Drillium help needed

[M_Chavez]

Hi All,

Building a weight weenie bike for one of my weans and the crankset appears to be a particularly difficult part to put on a diet. When magnesium and titanium are not available, it is only natural to start thinking of drillium.

I have a spare frog crankset in the right size, but the cheap buggers have gone for a riveted steel chainring. I'm no drillium expert, but I'm pretty sure that you don't need so much steel even for an adult's torque, never mind a young kid.

I know that the topic of drillium can be controversial, but putting the chainring on a diet has got to be one of the safest modifications one can make.

Is there a general guide to drilling out chainrings, i.e. how much material to leave? Never get within X mm of the edges, etc? Or should I just eyeball it?

Thanks.

 

[ishaw]

Re:

No idea,but take a look at renthal rings as these are modern and drilled. Might give you an idea as to what they deem safe. That said, different material and thickness.

If it were me, I'd try to find a better chsinset with a spider that accepts 104bcd rings. What arm length are you after? Spa cycles do a good range of cranks for not much money and you might find what you need.

 

[M_Chavez]

Re:

Well, I've got two of these frog sets, so I decided to go ahead and drill them. Plenty of thick steel left - I'll give them a very good kick myself, and if they can stand a 110kg gorilla jumping on them, they'll be fine for a 25kg kid. Will chuck the drilled ring out at the slightest sign of deformation.

- Ditched the back of the chainguard (33g)
- Drilled out the front of the chainguard (20g)
- Drilled the steel ring (20g).

Yes, I still want the chainguard, as the 13g it is adding is not worth the safety risk of not having it.

Overall, 73g dropped, which is not a lot, but all grams do add up. More than swapping an alloy brake caliper for a titanium one.

Right, I'm off to drill the mudguards.

 

[M_Chavez]

Thanks ishaw.

I'm after 115mm at the moment, but will need a 125-135 solution soon. I know HUP and Stronglight do 130, but not sure if stronglights have a reduced q factor for shorter cranks. The good thing about frogs is that the pedals swoop inside, towards the frame, so the kid does not end up sitting on a bike like John Wayne.