cmcb

Second Trek SL5 62cm frame kaput due to dropped chain. Trek recommend a different model (SLR or Checkpoint) with a stiffer frame/bb. Wondering if I should be looking at a different manufacturer. Any thoughts? I'm 6'6" 250# so shop mechanic suggests there's too much of me for the SL frame. TIA for any input you can offer.

FBinNY 

Stiffer how? Are you dropping chains because the seattube and BB are twisting under load? or simply looking for something stiffer to improve handling?

It's important to understand the EXACT issue you're trying to resolve before spending money on something that doesn't actually solve anything.

The reality is that unless you live in hill country, BB loads aren't actually that high, and the problem may actually be your riding style, vis. pushing high gears at low cadence, which will disappear completely if/when you address that. (plus your knees will thank you later).

Andrew R Stewart 

To follow up on another recent thread's content- The industry's response is to eliminate the ft der all together. Of course other features need to be improved when there's no ft der, like rings with wide/narrow teeth that can only be had in even tooth counts. It's like a dog chasing its tail. Andy (who rode 50 fairly flat miles today and will likely be VERY glad the bike has a granny tomorrow when climbing out of Springwater, NY)

alcjphil

So, "Trek SL5" What? which model are you asking about? I have seen many riders who I consider to be abusive. I don't think that your problems are because of a frame that isn't stiff enough, it may be because of unrealistic expectations of what you are going to be able to do while shifting your front derailleur

cmcb

Thanks for contributing to this post. I'm mostly repeating what the tech at the Trek shop shared from his conversation with Trek folk. Sounds like the bb is twisting under load. I live in SLC so do have elevation change. The most recent dropped chain was 25 feet off of a dead stop, seated, and switching to the large chain ring as I quickly moved off the line and gathered enough speed to warrant the gear change. I was in the middle of the cassette's gears as I did so.

alcjphil

"I live in SLC "
Please for the uninitiated, Where is that?
If failure happens while changing front rings it is often caused by not easing up for the half second it takes to make the change. Frame or bottom bracket stiffness has little to do with this

cmcb

My bad. SLC is Salt Lake City, Utah.

alcjphil

acronyms only mean something to someone who is familiar with them. I have included the city I live in in my profile in plain language. No acronyms

FBinNY 

Pardon, but I'm struggling to connect the dots here.

As you describe the event, it seems like a straightforward shifting under load issue.

How did you get to a frame stiffness issue, and how exactly did the frame break (if it did).?

Without very detailed and specific descriptions, we're shooting blindly on a dark foggy night.

alcjphil

I agree, I think that the OP is pretty vague about the actual failures they have encountered with shifting and how they happened. Bike dealers can sometimes be quite conciliatory to customers if they think that the manufacturer will approve a warranty claim even if they might believe that the customer was at fault. If they can pass the problem up the line they will do so to satisfy their customer. If the ultimate suggestion of the retailer is a "better" bike I would be very suspicious.

FBinNY 

It seems that the OP, Trek and the dealer are all agreed that it's a frame stiffness problem. If so, and if all the OP wants are suggestions for stiffer frames, I can't help him, though I'm sure there are others here who might.

OTOH- Based on the tiny bit of info provided so far, I'm skeptical that a stiffer frame will change anything. At this point I'm forced to wonder if this is a case of taking Pepto-Bismol for a headache.

CliffordK

Please explain what you mean. Is the frame broken? Did you drop the chain and tear something up with the dropped chain? Ripped off the rear derailleur? Broke the chain? Dropped the chain between chainrings?

There are a variety of drop stops and chain catchers available that help prevent dropping the chain on the inside. A bash guard will help prevent dropping the chain on the outside.

CliffordK

I have heard complaints that some carbon fiber frames from about the year 2000 to the year 2010 were considered overly stiff. So that might be something to try, and might be cheaper to experiment with.

You could look at a steel frame with oversized or double oversized tubing. However, they tend to make the tubing thinner with the bigger sizes.

You could talk to a frame builder, and see if they had any suggestions. Perhaps use oversized MTB or touring tubes on a road bike build.

If you do want to stick with carbon fiber, I'd look at some of the cyclocross frames.

rydabent

A RANS Stratus has the bottom bracket secured in place by no less they 5 tubes.

t2p

agree with replies above that suggest this might be other or more than a frame / frame stiffness issue

but for stiff frames for big / strong riders - back in the day there was one popular choice in our area : Cannondale

many of the big / strong mashers were on Cannondale frames ... both standard frame and the crit frame (with the even larger down tube)

70sSanO

As others have said there is too much going on to know the exact cause, other than the Trek dealer's response; which may or may not have analyzed the situation.

If I understand the latest situation, you basically accelerated two crank revolutions, (Estimate 7' circumference at 2:1 ratio = 28') and shifted under force and dropped the chain that (I'm guessing) tore up the chainstay. Generally modern drivetrains with ramps and pins to assist chainring shifting have eliminated a lot of issues, but trying to make a 16t jump (compact crank?), especially after multiple hard shifts may be more of a culprit.

Regardless of the actual cause, to be specific to your stiffer question, I'm not sure there is a lot of published data of bike frame lateral stiffness. Apart from that you may only get educated and uneducated guesses. I did find an article that provided some data, although it is 6 years old, it does give tested numbers for some frames. My recommendation is to pursue data driven recommendations and hopefully find testing of current frames. Possibly contacting "Tour" magazine for more current data.

https://www.cyclist.co.uk/in-depth/1...rame-stiffness

John