chainstay bridge?

[hgrd]

Hello!:) New to this forum, so in short I am a 30 year old teacher from sweden who has a passion for cyclocross, mountainbiking, and drinking coffee in my armchair in between. I am building my second frame, which is for a cx-bike, and wondered if you would say that the bottom bracket I have eliminates the need for a chainstay bridge? This is what it looks like:
//Henrik

[pruckelshaus]

Theoretically, yes, but the cat who designed that shell will probably be in soon to smack me around a bit. However, it will also probably depend somewhat on chainstay length. If you're at 415 or less, you're fine.

[Curt Goodrich]

There is no "need" for a chain stay bridge. If you have no desire to ever mount a rear fender I'd leave off the chain stay bridge. On a cross bike the bridge tends to collect mud. On my race cross bikes I leave off the bridge and leave the bb shell web. If I use a bridge then I remove the bb shell web. There is no wrong answer though. Well unless you put in a bridge and leave the web. That just looks silly.

[e-RICHIE]

...to smack me around a bit.

[Will Outlaw]

There is no "need" for a chain stay bridge. If you have no desire to ever mount a rear fender I'd leave off the chain stay bridge. On a cross bike the bridge tends to collect mud. On my race cross bikes I leave off the bridge and leave the bb shell web. If I use a bridge then I remove the bb shell web. There is no wrong answer though. Well unless you put in a bridge and leave the web. That just looks silly.

I prefer the term "redundant."

But yeah, now that I look at it closely, it does look silly. Son of FrankenFrame is ready for tacking and will go without a bridge.

2012-05-11_09-39-50_874.jpg

I'm glad someone asked the question because I was wondering the same thing and just went ahead and stuck one in, "just in case."

[hgrd]

[IMG]dirty ritchie[/IMG]

Even if there is an atlantic ocean to keep me safe I might have to put a bridge on there just to sleep at night. ;)

From Curt's answer I take it that chainstay bridges are not primarily about adding strength/stability, but more about mounting fenders?

[pruckelshaus]

I always thought they added a bit of stiffness, especially with oval chainstays.

[e-RICHIE]

I always thought they added a bit of stiffness, especially with oval chainstays.

You're over thinking it atmo. On one hand, no one has ever ridden the same bicycle with/without the bridge - at least, it hasn't been done enough to make a conclusion. On the other, the bridge is a vestige that's part of the 22.2mm chain stay era and one in which the cross-member was not yet cast into the shell. According to my opinion, the word redundant is applicable in this thread because the web in the shell IS the bridge. If, in the future, the end user desires a method to attach a fender to the frame, by all means adding a bridge is a safety net. But I gotta say - it won't add stiffness to the point that it's discernible.

[Proparc]

I feel an extra bridge is imparting unnecessary heat into the picture. Especially in an area that sees a lot of heat.

[jamaris]

You're over thinking it atmo. On one hand, no one has ever ridden the same bicycle with/without the bridge - at least, it hasn't been done enough to make a conclusion. On the other, the bridge is a vestige that's part of the 22.2mm chain stay era and one in which the cross-member was not yet cast into the shell. According to my opinion, the word redundant is applicable in this thread because the web in the shell IS the bridge. If, in the future, the end user desires a method to attach a fender to the frame, by all means adding a bridge is a safety net. But I gotta say - it won't add stiffness to the point that it's discernible.

So not stiffness, what about ride quality according to this guy: Kirk Frameworks Custom Bicycles - Blog

Is that the same for steel frames or a ti exclusive thing?

[e-RICHIE]

So not stiffness, what about ride quality according to this guy: Kirk Frameworks Custom Bicycles - Blog

Is that the same for steel frames or a ti exclusive thing?

I am not sure what the question is, but my comment of one not knowing even if his frame has a bridge or not still has value. I mean, short of a visual, how would we know it's there or what it's doing? Has someone ridden the same unit with and without the cross member? More to the point of this thread, I don't think brazing in a cross member to a frame on which the bb shell already has one cast in makes sense. And I am reiterating that I don't think it adds anything except a mounting port for a mudguard.

[Pi Guy]

Has someone ridden the same unit with and without the cross member?

I think that I am going to have to give this a go on my next build. I'll report back. Although, the BB that I am using does not have a cast in bridge.

[jamaris]

I am not sure what the question is, but my comment of one not knowing even if his frame has a bridge or not still has value. I mean, short of a visual, how would we know it's there or what it's doing? Has someone ridden the same unit with and without the cross member? More to the point of this thread, I don't think brazing in a cross member to a frame on which the bb shell already has one cast in makes sense. And I am reiterating that I don't think it adds anything except a mounting port for a mudguard.

Thanks for the reply. I think I am deviating away from the original post too much. Sorry about that.

[e-RICHIE]

I think that I am going to have to give this a go on my next build. I'll report back. Although, the BB that I am using does not have a cast in bridge.

Cool atmo. When I transitioned from 22.2mm stays to fully oval versions in the mid 90s, the casting I used had no web or bridge. To my senses, there was no difference from style to the next. When cast versions of the shell with a bridge were made available by Y2k, I started using them. No changes felt or sensed. Certainly something must be different because you are adding a span between the stays, and also the necessary heat needed to install it. Would one of these cancel out the other? I mean - even if the cross member changed a dynamic such that you could say, "Yes, I love it with the bridge", is any of that tied to the heat cycles applied to what are fairly thin walled pipes to begin with? With a R.O.R. section at least the contour of the part is curved and the area covered by a bridge can be spread out.

[Dave Anderson]

I have actually tested this. I try to test everything to the extent that I can. I have deflection test data going back years for frames and forks, especially with respect to deflection/stiffness comparison data between many different brands and models of forks. I have tested several frames....the exact same frames...with and without a chainstay bridge. Whether cast into the shell or not, there is no (zero) difference in stiffness in a typical race frame, whether it has a bridge or not. On the frames I have tested to date, deflection measurements under a given load have been essentially the same with or without a bridge in nearly every case. Again, I am talking about a road/race style frame with the typical 408 to 415mm rear end. There is a slight difference when dealing with a very long rear end, like a touring bike with 460mm or etc. stay lengths, but only if the bridge is located rearward of the bb shell quite a distance, but even then, the difference is small and would not be perceptible IMO.

If you are worried about rear end stiffness, you need to look at the whole unit as an assembly. Chain stay diameter, shape, wall thickness, length, seat stay configuration, & etc. This is where its at.....not the bridge. As has been said...if you want to mount a fender, put in a bridge. If not...you don't need it IMO.

Just my $.02

Dave

[pruckelshaus]

Well there you go.

[Dave Anderson]

I should add that I have only tested steel frames.....

[hgrd]

Fantastic! Thank you guys! There is such a wealth of knowledge here.

[jon_norstog]

The bridge was one of the pieces Kieth Bontrager lost on his race frames. That was before he sold out.

jn

[prolix21]

We were discussing this at UBI today and everyone pretty much agreed that on most frames it makes no different. One thing that did come up was that re-aligning a frame with a chainstay bridge was quite difficult, but in a riding situation it probably isn't noticeable b/c there are so many factors that go into stiffness, bb deflection, etc.