chainguide advice for 48t on dh comp chain falls off alot

[Duc998]

Hi guys,

I'm stuck and need some advise. I've got a 48t Shimano Deore Chainring Outer on my DH Comp. If I just off the pavement onto the road half the time
the chain comes off, Mostly towards the outer side of the chain. Even droping down the gears quickly will sometimes result in the chain coming off. Annoying
when commuting to work

Now I know I need a chainguide / tensioner but stuck to what will fit on a DH Comp 03 model with a 48t. Also could having a 48t "Outer" ring be the issue? I say
this as I notice some of the teeth a shorter then others. Its not the wear on the chainring but maybe by design from dropping to the middle / inner rings.

Can anyone recommend what to get?

Pic attached...

 

[amberwolf]

Sounds like your chain is too long or your rear derailer spring isn't taut enough.

 

[Chalo]

Sounds like your chain is too long or your rear derailer spring isn't taut enough.

Or the chainline is bad.

 

[mdd0127]

Or your link bearings/bushings are bad and allowing your frame to flex.

You could copy something like this and extend the bracket for the larger chainring. I used lathed down skateboard wheels on mine.
http://www.universalcycles.com/shopping/product_details.php?id=39938&category=363

 

[full-throttle]

Your chainguide has the roller on the bottom only. You need top and bottom.

48t are rare. I've got a brand new, still in box 50t (didn't fit my bike) if you need. Does your Giant have ISCG mounts?

Also, using straight cut chainring helps. The one you got is designed for shifting..

 

[Duc998]

Tks for the advice all, I've attached a pic with the bike in low gear. can anyone tell by the pic if the tension is not right on the derailer? it does
seem a little floopy on the top but I'm no expert and unsure as to how it should be.

I'm afraid if top roller the chain will rub on the swingarm. If I can find a chainbash guard/guide for a 44t I'm happy to drop down. I think 44t is the lowest
I can go with kiwi's dropout adapters.

 

[Chalo]

Tks for the advice all, I've attached a pic with the bike in low gear. can anyone tell by the pic if the tension is not right on the derailer? it does
seem a little floopy on the top but I'm no expert and unsure as to how it should be.

It looks like it could stand to lose some links out of the chain. (That's high gear, by the way.) If you wrap the chain around the big ring and the big rear sprocket, determine how short you can make the chain and still close it, and then add a link (Inner and outer), you'll have the minimum practicable length for the chain. That should improve chain retention. But it won't completely eliminate the problem. For that you need positive retention like double guards or a dummy front derailleur.

Chalo

 

[HombreNeuvoElectro]

You have the same cranks as me and i am also looking for a solution to a larger chainring.

Maybe a 48T guard on the outside and N-Gear Jump Stop on the inside http://n-gear.com/

Does the Deore ring have a 104 pcd? Do you have any clearance issues with the 48t ring?

Full-throttle: The 50T did'nt because of seat stay clearance issues?

 

[Crash Machine]

The quickest way to get results is to buy a single speed chainring. No ramps on the chainring. Rocket rings makes a good one:I think its Origin8 now but they come in black or aluminum finish and have holes for 160 and 130mm mounting. Then find a good outer chaingaurd/bashgaurd Origin8 makes a good one of those too but I am not sure if it will bolt to your crankarms. Then finally the third eye chainwatcher for the inside but mine never falls off the inside either. I have the same setup as you and don't run any tensioners. Also chain length like someone else mentioned.

 

[full-throttle]

Full-throttle: The 50T did'nt because of seat stay clearance issues?

The guide is adjustable from 36t to 50t, it didn't fit to my frame as it interferes with the swingarm. 48t works fine though.

Mark is running a 56t on his Giant Comp

Notice the guide on the top

 

[GMUseless]

It looks like you've got a short or medium cage rear deraileur. With a 48t you probably need a long cage. That cage length is there to prevent the exact problem you are experiencing.

Also look at your suspension. Make sure your static load sag is roughly 25% of your travel. Regular full travel of the suspension can drop the chain as well.

But most likely that short cage rear D was originally matched up with a front cog in the 30's.

 

[Chalo]

It looks like you've got a short or medium cage rear deraileur. With a 48t you probably need a long cage. That cage length is there to prevent the exact problem you are experiencing.

Nope. The longer the derailleur cage, the larger the gearing range can be. The only reason for a long derailleur cage is to provide more wrap. But the shorter the derailleur cage, the higher the chain tension can be. So all else equal, the chain will drop less with a short cage derailleur that is properly set up.

The OP has a single ring crank, so he has no reason to use a long cage derailleur.

Chalo

 

[GMUseless]

Damn Chalo! Did you used to work in a bike shop man?

Now that you mention it, though the rear D's have a T rating, I think there was some kind of formula for that; like the sum of the range differences on both sets of gears.

But even so, isn't the rear D what's mostly responsible for chain tension? And wouldn't a longer cage provide a larger arc of motion and thus provide better take up as the suspension compresses and slackens the chain? I guess I always assumed the springs in the longer cages were tensioned higher to make up for the larger moment arm.

Also, isn't there a way to tighten the spring in the rear D to speed up the chain take up?

 

[Chalo]

Damn Chalo! Did you used to work in a bike shop man?

Now that you mention it, though the rear D's have a T rating, I think there was some kind of formula for that; like the sum of the range differences on both sets of gears.

Right. The capacity of a rear derailleur is expressed in teeth. So a long cage derailleur with 43t capacity would be able to accommodate (for example) a crank with 28-38-48 rings, amounting to 20t of required capacity, plus a 11-34 cassette, which needs 23t capacity by itself. With a single ring and the same cassette, you'd only need 23t of capacity.

But even so, isn't the rear D what's mostly responsible for chain tension? And wouldn't a longer cage provide a larger arc of motion and thus provide better take up as the suspension compresses and slackens the chain? I guess I always assumed the springs in the longer cages were tensioned higher to make up for the larger moment arm.

Not so far as I can tell. If it were feasible for a long cage rear derailleur to apply as much chain tension as a short cage without a weight or reliability penalty, I don't think we'd even have short cage MTB derailleurs.

There are some circumstances where a long cage might have better tension in some gears. For instance, a short cage that is pushed to its limit might have lower tension in its smallest sprocket combinations than a long cage one with the same gearing range, because the long cage wouldn't be anywhere close to its limit. But in most of the gears, the short cage would still have higher tension.

Also, isn't there a way to tighten the spring in the rear D to speed up the chain take up?

In the early days of the downhill MTB fad, there were some external "helper springs" you could buy and install on your rear derailleur. These consisted of a little arm hanging off the back of the right dropout with a small screen-door type spring hooked to it and to the bottom end of the derailleur cage.

You can also tighten the B-tension screw at the top knuckle of the derailleur, but slightly higher chain tension is only a side effect of doing that rather than the purpose of the B-tension screw. It is designed to adjust the clearance between the top pulley and the sprockets of the cassette as the derailleur swings across. Tightening it all the way up would add a little chain tension, but would also move the pulley farther away from the sprockets, and shifting could suffer as a result.

Chalo