boxrick

Hello!

So I have recently rebuilt my bike on a new frame and I have an issue which seems to have carried across:

When I'm pedalling, particularly in the large chain ring on the front, I feel but don't hear a grinding vibration feel through my crankset. This also happened with my previous frame.

This is a completely new bottom bracket
The wheels and cassette are brand new
The chain is different
Rear hanger is brand new
Cables and outers all new
Right hand ( rear shifting) STI shifter is new

So I'm thinking potential causes are:

Crankset
Pedals
Rear derailleur

Is there anything obvious I should be checking for our any ideas what this may be caused by?

dedhed

Try different pedals first

Troul 

big front, but what cogs are being used in the rear when it occurs?

boxrick

It is worst in the smallest rear cog, but continues noticeably all the way and lessens with the largest cog. This can be felt with the bike upside down and pushing the crank arms directly ( so its not likely the pedals ). It feels like it is coming from the derailleur / cassette then passing up through the chain so I can feel it during pedaling.

I am half thinking of switching out the rear derailleur to see if it helps, but any steer would be useful

Iride01 

It you feel it the most with in the small cog on the rear, then I'd tend to think it's normal stuff that some of us feel and others don't. It's just the chain links slamming the rollers into the cog as they have to bend and travel more as they wrap around that tighter radius.

I typically feel it when cruising at a reasonably fast speed putting out a fair amount of power. However if you are feeling this at low power and slow riding, and in all gears, then you probably have something else going on.

Clean cassette and chain seem to make it even more noticeable on my bikes.

boxrick

This is a brand new cassette and freshly cleaned chain....

I did swap over the derailleur out of curiosity and same issue, so this really has me stumped.

adipe

maybe derailleur hanger is bent...

Iride01 

I don't know what to make of your response. A brand new cassette and freshly cleaned chain are exactly the times I will feel this the most for the reason I wrote about.

Are you thinking it is still something else? Then give something other than more reasons that seem to point to my reason being correct. For certain, I don't even know how much you are feeling, so that might well make it something else.

Have you threaded the chain through the rear DR cage wrong and its rubbing on that little tab between the jockey and pulley wheel?

Bigbus

If the chain is routed correctly, I would definitely check the DR hanger. It sounds like the chain is being pushed ever so slightly to one side or another against the cogs but not enough to change gear. Other than that, I would have agreed with Iride01 about the clean chain and cogs, I feel that all the time and especially when I'm hoofing it down the road with force.

tFUnK

Crank spindle might be ovalized?

thook

is the chain correct for the speed of drivetrain? iow's, i can't tell if it's 10spd or what, but do you have the correct one for it?

smashndash

Afaik this is normal. If it's much smoother when you're in the bigger rear cogs, then that's just the chain meshing.

The real question is, have you ever ridden a bike that didn't feel like this? Was your bike ever not like this? If so, then maybe we have to investigate further.

EDIT: Your post says your rebuilt the bike onto a new frame so never mind lol. Not sure what to say anymore.

Double edit: my reading comprehension is trash. I guess it happened on your old frame too so my question still stands.

79pmooney

What is your cassette? Or more specifically, what is the smallest cog?

If you go to engineering references for least number of teeth for machinery driven by steel roller chains, you will see that bicycles go way below that number. We want our high gear ratios and don't want huge chainrings, so we settle for tiny cogs no respectable engineer would ever spec other than as an idler cog. The reason? Small numbers of teeth do not allow enough "sharing" of the load between them so you feel the shifting of the load from the last link on the cog to the one behind it. (They also wear both the chain and cog teeth far faster.)

I ride a fix gear that I have all the cogs for from 24 teeth to 12. Between the 24 and the 18 I cannot feel the difference but as I go smaller, the feel gets rougher each cog. The 12 is pretty crude. (I have a traditional fix gear hub. It's thread diameter means going smaller than 12 is just barely structurally possible and not feasible for an off the shelf item. I suspect an engineer had some say in the threading diameter and knew this.

Now we have 11s and even 10s. I suspect there are engineers shuddering in their graves. (Now the English time trialists used to love very high gears. What did they do? They mounted 60 tooth chainrings and stayed with 13 tooth cogs.)

As others have said or hinted at, one way to mask the roughness of very small cogs is to use a thick lubricant. (Dirty qualifies!) Really clean chains and light or dry lubricants do an excellent job of transmitting vibration and sound - the two are very closely linked.) I say dirty deadens the feeling. I will not say that dirty is good.

Edit: Chains vary. Small differences can mean they feel quite different running over the cogs, especially the smaller one. This doesn't mean necessarily that the rougher one is "bad"; just that it's noisy. On my fix gear I run the Isuze Eco chains because they are a rock solid, reliable chain. They run rough on cogs. But they last a long time and the cogs I run them over do too. By contrast the KMC equivalent runs far smoother, especially on the smaller cogs.

boxrick

Thanks for the replies everyone, I just swapped out the chain for a different one and a new hanger without any difference.

I think it’s just the fact that I can feel and hear the small teeth more noticeably as people have stated here. It all runs perfectly and given I’ve swapped basically everything now.. I pulled out my road bike and whilst less noticeable I can feel the smaller cogs there too.

For reference the cassette I just fit is a Shimano 11-34 my old one was a SRAM 10-42 but I could never quite get it shifting properly on this bike sadly..

dsaul

Google "chordal action"

oldbobcat

How often are you using the small cog with the big ring, and how fast (cadence) are you turning the cranks when you're in this combination?

One thing to remember is the chain only meshes with less than one half of the teeth on any cassette cog at a given time. So if you're using big-small with a 50-11, that chain is working with 4 or 5 teeth at any given moment. That's not a lot of contact. If you're driving piles trying to push this gear at 50 RPM, you're going to create a harmonic vibration. Tell us what happens when you try to match your speed turning a smaller gear, say 50-16 or 50-18, following the pedals around the spindle rather than applying all your power on the downstroke.

djb

I figure I can't be the only person who has done this......

but ya, new chain and cassette tends to do this, and of course the smallest cogs

Iride01 

I use my 11 cog a fair bit. The route I ride the most is constantly rolling terrain so when I'm going downhill I'm in the 52/11 or 12. Can't bring up my average speed if I'm only going to coast downhill. My cadence might be anywhere from 70 to 100 in the 11 cog if I'm not wanting to see what my max speed is that day.

On the smaller cogs the chain is meshing with fewer teeth altogether. The smaller radius, fewer teeth, the geometry of the chain roller as it engages the cog, and the changing position of the roller on the cog as it goes from little tension to a lot of tension just before it releases from the cog.

I'm not saying this is harmonic vibration. To me the above reasons are plenty for there to be vibration for other mechanical reasons from the chain just shifting position more so on the small cogs than the bigger cogs which have a larger radius and many more teeth engaging the rollers.

As I shift to bigger cogs this goes away gradually. I don't know for certain when, but probably around the 15 maybe 17T cog. If you want to go experiment and match speeds you can do that yourself. It's a normal thing to me, and I'm not really interested in proving whether it's harmonic or another type vibration.

oldbobcat

90 rpm on 50/11 gearing is about 32 mph. Are you really hitting 32 mph on rolling terrain?


Generally, I don't shift up to my 11 until I'm doing at least 32 mph and expecting to go faster. My knees really can't take pushing a gear that big any slower than 85 rpm. In my days as an ignorant tourist I used to chuff along in a 52/14 as soon as I got the bike rolling. I would experience a similar resonance, less pronounced because my chain was covering two more teeth, whenever I tried to push it. That would be until I blew my knees out. After a year of no extended riding I took up with a racing club where we'd spin for hours in the small ring, never using the big one until a serious downhill came up. My knees and my drivetrain recovered.


Try saving your 11 for bursts past 31 and then tell me if it's still doing it. If it is, I might concede the problem might be mechanical.

Iride01 

oldbobcat Yes I'm exceeding 32 mph in rolling terrain. I have to hold back some because of the limits to visibility at the bottom or intersections. Those speeds are downhill speeds for me or long straight areas with a -1% slope, not my paltry average speed. On the other route I go occasionally, I exceed 40 mph. But it's one of the longer downhills.

Are you talking to me or the OP? I'm not the one with a problem. The slight vibration I feel isn't causing me any issues. I see no reason to worry about it. And if the OP's vibration is the same as mine, I see no reason for the OP to worry about it.

I have at times wondered if the vibration is spokes fluttering in the wheel when at those speeds. Maybe even when the wind is at a certain angle to the bikes path. But it seems more in the drivetrain than the frame.

jccaclimber

I'm guessing this falls under "that's what roller chains feel like on small sprockets", but I haven't seen the condition of the front rings mentioned. My first guess was going to be the chain, but that has been replaced.

As for using an 11t rear, I can relate. When I was younger with better legs and lived in a flat area with strong winds I would regularly use my 53x11 on downwind and long downhill stretches. I remember swapping a 12-23 cassette for an 11-21 because I didn't need the 23T and was annoyed by occasionally spinning out the 12T.

boxrick

So, I believe there is possibly two problems at play.

Firstly the grinding I mention in the smaller rings which is more constant.

And a second “blip” of a feeling which it seems to do in all gearings. Probably around once per peddle rotation and feels more like a worn bearing. I’m wondering if this is to do with my bottom bracket fitting.

It’s a BB386 evo with GRX800 cranks fitted with one of these bottom brackets: https://www.wiggle.co.uk/rotor-pressfit-4624-bottom-bracket-steel

I’m wondering if my fitment was poor, I may have over pressed the bearings or used the wrong spacer. I would have preferred one of the Praxis adapters but they don’t make one for BB386.

Any thoughts around this?

Iride01 

Swap pedals with something first to see if it might be the pedal bearings needing grease. This also will be felt as a click once per revolution. Some pedals are easy to lube if you have the tool to remove the spindle/bearing assembly from the pedal body.

oldbobcat

Oops. I'm sorry.