drilling cranks, how risky ?

botfood said:

I am messing around with different ways to get my kids set up stoking
on a tandem. Found a great deal on a kit at chucksbikes.com for under
$100 including a nice AL 4-bolt BB shell, BB, chainrings, and 155mm
pedals.

155mm is probably ok for my 8yr old, but half the time my 5 yr old will
be on and would probably be better with 125mm cranks. I hesitate to get
the crank shorteners since they add a good 1/2" to q factor and I would
be afraid for their knees.....

what says the group regarding crank shorteners versus just drilling an
extra hole at 125mm and tapping for pedals? The kid cranks are solid
AL, and kids dont push that hard, so I'm thinking it would be fine.

second option is to forgo the kid cranks and move the whole stoker BB
and adult crankset up tot he kid BB shell to avoid the extra chain and
pedal set.... bu then I would HAVE to go with crank shorteners or drill
into a nice new Ultegra tandem crankset... probably not a great idea
later with adult use later? besides, arent the Ultegra cranks 'hollow'
at some point? comments?

If you were drilling cranks and tapping 'em for pedals for use by your
partner for tandem track racing, I'd say don't do it. But, your 5 year
old isn't going to put hardly any force on the pedals, so I'd say the
odds of him breaking a crank after you've drilled/tapped it is very
rare, if not nonexistent. I'd say go for it.

If there's room, drill it out, drop a helicoil in there and you're
good. Even your 8yo won't cause any problems. Unless you're standing
(which is hard on a tandem) you just don't put that much force on the
crank.

botfood said:

I am messing around with different ways to get my kids set up stoking
on a tandem. Found a great deal on a kit at chucksbikes.com for under
$100 including a nice AL 4-bolt BB shell, BB, chainrings, and 155mm
pedals.

155mm is probably ok for my 8yr old, but half the time my 5 yr old will
be on and would probably be better with 125mm cranks. I hesitate to get
the crank shorteners since they add a good 1/2" to q factor and I would
be afraid for their knees.....

Let the 8 year old try the 155 cranks. You may be surprised.

Quoted message said:

what says the group regarding crank shorteners versus just drilling an
extra hole at 125mm and tapping for pedals? The kid cranks are solid
AL, and kids dont push that hard, so I'm thinking it would be fine.

The typical crank's web is thinner than the end, and also isn't flat.
You would have to drill and face the location to provide a shoulder
for the pedal shaft to seat against. In the process, you're creating
a potential breakage point that the 8 year old might be able to crack
if he stands, and he's only going to get stronger and heavier with
time. I'd advise strongly against this course of action, no matter
what commercially-available aluminum cranks were chosen. It might be
possible to fabricate some suitable ones that had a widened and
reinforced boss at the 125 position, but I do not think any commercial
product is safe for this treatment.

Quoted message said:

second option is to forgo the kid cranks and move the whole stoker BB
and adult crankset up tot he kid BB shell to avoid the extra chain and
pedal set.... bu then I would HAVE to go with crank shorteners

Two sets of shorteners, actually; one to achieve 155, one at 125.
Doesn't sound like a stunning move, chain path problems aside. (I'm
pretty sure that you would need an idler at the lower BB anyway to
keep the lower chain run from hitting the chainstay, among other
problems.)

Quoted message said:

or drill
into a nice new Ultegra tandem crankset... probably not a great idea
later with adult use later? besides, arent the Ultegra cranks 'hollow'
at some point? comments?

Definitely the worst of the ideas. Tapping an extra hole in an
Ultegra would result in swift failure in my opinion. They're in the
group that's engineered near the limits of prudence in material
removal already, and installing another big hole where it's not wanted
would make them into a failure waiting to happen...and probably not
waiting long.

--
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botfood said:

I hesitate to get
the crank shorteners since they add a good 1/2" to q factor and I would
be afraid for their knees.....

If you measured the Q on the one-piece cranks you probably grew up
with, you wouldn't worry about it.

Since when have little kids been able to do anything to cause their
knees harm? You only have to reflect for a moment how lightly loaded
the little suckers' knees are before you see that this is a non-issue.
I've watched kids jump up and come down lock-kneed on concrete for
uncannily long periods of time. I could do that for five minutes (if I
made it that long) and be ready for knee replacement surgery.

Quoted message said:

what says the group regarding crank shorteners versus just drilling an
extra hole at 125mm and tapping for pedals? The kid cranks are solid
AL, and kids dont push that hard, so I'm thinking it would be fine.

It would be fine, but it's not as easy an operation as you make it
sound. You'd need to spotface the ouside of the crankarm to provide a
square and flat surface for the shoulder of the pedal spindle, and
you'd have to take care to drill and tap the hole quite parallel to the
BB spindle, or else the pedal will wag around as if it is bent. It
helps to have a vertical mill-- and the expertise to use it well-- to
do both of these things.

Quoted message said:

bu then I would HAVE to go with crank shorteners or drill
into a nice new Ultegra tandem crankset... probably not a great idea
later with adult use later?

You'd have to be on crack to drill a spendy crankset like that for a
kid stoker! What are you thinking? Look, just go to Danscomp.com and
get some of these:
http://www.danscomp.com/cgi-bin/hazel.cgi?action=DETAIL&item=452006
They come in sizes down to 140mm, and they only cost thirty bucks.
Drill and tap cranks if you like (and if you are competent to do the
job without bungling it) but for God's sake use some cheap cranks for
the purpose. It's not like a tiny child is going to need a stronger or
more refined crank than that.

Chalo

botfood said:

I am messing around with different ways to get my kids set up stoking
on a tandem. Found a great deal on a kit at chucksbikes.com for under
$100 including a nice AL 4-bolt BB shell, BB, chainrings, and 155mm
pedals.

155mm is probably ok for my 8yr old, but half the time my 5 yr old will
be on and would probably be better with 125mm cranks. I hesitate to get
the crank shorteners since they add a good 1/2" to q factor and I would
be afraid for their knees.....

what says the group regarding crank shorteners versus just drilling an
extra hole at 125mm and tapping for pedals? The kid cranks are solid
AL, and kids dont push that hard, so I'm thinking it would be fine.

I agree with other comments that this is unlikely to cause any problems
but that a large Q-factor is also unlikely to be an issue. When our
daughter was 4 I converted our tandem to kid-back use with a bolt-on BB
shell. It didn't come with anything else so I stopped by a local bike
shop and rummaged around for some old cranks from kid's bikes. Came up
with two that would fit with different lengths so I could swap them out
as she grew. Wasn't the most elegant solution since the pedal
threadings were the wrong way around (lock-tite solved that issue) and
the number of chainrings didn't quite match the cross-over ring on my
cranks. So we rode with a constantly varying phase which looked a bit
wierd to other riders but wasn't even noticeable to us.

We attracted quite a bit of attention at first when I'd drop her off at
kindergarten on my way in to work each morning and pick her up on my
way home - not all that many kidback tandems on the roads.

yeah... after more looking and thinking... the current plan is to leave
my shiny nice ultegra cranks alone and look for shorter (125mm) cranks
I can use on the kid BB. Even if they are 'wrong sided' for the tandem
since I need the spider on the left, I can locktite the pedals.

I dont think the triple would shift well after raising it almost 7
inches as the angle of the chain through the front derailler would be
way off where the chain usually passes through.

A bigger problem is that on close inspection i note that the adult left
sync chainring uses the outboard side of the spider because the inboard
side has almost no clearance to the chainstay... its a big fat oversize
AL chainstay. SO... I think I am going to have to put the kid chain on
the outside with spacers.

botfood said:

yeah... after more looking and thinking... the current plan is to leave
my shiny nice ultegra cranks alone and look for shorter (125mm) cranks
I can use on the kid BB. Even if they are 'wrong sided' for the tandem
since I need the spider on the left, I can locktite the pedals.

I'd second the thought that 155mm cranks are too long for an 8-year.
If 170mm cranks suit well 170cm tall person, than guess which is
OK for a 120cm 8 year old child. Our Nexus-7 kidback tandem
(http://konstantin.shemyak.com/cycling/Tandem)
currently has 135mm rear cranks and the stoker is 9 years old and
130cm long. She sometimes complains that the pedals are going
too fast (and there are toe clips), although for me our cadence
is always too low.

Konstantin Shemyak