Anything I can do to Stiffen the BB of the Frame?
 

I recently got a new (to me) Ti Merlin Solis with carbon seatstays. Coming from my Al Fuji, the rear of the bike feels way more flexible. On a hard sprint, I can even feel the rear wheel camber. It's not slipping in the dropouts, but the entire rear triangle of the bike just flexes towards the drive side. So... I was thinking, is there anything I can do to stiffen that part of the bike? I was thinking of swapping out the Ultegra Octalink crankset and bb for a 105 Hollowtech II crankset and bb (I have the crankset on my Fuji, but it has a pressfit bb, so I'll need to get an external bearing bb for the threaded bb of the Merlin). What benefit would this bring, if any?

THe other day, I was taking off the crankset from the bb (it uses a self extracting 8mm allen bolt), and when I was putting it back on and torquing the crankarms onto the bb, I could see the bb flex towards the NDS. It flexed quite a bit (maybe 2cm). I'd estimate the torque I put on the bolts at about 35-40Nm, so that should give you some sort of a sense of the flexibility of the bb.

Your new mantra: "I am laterally stiff, but vertically compliant".:D

Just say it over and over to yourself each morning and you'll be fine.

Buy a frame built from Columbus MAX tubing.

Remeber your pushing on one side of the frame with al your 150lbs. The bike is designed to have your weight evenly distrabuted between both cranks and acting against each other.

Standing next to a bike and pushing against the BB area tells as much about the bike as bending a fly rod against the floor Nada!

I doubt the crankset and BB you have fitted will make much of a difference to the stiffness of the frame, which is the issue here if you're flexing the rear wheel. If you're sure the frame's not broken in some way, that's just how stiff that particular make of frame is.

A stiffer bottom bracket won't stiffen the bottom bracket shell and certainly won't stiffen the rear triangle. Save your money and either learn to spin lower gears or get a different frame.

BTW, you said the bottom bracket flexed 2 cm under torque? That's almost 7/8"! Are you sure you didn't mean 2 mm? What you were seeing is the bb shell moving.

I said 2cm. Not 2mm. It's probably less than 2cm, but it certainly is at least 7-8mm.

Oops. I just went out to the bike and put some torque into the crankset with the 8mm allen wrench and it only flexed a couple mm. I'll upload a video soon,

Swapping cranks will not solve the problem you're having.

If your rear wheel is a noodle swapping in something stouter might help, but I would try to borrow one before spending any money.

No fix, get another frame.

I sold my Fuji Ti because the BB shell had so much lateral flex that it would cause the chain to scrape the FD cage during hard pedaling. Quite disconcerting to listen to a hard metallic scrape with each revolution.

It's a Merlin Ti. Give me a couple minutes and you'll see how much it flexes.

Here you go:

Admit its Titanium, and that is a characteristic of the Metal..

should have Brought the Jedi, to say: "this is not the Bike you are Looking for"

N+1, next bike one of those with a massive stack of Carbon fiber sheets around the BB
and a frame designed to be stiff.


alternatively.. add More Titanium .. double tubes and add gussets.. More Welding..

It's not a bug, it's a feature!

This bike is N+1. I have a Fuji Roubaix 1.0 with full 105 5700 which I treasure, but I got this bike for basically free from my school's college counselor's husband. Our agreement is that I just have to provide them with computer assistance services in exchange for the bike. It was sitting in their basement for a while.

Buy a frame built from Columbus MAX tubing <-- have to agree but the op sure will complain about the weight, columbus max is not super light either.

What I would try is to get stiffer wheels, besides that there is no way to make a bb area stiffer unless you add carbon around it maybe.

I'm not interested in buying another bike/frame. I got this bike for free, it's my N+1 bike and I don't care all that much about it. Though it rides like a dream.:thumb:

Edit: Also, I'm not a weight weenie. With the Ultegra 6500 groupset, the bike weighs in at 19.5lbs. Not light at all for a titanium bike. I'm sure if I were to swap the 5700 group from my Fuji onto it, it would drop at least half a pound.

Yeah, it's flexy all right. I have three Litespeeds, a 1996 55cm Catalyst, a 2006 57 cm Tuscany and a 2006 57 cm Firenza. Two of them have the same BB6500 Octalink bb and Ultegra cranks you have and one (the Tuscany) has a Campy Chorus square taper bb and crank. None of them have anything close to the deflection your Merlin shows in the video even with my full weight on an 8 mm hex bit in a full length 3/8"-drive ratchet.

All I assume is that the Merlin must be an Ultra Light and those carbon seatstays contribute very little to the stiffness. Is it a big frame, say 60 cm or more? As noted, a different bottom bracket and crank aren't going to improve the situation. This bike is an excuse to learn to use a fast cadence in lower gears. :)

General Mechanics : want stiffer , add gussets and thicker material..

Butting is the opposite : tube center, not stressed, can use less material, so make it thinner.

In CO, Maybe Moots can add some Metal on it ..

It's a 51cm frame, not large at all. I'm 5'5" and weigh around 135-140lbs. I'm not a masher, I typically try to ride with a 90rpm cadence. I only notice the flex when sprinting (which I don't do often) and when I leave my house or start up from a stoplight (yeah, some cyclists do that :rolleyes:). If I'm already going 10-15mph and I decide I want to speed up to 20-25mph, the flex is noticeable, but not nearly as bad as when I start from a standstill.

To get an idea of my ideal cadence (not gonna use the triple on this merlin as any sort of a reference because I despise triples) : on a flat road with still wind, I ride with a cruising speed of 17mph with my Fuji and I like to ride in my 50t chainring and my 21t cog in the back.

i don't care what they say, TI is more flexible, in a real world bike application, than any other frame material i have had experience with (i.e. aluminum, steel, carbon).

i've got one, it's pretty smooth, but ya, it's flexy. mine is oversized, and overalized (so they tried), but it's still too noodle y.

OP: Recommend a good steel or aluminum frame from a reputable builder. Add some decent wheels and reuse the rest of the stuff off the flexible flyer.

It's not the crank set that's flexing when you exert force on the spindle via an allen wrench, or even the BB itself, but rather primarily the rear stays. Adding a stiffer crankset would actually increase the flexing, as more of the pedaling forces would be transitted to the frame rather than being dissipated in flexing of the crank arms/spider/chainwheels.

your wheels flex too

This is the nature of the metal the frame is made of, Ti.

I have been on two Ti bikes (different brands), and I could not believe the amount of flex in the BB area. Now, whenever I see a Ti or steel bike with pretty lugs, I just admire/ogle the weld quality of the Ti and the lugs on the steel (no, me want). I always always will prefer a stiff, but not teeth-rattling stiff bike, as I am 210Ibs. Carbon frames for me right now, until some other frame material comparable to carbon in ride-quality shows up.

The only way I'll ever consider a Ti bike is if it were to be custom, and even then the fabricator must be someone with a reputation for making stiff-enough frames.

Don't fret the flex. As you point out it only happens when sprinting, and just about every good bike flexes then too. It'll also flex if you charge steep hills. Either way, it won't affect handling or safety because you're usually out of the saddle at that time, maybe rocking the frame, and so any sense of precise handling is already gone.

Ther are stiffer frames out there, but not necessarily better. In many ways race bikes are like sausages, you can enjoy them more if you don't know too much about them.

BTW- if you want to increase the BB stiffness against pedaling forces, you can do so by buttressing the seat tube in the inside. One way is to turn an old seat post down to the ID at the base of the seat tube and jam it into place, possibly with some bonding. Seat tubes are generally single butted with thicker walls at the bottom, but I don't know if your Ti tubing is.

Buttressing the tube is something you want to do right because you don't want to create a stress riser at the top of the buttress. In any case consider this only if after riding it a while you decide that it is in fact more flexible that desirable. (Based on riding, not experiments).

External BBs in general are the stiffest out of all BBs. If you swap cranks with stiffer ones, you will get stiffer cranks, not a stiffer BB:bang:; that will put more stress on the rear triangle making it flex more.