Chainstay philosophy?
 

I have a weird topic that I was thinking about during the break in March Madness. My wife has basically given me the green light to get a new frameset in the next year sometime. I have 58 fierte that I really like, but sometimes I have illusions about my abilities and long for a race bike. It was a sunny but cold day in Michigan, so we went looking at some different bikes. I have the green light to spend about 3 or 4 grand. I've been to the Serotta dealer numerous times and am considering a custom concours that is a little racier. We also went to the Orbea dealer to look at the new Orca. What a jaw dropper. The guy offered to sell me it in a 57 for 2340 dollars. That is frame fork and headset. I explained I was just looking at then we went to the Seven dealer. I was looking at the Seven's (most of which were signature series) and noticed on every single one had much beefier chain stays than the Fierte. I didn't think I was imagining things, but it seemed pretty obvious to me. My question is there a chainstay philosophy, and what do differently designed chainstays offer. The Seven chainstays were very stout. I weigh about 200 lbs on the nose, and got wondering if I would like the ride of a bike with a thicker set on chainstays. The Fierte's stays are on the long side (42cm) so I don't know if that is a factor in what I was perceiving. I also looked at alot of Cannondales, and other "race bikes' and those stays were a lot beefier as well. I know the chainstays are only one component of the bicycle, but do different manufacturers have different ideologies about the stays. I was reading some of the reviews on pez cycling and the author test rode a Seven (I can't remember what model) and they gave him oversize stays. He asked Serotta for the same, and they rejected his idea. They built him a bike with normal stays that was exactly what he was looking for. Both bikes were designed to be very stiff race bikes. Any ways back to my question. Do different manufactures have different ideas, and what are benefits or weaknesses of each design.

Hey,

This is a favorite subject of mine.

I'll say up front that I have no idea what bike you might like best. I'll leave that alone.

First I'll say something that you won't hear from many builders.......curving or bending the stays into any configuration other than straight will make the stays flex more. There are very few absolutes in life but this is one of them. Curved stays flex more than the equivalent stay without the bend. They won't flex a lot more......just a little. It depends on the type of bend and the degree of bend. One thing is for sure, the bent stay can never be stiffer due to it's bend. The biggest reason to bend a stay is to allow for better clearances with tire and chainring. This is a bigger deal on mountain bikes than road but true nonetheless.

In any given material, stay diameter is the biggest factor in determining the stiffness. The bigger the diameter, the stiffer the stay. The problem is that if you make the stay bigger in diameter you run into clearance issues with both the chainrings and tire. So..... most stays are about the same in this regard. One can use huge stays (Big Leg Emma) but then you need big, deep dents to allow for the clearance room and the dents go a long way to negating the gains in stiffness made by the larger diameter stay.

There is a current trend in frame design to control BB flex with a big-arse downtube. This may feel like a good thing at first and it will certainly help with front derailer rub but in the end it does not make for a stiffer bottom bracket. It's often overlooked but it's the chainstays that make the drivetrain stiff. A big downtube might be good for other reasons but it doesn't do squat to make for a stiff drivetrain. A Slingshot frame is a great example of this. It's simple when you think of it. The energy goes into the frame at the BB and goes to the rear wheel.........and the only thing between the BB and the wheel is the chainstays. This is one of the big issues in designing a good Ti bike for a big or heavy rider. Titanium is much more flexible than steel. The way to make it as stiff as steel is to make the diameter much larger. But this has it's own clearance issues. So they make the stays oval (making them stiff in the vertical direction and soft laterally.......not ideal) or they put in huge dents which has it's own issues. So most Ti builders compensate for the wimpy stays with a huge downtube. And like I stated above this has little effect on true drivetrain stiffness. This is one of the reasons that many larger folks feel that Ti bikes don't have the snap they are looking for and why it's fallen out of favor with many racers.

When pedaling a bike out of the saddle chainstays are asked to do a number of different things. They undergo compression which is easy for almost any stay design to deal with as the loads are low. They undergo a torsional (twisting) load which most stays deal with fairly well regardless of shape. Larger diameter helps a good bit with the torsion. The other thing that happens to stays is that they see a lateral bending load as the BB is pushed from one side to the other. This is where ovalized stays can get in trouble. An oval tube has a major diameter and a minor diameter. There is a pretty good rule of thumb that addresses how an oval tube will flex compared to a round tube. The oval tube will flex about the same as a round tube that has the same diameter as either the major or minor diameters of the oval. In other words if you have an oval tube that is 30mm by 17mm in cross section it will flex about the same as a 30mm round tube in one direction and about the same as a 17mm tube in the other direction. So if you think of oval chainstays you in effect get the lateral stiffness of a rather small round tube.

For all the above reason I feel that a round stay is best. You get the most bang for the buck in ever direction. You get good clearances with minimal denting and you get a nice stiff stay and drivetrain.

Wow............that's more writing than I thought it would be. Thanks for sticking with me.

Dave

Dave,

Super engineering description on the role of chainstays. Enjoyed reading it. Keep writing.

Back to March Madness. Go Bucks!!!

Thanks
 

Thanks for the education, waiting for my hat and my first ride on my Terraplane!

Henry

My Cannondale has pretty beefy chainstays. I always wondered how stiff the BMC SLC is.

snipped

Wow, I wonder what Ben would say about this. His marketing literature from years past indicated otherwise.

Regardless, great post Dave.

wonderful post on the subject.

My experience with a famous brand ti frame is that it's true, the material doesn't have that "snap" you're describing, but I only see that as an issue if I were racing crits, and it looks like I don't do that much anymore. Otherwise, I think the flex is pretty nice to have.

I remember in the 70's, lots of French bikes had round stays with all kinds of dents and crimps, and the whole purpose of going to oval was to escape that crimping. . .

there's such fiery arguments about straight vs. curved forks, it makes you wonder why we don't have similar interest in the disposition of the rest of the bike.

Thanks
 

Thanks for the info everyone. Your responses are worded much better than my question was. Thanks again

Concerning the BMC, they flex. They are not known for being real stiff. Comfortable ride though.

one other factor worth considering is the drop-outs. if the stays taper down to nothing around the rear wheel save a piece of stamped aluminum a fixed to the seatstays via an m4 bolt the fatest chainstays in the world aren't going to be any better than anything else. part of what makes a pegoretti or a serotta legend ti such wonderful bikes while cornering hard and descending fast is the meaty drop-outs.

in the mountain bike world through-axles are employed to stiffen up the rear ends on downhill and freeride bikes for the same reason.

obtuse

Word.

Dave

I think Dario Pegoretti said more or less the same thing when he came by Austin a week or so ago...in fewer words. :D

Round is good.

In Italian? :D

No, Round is great.

Being Round makes the world go Around. :D Yes

Great post Dave. Thanks.

Kevin