drilling multi-hole cranks for easy pedal length options

Unicycles and Equipment
1

I’ve found a professional machinist (father of my daughter’s friend) who says he can drill extra pedal holes in cranks for me like:

What lengths do you guys think would be best and/or possible?

In preparation I bought new KH Moments Duals (165/137) (If I had read all the posts I probably would have taken KH Moments Single to not have the second chromoly insert and thus have more space).

My plan was 2 additional holes for the Dual 165/137s:
165/137/117/100 (20/17 : distance between holes)

With 20mm distance from the 137 home, because most threads here say that the KH chromoly inserts are 20mm apart.

But in the German forum, Cyc wrote that he would recommend a minimum of 21mm distance next to the chromoly inserts of the KH duals as he ran into problems trying 20mm (although if I understand right, he was able to do 20mm):

That would mean:
165/137/116/99 (21/17)

Any opinions/experience? i.e. is 20mm enough? I guess the problem is that if the distance is exactly 20mm and you space it at 20mm and are even 0.02mm too close then it could be a problem.

So should I try?
165/137/117/100 (20/17)

OK, I know it’s lots of personal preference, but what are some opinions on the lengths?
I’ve been commuting for about 3 times a week for the last 6 months with 127mm for my 16km x2 commute and I have great control and also feel like I could go faster, so I think I could go a little shorter as my standard length for commuting. A few weeks ago I did the commute 2 times with 100mm cranks, and although I was in sum slower as I was not used to the pedals and struggled a bit with mounting, it felt fine and I think with practice it would be ridable.

And for Munic I am presently riding 170mm (I found 150mm was too short, although that was months ago and I’m better not), but I think 165mm should not be such a big difference.

Therefore:

  • 165 for Muni
  • 137 as "bailout" for commuting, when I'm dead, (or for fast XC)
  • 116/117 for commuting
  • 99/100 for fast commuting and practice
Or is 99mm not really practical on a 36er? And/or should I go even shorter?

Or should I exchange the duals for KH Moment Singles so I can do something more moderate:
165-127-110 (38/17)
or
165-125-108 (40/17)

I will see the maschinist this weekend and am hoping to try and set up an appointment beginning of the week.

2

Moment are not ideal for such machining because of thread inserts
I made 3 holes cranks from Spirit 127/150 + additional hole at 110, this is perfect for my 36 Unguni

3

Telescopic cranks similar to telescopic crutches would be nice. A row of holes in the outer sleeve with a pressured pin on the inner sleeve which would engage into a selected hole affording different length settings.

I think it was Roger@UDC,UK, who designed the Nightfox unicycle with it’s telescopic forks. Maybe he would use his genius to design telescopic cranks.

:D:D:D

4

To get a telescopic crank that was sturdy enough and had little to no slop would be very expensive I would think.

5

…hmmmm titanium, that’d be awesome if it works! :sunglasses:

6

Yep, I think titanium would do the job. There might be some flex with titanium but in my case I’m thinking about road riding where there wouldn’t be much side pressure on the cranks.

If the sleeve tubes are a tight fit I think the slop would be minimal.

It would be great not having to plan crank lengths before going for a ride. It’s not just the hills. Sometimes a strong wind may suddenly start blowing in which case it would be nice to just adjust the crank length with ease.

Also, telescopic cranks would, in effect, give you more ratio choices than the existing Schlumpf geared unicycle hub. :smiley:

One set of telescopic cranks would be worth several sets of normal one hole cranks and I would be willing to pay a fair price for them. :slight_smile:

7

I have already given considerable thought to variable length cranks. It certainly would not be done through telescoping. Far too weak for the forces involved.

The effective length of the crank is the distance between the wheel axle and the pedal axle and there are other more practical ways to adjust this.

Mount an ordinary crank offset from the wheel axle by thirty millimetres. If it were fully able to rotate about this point, the effective crank length could vary by up to sixty mm.

In a practical construct the end of the wheel axle would limit the rotation because mounting the crank beyond the end of the wheel axle would result in a ridiculous Q factor. However the offset could be much larger to compensate.

The intermediate positions could be pinned but ideally I would like to come up with a continuously variable position. Better still one that responded automatically to pedal pressure that could be set with a lever.

I have contemplated such mechanisms. A forward only system with a sprng would be quite simple but the requirement to work on pressure in both directions seems intractable.

8

Are you talking about a crank that would vary length while riding or one that could be adjusted and locked in place without removing and exchanging them?

9

I originally started thinking of a system that could be adjusted without tools when stopped but the possibility of a dynamically adjustable crank length soon started to look like a very attractive possibility.

The stop and move system would be simple to build from ordinary cranks and some metal bar.

Take a crank and weld a short strong flat arm to the back if it. In the end of this arm, fix a stout pin cantilevered and parallel to the axle. I call the arm a “metacrank” for want of a better name.

Around the pin and rotating upon it, fit the crank interface of a hub and retain it. Now this “metahub” can be fitted with a range of ordinary cranks.

By rotating these cranks on the metahub we change the effective crank length. The extent of that adjustment is just a matter for geometry.

Another opposite arm (or even the original crank itself) on the metacrank retains a stay of adjustable length that joins the main crank. Altering the length of this stay changes the effective crank length.

Replacing the stay with small hydraulic rams could allow adjustment on the move if a suitable control system could be envisaged.

I expect the system also needs to synchronise the movement of both cranks. However the possibility of independently adjusting crank lengths could be considered. Perhaps it could be used like a trim knob to overcome camber thrust by having a longer crank on one side.

The control system could be designed to be pressure sensitive. Rider sets the pressure and the cranks change with the terrain to that pressure. Dampened of course.

A further possibility would be cyclic crank adjustment so the dead spots were traversed with effectively shorter cranks.

The change would feel wierd. Not only the length but the phase of the crank would be changing due to the geometry.

Lots to think about there. Anyone ever ridden with two different crank lengths? I would love to know what it is like.

One way or another I am sure this is a way forward in unicycling. I think 100 to 175 Zoom cranks could catch on.

10

I think for road riding, the telescopic cranks would be fine. Chromoly telescopic tubular cranks would be another option. Chromoly tubular cranks are already used by bmx riders. My BMX GT Performer came fitted with chromoly tubular cranks and they are strong and rigid and I’ve had no problems with them. :slight_smile:

11

The elusive adjustable crank discussion…

This comes up every once in a while, and there have been riders who made various versions. I cant find the one that has the knee joint as described by onetrackmind, but i’ll keep looking.

Here’s a telescopic set that Nathan Hoover was playing with:

12

Thanks for your post, jtrops. Nathans experiment is very encouraging and his telescopic cranks look really solid. :slight_smile:

13

it was 10 years ago and since this nobody launched a similar system in the market … Dual or Triple holes cranks is the easiest and lightest solution

14

Quite true. All of the adjustable cranks I’ve seen look very heavy. Not to say that you couldn’t make some of the designs lighter, but at what cost?

What I’d like to see is a triple hole crank with zero offset, and built in quick release pedal mounts. That way you could quickly swap between sizes without tools. The mounts would give a little offset to the pedal position, which would be consistent at each length.

15

back to topic

Interesting ideas here, and yes, I’ve contemplated the same thing, more of a long channle slot running vertical along the pedal with a click-in mechanism with fixture points every so often, maybe with a spring to pull out and move. Anyhow just ideas…

However, you guys haven’t given many real answers to my concrete project. The only real answer being here:

Due to a shipping mishap, I actually do have both KH Spirit Dual 165/137 and KH Moment Dual 165/137 cranks (i have to send one back).

Prior to bouin-bouin’s comment and based on some other older threads where people drilled Moments (either to save weight or for multi-length crank options), I thought the Moment was the better (or rather only) choice as it’s stronger and easier to drill as it’s straight.

Can you really safely drill the Spirits? Or are they hollow somewhere? I’m wanting to use them for road and 36er Muni, no trials, but they still have to be pretty strong.

And I know it’s a lot of personal preference, but still looking for opinions at to which holes for cranks lengths and why?

Moments with 165/137/116/99 (21/17)
or
Spirits with 165/137/120/103 (17/17)

16

Spirits only triple, no quadruple hole?

I just pulled out the Spirits and from about 20mm to 99mm (from the crank axle) it has a recessed area (on both front and back pedal sides). A hole at 103 will go down to about 95mm, so definitely into this recessed area. That doesn’t look good. So I think if I use the Spirits I can only drill one hole. If I did 165/137/117 then I could probably achieve my main goal, which is not having to switch cranks for my commute (currently on 127s) and offroad (currently 170s), as I could just switch the pedals from 117 to 165 (much easier than changing the cranks). But I won’t be able to play with really short (near 100mm) cranks as well (I have 100mm cranks but I don’t use them often, as if I’m tired then I become slow with many dismounts and so).

17

Shall you link to the video showing Martin’s cranks? The one that was posted on the french forum?

18

Pierrox, I would like Martin CHARRIER to present it here and to explain his concept, from next week we will be neighbor at office so I expect him to joint unicyclist.com

19

Spirit crank forging tool is unique for all Spirit lengths, this means you can drill a 127/150 Spirit and add a hole at 110 as 110 Spirit is existing

3 holes cranks with zero Q factor are already existing : see Mad4one model 110/125/150 cranks (nice but expensive)

20

That’s great news!
Say hi for me.

Guys, it’s a pretty cool testing prototype that Martin made. :slight_smile: