Chainstay length, head angle etc. all contribute to a bikes handling.
On a bike with longer stays you might get away with a little shorter toptube, whereas on a bike with short chainstays you need the longer TT to make up for the shorter CC.
Going by geo numbers is really only recommended if you have some experience and know what exactly you need/want.
For a beginner i´d suggest either a testride or going by sizing numbers. Maybe compare reach measurements to a bike you know you´re comfortable on.
If possible, always go for the bigger bike as long as you feel comfortable on it or the reach measurements are within a certain place around those you know will work for you.
Don´t go for a small bike for handling purposes. Smaller bikes feel only better handling as long as you don´t improve and will in fact hinder you progression by putting you in a cramped position and therefore hindering your ability to throw the bike around.
flag JRadM
Posted: Jan 9, 2015 at 12:39 Quote
Thanks for the info Loki.

I was recently offered what sounds like a great deal on a 2009 Rocky Mountain Flatline - in a medium frame size. The seller says it fits large for that size though and my google searching seems to back that up.


I also found this article which gave these geometry numbers:

Head angle 64.6
Seat Angle 69.2
EFF Top Tube 23"
Chainstay 18"
Bottom Bracket 14"
Wheelbase 47"
Actual Frame Size (effective seat tube length) 17"
Standover 29"

The article said the standover height measured at the center of the toptube is slightly higher than Rocky's given number at 30.25", with a BB height of 14.25". it also said the reach was 17.25" and the stack was 22.25".



Any thoughts on this bike? Might it fit despite the tagged "medium" size? It seems like a great deal!

photo
flag Loki87
Posted: Jan 9, 2015 at 15:51 Quote
I´d strongly recommend a testride.
The real problem with those measurements is, that they are, more often than not, just plain wrong.
I can´t tell how often i sat on a bike which claimed to be really large by it´s numbers, but really just didn´t match in reality.
Same thing goes for headangles etc.

Sadly i´ve never seen a rocky flatline in the flesh so i can´t give an informed opinion on this one.
It might as well be as you say and that thing is bigger than most L sized frames from other manufacturers.
I´m a little smaller than you and ride a bike with comparable numbers, so i guess you should be fine with the rocky IF those numbers really reflect how the bike fits.

What seems odd to me is the chainstay length. 18" is really fukn long!
And despite being the same reach and longer chainstays my bike is 70mm(!!!!) longer in wheelbase. Granted it has a 2.6degree slacker headangle, but that can´t account for 70mm in wheelbase.
That´s not really helping you i guess, but those numbers seem a little off to me.

I really do understand that it´s tempting to just jump on it when there´s a good deal and you just wanna start riding, but i can´t stress one thing enough.
The most important thing is the bikes geometry and how well it fits you.
I´d rather ride a bike with proper geometry and in a fitting size but with crappy components, than a great specced bike that doesn´t fit.

If in any way possible go and demo the bike you want to buy before actually buying it.
With used bikes i´d actually always recommend to not buy "blind", because there will be some sort of defect. be it worn out brake levers (lever bearings worn or spongy feel), worn out drivetrain components, suspension that hasn´t been serviced etc.
Look at the bike closely and maybe there´s something that warrants talking about the price again. Nobody would buy a car without test driving it, i can´t understand why so many bikers are willing to just buy used bikes without knowing what was done to them before.
flag JRadM
Posted: Jan 12, 2015 at 9:52 Quote
Thanks again Loki. This was a bike I could see in person. Since I'm a noob, I just didn't know if I should trust how I felt on the bike or if I should rely on the stamped size above all else.

I did go see the bike and... I bought it! Big Grin It felt good to me. I'm in the process now of tuning it up.
flag Loki87
Posted: Jan 12, 2015 at 10:23 Quote
Great, have fun with it! tup
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