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pro & con's of longer chainstays ??

preppie

Monkey
Aug 30, 2002
379
0
Europe
What are the advantages and disadvantages of a longer chainstay on a DH bike?

For example :
The IH Sundays have a 17.25" chainstay.
The Commencal Supreme DH has a 17.79" CS
The Cannondale Gracia DH has an 18" CS.

There's a big difference between these 3 top DH bikes chain stays, but why?

I personally noticed that my 17.8 CS DH bike is more stable at high speeds, but it turns slower than bikes with a shorter CS.
I also have to use a lot more 'body English' to get the bike into corners and through rock gardens.

Any info on CS length is welcome.
 

bikenweed

Turbo Monkey
Oct 21, 2004
2,435
0
Los Osos
CS length changes with how the suspension works, a single pivot with a rear-ward axle path will have a longer measurement from axle to BB at mid travel and when bottomed out than will an FSR type bike with a nearly vertical axle path. The Commencal and the Gemini are single pivots with linkage driven shocks, and the Sunday is a DW link. The Commencal and the Gemini have different pivot locations. That's why they all feel different when cornering, accelerating, puming, manualing, and bunny hopping.

Shorter stays are better for techy stuff, longer stays better for fast stuff. I like bikes that allow for adjustments to the chainstays, like the old M1's. I don't know of many current bikes that allow this, though.
 

thaflyinfatman

Turbo Monkey
Jul 20, 2002
1,578
0
Victoria
Basically longer = more stability, but stability is basically the opposite of agility. Personally I like to have them short (about 17" on a dh bike, or as low as 16.5") and run a relatively long top tube (well, reach) for a given wheelbase. If your chainstays are too short the rear end doesn't track very well, if they are too long then it's hard to throw around.
 

Biscuit

Turbo Monkey
Feb 12, 2003
1,768
0
Pleasant Hill, CA
preppie, I think you hit the nail on the head in your original question.

Long cs = stability
Short cs = manuverability

Depending on the suspension design, most people (IMO) will be better off with short cs. That is unless you ride mostly open, super fast stuff. Or are a really powerfull rider who can muscle a longer bike through a corner.

It becomes a design issue on downhill bikes because you need room for larger tires, and don't want said tire to hit the frame/seat when the suspension bottoms out. This is why most dh bikes have slack seat tube angles that intersect the frame somewhere on the downtube, not at the bb. To allow proper positioning and seat height adjustment, and shorter cs without the tire hitting the frame.
 

bizutch

Delicate CUSTOM flower
Dec 11, 2001
15,922
8
Over your shoulder whispering
i would also assume the single pivot c-dale & commencal HAVE to use longer stays b/c at max travel the wheel begins to move toward the seat mast and may contact it if the stays were the same length as a Demo or sunday?
 

Spunger

Git yer dumb questions here
Feb 19, 2003
2,258
0
805
On my M1 I notice a difference between the 2 settings, mainly in corning and in feel. The shorter chainstay setting I can it rails on turns. Not as stable though the rough stuff though. The longer setting isn't that much longer, but it helps with the stability of everything. I think bikes with long chainstays to begin with typically have a better running everything over approach to riding. Shorter bikes are more nimble and quick. Other than that I've noticed nothing else with the chainstay lengths. I just ride.
 

ohio

The Fresno Kid
Nov 26, 2001
6,638
4
SF, CA
I used to think shorter chainstays were better, until I got my Gemini DH. Now I love the longer chainstays. More stable at speed, more balanced weight distribution front to rear. The bike still corners unbelievably at anything faster than 10mph... I've ridden it as a trail bike and it was still pretty good, but the bike feels big at low speeds. At high speeds it just disappears... beautiful.

For a DH race bike, I won't ever ride anything below 17.5" again.
 

ohio

The Fresno Kid
Nov 26, 2001
6,638
4
SF, CA
bizutch said:
i would also assume the single pivot c-dale & commencal HAVE to use longer stays b/c at max travel the wheel begins to move toward the seat mast and may contact it if the stays were the same length as a Demo or sunday?
Nah, that's more of a problem on the Demo than either the Gemini or Commencal.
 

Jeremy R

<b>x</b>
Nov 15, 2001
9,513
577
behind you with a snap pop
thaflyinfatman said:
Basically longer = more stability, but stability is basically the opposite of agility. Personally I like to have them short (about 17" on a dh bike, or as low as 16.5") and run a relatively long top tube (well, reach) for a given wheelbase. If your chainstays are too short the rear end doesn't track very well, if they are too long then it's hard to throw around.
A real headscratcher when it comes to geometry is the demo 8.
It has short 16.8 chainstays, a regular size top tube, but a long wheelbase.
Compared to my DHR it has shorter chainstays, the same size top tube, but it is about an inch longer overall.
Weird, but the bike rips though.
 

Biscuit

Turbo Monkey
Feb 12, 2003
1,768
0
Pleasant Hill, CA
Jeremy R said:
A real headscratcher when it comes to geometry is the demo 8.
It has short 16.8 chainstays, a regular size top tube, but a long wheelbase.
Compared to my DHR it has shorter chainstays, the same size top tube, but it is about an inch longer overall.
Weird, but the bike rips though.
I havn't compared the numbers, but I would attribute some of this to head angle, fork offset (diff co's could use diff assumptions), and seat angle/position.

Look at a side view of both a demo and a dhr. Check out the seat angle, and where it intersects the frame relative to the bb (at the downtube). When measuring tt a horizontal line is drawn from the bars to the seat. Downhill bikes are generally set up with the seat lower than the headtube. So, differing seat tube angles, and seat tube locations will greatly affect where the seat-tube-line intersects with the horizontal tt-measurment-line. Which will affect the top tube measurement.

It's easy to think yourself in circles when trying to digest this kindof crap. Instead I just calc the front center (fc) measurment by subtracting the chainstay length from the wheelbase. If you take into account differences in head angle, you have a good idea of the actual "cockpit" size (bars to bb). And since you are usually standing when riding a downhill bike, this is the important sizing dimension.
 

bizutch

Delicate CUSTOM flower
Dec 11, 2001
15,922
8
Over your shoulder whispering
Spunger said:
On my M1 I notice a difference between the 2 settings, mainly in corning and in feel. The shorter chainstay setting I can it rails on turns. ....
that's because in the shorter setting, your BB is WAY lower and you're lower in the bike's center of gravity. :love:
 

mack

Turbo Monkey
Feb 26, 2003
3,675
0
Colorado
bizutch said:
i would also assume the single pivot c-dale & commencal HAVE to use longer stays b/c at max travel the wheel begins to move toward the seat mast and may contact it if the stays were the same length as a Demo or sunday?

I had the spring off my DHR last night, and the wheel comes pretty damn close to my seat. And my seat is pretty high.