Small wheels and large gears

[Cupples]

OK, so I've got a 11-40t 9 speed cassette ordered for the 20" rear wheel on my KMX. The rear mech is a Microshift, looks to have a medium or long cage. But I'm a bit worried about all that extra chain that's going to be flapping around whenever I'm in the lowest gears. As it is, there's a bit of slack in the lowest gears, and I get the distinct impression that the chain or chain tubes is smacking off the road when I go over speed bumps etc. Occasionally (chainset dependent), the chain gets thrown off the chainring.
So, I've been looking at maybe changing the rear mech to one with a clutch, maybe a SRAM GX, or similar Shimano or Microshift model. Any experiences with this sort of setup?
Andrew

 

[voyager]

check the maximum number of teeth difference the mech can handle along with the max sprocket size

 

[classic33]

Is the cage one of the long ones?

 

[Tigerbiten]

Do you have it the right way around ??
If you're in your lowest gear then you'll be on the 40t sprocket and the chain will be tightest.
If your on the smallest sprocket then you're in the highest gear and the chain has the most slack.
Or are you worried on how close to the road the chain will be when the derailleur is sitting vertically.

On my ICE trike, I use an elasticated strap which just supports the lower back chain tube about halfway down it's length.
The ICE trikes have a joint in the rear chain tubes due to need to fold and this is where I support the lower tube.
The advantage of the strap is it can angle slightly left and right to help line up with the gear changes
A small bracket from the top of the frame will then allow you to line a strap up with the chain tubes when you are in a middle gear.
If you don't use chain tubes then it's either to make a bracket to hold a sprocket wheel off a single speed type chain tensioner by it's post in the right place.
Or the use a bracket to hold a post on which a standard jockey wheel can slide on, the advantage of this is the jockey can slide sideways as you change gear for less chain wear.

The easiest way to keep the chain on the chainring is to just fit a front derailleur.
The more simple the cage is the better as they tend to be narrower.
That will help keep the chain on the ring even if it not attached to anything.
Just use the limit screws to line it up with the chainring.

Luck ........

 

[Cupples]

Doh, I meant high gears, of course! I've measured the cage as being 10cm in length, top to bottom, and it's a heck of a lot longer than the Campag mechs I usually use on my road bikes. There are no markings on the mech that I can find, so I can't say which model it is.
Tigerbiten - I have nowhere to fit a front mech! There's no pillar on the boom. I know the easiest thing to do would be to get a new boom with a pillar then whack on a front mech, but that's no fun! Should have said that I'm running a single chainring up front. That's a great idea about an elasticated strap to support the chain tube, though - I'll try that.

 

[Tigerbiten]

How big is your chainring ??
For the chainring the only thing I can think of is to make a couple of bashguard/chainguard type plates slightly bigger than your chainring and sandwich it between them.
Doing that will force the chain to jump further before it un-ships.
Look at the front of kiddies KMXs for what I mean.

 

[voyager]

Assuming the front boom of the KMX is steel ( most are I believe ) why not weld a stub of tube to make a front mech mount . Not a hard job , Then you wont have a problem - Jobs a good one !

Happy Xmas guys

emma

 

[Tigerbiten]

KMX trikes use an aluminium front boom .........

2 plates with holes in the corner for 4 bolts with the derailleur post welded to one of the plates is probably the easiest way.
It all depends on the spacing between the outside of the bottom bracket and the inside of the cranks if you can get it to fit.

 

[voyager]

Plate fitted between the rh cup and bb tube and use a lug mount front mech -

 

[Cupples]

Tigerbiten - Currently ok on a 42T, main problem was on a road 53/39t double. I've got a 56T single chainset with built in bash guard ordered from somewhere in China, hopefully that'll help.
Voyager - I've seen some front mechs that attach to a bracket which goes beside the BB - E-type front mechs, I think they're called - that might be an option, if I could get something to fit. Failing that, a dirty great bit of metal with a mech bolted to it might work too...

 

[Andrew1971]

Hi All
@Cupples A 56T you must have some BIG leg's to pull that size. When i had a 20inch rear wheel in my KMX my big ring was 48T that was about
25mph ISH in 7th gear. I bought the boom with delraileur post on £76 from KMX about 2 year ago.
I am upgrading my KMX Tornado with a venom wheel set 20in front. 26in rear. new triple crank set 22/32/44. and 8sp rear cassette 11/28.
3x8=24 gears. I never had a chain problem with mine must have been long enough for it to work.
You will find that you spin more on a recumbent than on a DF bike it feels more comfortable to me.
And Have you got GRIN yet.
Andrew

 

[Cupples]

I've got the cranks on, and the cassette ready to fit. I'm going to need a lorra lorra extra chain to get round these big rings!
By my calculations, the lowest gear will be near enough to my old 42/32 combination. The highest gear... Well, that should be a good but higher. I do have big legs. Sometimes I get them stuck in my trousers...

 

[Andrew1971]

Good going. When's the test ride
And I just finished mine today as well.

 

[Cupples]

Doh. Two snags. Firstly, I'll need a Goatlink (or cheap generic alternative) to allow enough clearance between the upper jockey wheel and the largest cog. Secondly, the bar end shifter doesn't fit in the end of the bars! Never even thought that there was a difference, but the diameter is sufficiently different between drop bars and flat bars (or trike bars in this case...) to mean that it ain't gonna fit. So, plan B is that I've ordered a 11-40t 8 speed cassette instead of 9, and will keep the Microshift twist shifter.

Now I have all this spare kit... maybe it's time to get a retro time trial bike going, with a big single chainring on the front and stick the 11-40 cassette on that, with the friction shifters on some flipped and cut down drop bars... hmmm......

 

[voyager]

buy an adjustable reamer and open up the i/d