crank lenght and roll outs

hi,

When using a trials uni (tuni), what’s better to use when rolling out of a drop? 125mm cranks or 140mm cranks? I don’t plan on doing drops more than 2.5 feet.

Thanks

Rod,

I don’t have much experience with rolling out of drops with different crank lengths but I would have thought that the 140mm ones would be better. For both drops onto a decline and drops onto flat ground they’d probably give more power for pedalling out of the drop. Are there any disadvantages in using 140mm cranks for trials?

Andrew

Andrew,

I think my issue was with smoothness with the pedal rotation out of a drop. I thought the shorter cranks would be smoother. But after I started to think about this some more, it probably doesn’t really matter because the difference in crank length is only 15mm, or little more than a 1/2".

The additional length probably won’t be noticed on rolling out of landing a drop, but could be noticed when establishing a “platform” when trying to land on a small based or tilted object like a rock and then dropping or hopping somewhere else.

Hmm? Dunno,

But, pedal strikes would be the only think I can think of if you are riding perpindicular to a slope. But maybe there’s more.

I know nothing about rolling out of drops, but I do know a bit about crank length and the maths related to it.

The important thing is NOT the absolute difference in lenght (‘only’ half an inch) but the percentage.

Take it to extremes to see the principle: A 150mm crank is 20 mm shorter than a 170 mm crank. A 90 mm crank is 20 mm shorter than a 110mm crank.

BUT the 150 is 88% as long as a 170, and a 90 is only 82% as long as a 110.

So ‘only’ a few mm can make 10 - 20% difference to the leverage that you can exert on the wheel. It also works the other way - if you have a shorter lever to drive the wheel, then the wheel is a longer lever to drive you - which is why riding a uni with short cranks down a steep hill can be so enervating.

I’m not answering the question you originally asked, but pointing out that what you see as a small difference could, in reality, be a crucial difference.

i’d say long is better,

i’ve become very aware of this just recently as i’ve been riding my freestyle unicycle more,

when you need to roll out of a drop you need leverage not smoothness.

shorter cranks tend to kick you off when you land, whereas you can apply much more force to counteract the kick with long cranks

I like 140’s for tirals the more spread out base helps with your stability. I didnt really get into doing a ton of drops with my Lasco 140’s so I cant really comment on how they are for that, I didnt have any problems doing the few that I did. Doing drops and rolling out with 125’s isnt bad to do, but pedal grabs do seem to be slightly more difficult. Also it took me a while to get used to them while jumping, I lost 4 inches in my vertical with the intial switch.

I would say go for the 140’s on the trials machine.

Chex

I say go short - especially if you want to your cranks to survive big drops.

Because of this I’d say that for the same crank construction the shorter cranks will be stronger. For example if you put 125mm Bicycle Euro cranks on your trials uni they’d last a lot more drops than 150mm Bike Euros which would survive more than 170mm Bike Euros.

I find riding with 140mm cranks on a 20" uni clumsy and un-precise. Maybe its because I’m used to 127mm for 20" trials and 114mm for 20" freestyle.

I, like Checkernuts, had the 140mm Lascos, broke them, and switched to the 125mm bicycle euros because of strength issues at his suggestion. (thanks) I preferred the longer cranks fro pretty much everything. I remember rolling out better with them, but perhaps I have not fully adjusted to the shorter cranks. Maybe I just felt more stable since you don’t move as much along the ground as you roll out with the longer cranks.

At 2.5 feet and under I can’t imagine that you would break any cranks unless you’re absolutely horrible with your commpression and roll out technique. Well, you might break the Lascos… yeah, you probably would. They suck. You could get the Dotek and test them out for everybody

I would suggest basing a decision on your MUni crank to wheel diameter ratio. If you have say 150 or 160 on a 24", the 140s might feel clumsy as Tony suggested. But if you use 170 or 175, the 140s would probably feel right.

I can’t really think of many disadvantages of 140s. Speed is affected, so perhaps rolling hops might be diminished in their height and distance. I sometimes scraped the ground with the pedals when making tight leaning turns.

Don’t have an ‘all things being equal’ comparison, but longer cranks have helped me. On scarry-steap decents, it was harder to controll spead with my 150’s, the wheel spinning up sooner durring the drop phase and much more difficult to keep foot on pedal when pulling energy out at the bottom than with the longer 170’s.

On the other hand, I planted a pedal on the asphault when cornering hard evading Tommy while playing Duck, Duck, Goose -an important consideration.

-Christopher

i’d aggre with zim and tony, shorter cranks are definatly stronger.
so on a cotterless hub, in hindsight i’d prefer to use 125s.
but then idealy you wouldn’t have a cotterless hub on a trials uni.

since i’ve got a profile i’m going to stick with the 145s
in profiles bmx parts catalogue it says that if you ask nicely then profile will make you cranks in custom sizes,
like 135 or 125. if you realy wanted. but why?

140s doo feel realy shaky and imprecise when you’ve been on a freestyle uni all week, but if you get used to them then you’ll apriciate the extra leverage and the extra locking effect when you’re in the hopping position

sorry pal,but once again i must tell you that Profile does make custom sizes but NOT SMALLER THAN 145mm!!

they will not make 135,they will not make 125

they will not make them in the rain,they will not make them on a
train and in any place that rymes with plane…they just will not make them…

145mm is as short as there machienes can go

145mm is as short as there machienes can go

145mm is as short as there machienes can go

Everyone thanks for the input!

Nick,

I rode with a 170’s on my muni with a 3.0 Gazz and then 158’s with a 2.6 Gazz and the percentage(thanks Mike Fule) of crank length to radius was very close when I did the math.

Your remarking on the similar feeling to the above setup, helps me out greatly. I was hoping someone would bring up that point.

I am going with the 140’s. Estimated purchase is late December.
Maybe by that time there will be some reports on the Doteks. Otherwise, it’s the Monty Cranks.

As much as I would like to drop off the 4’ brick towers that separate parts of my front fence, I am saving the bucks and going with the monty wheelset. Quite frankly, I am afraid of doing a drop like that and not walking(riding) away from it. I need to be on my feet all day long for my job.

Rhysling - what wheel and tyre size are using? I guess it’s a 24" or 26". Riding 150mm cranks on a 24x1.75" tyre is different to with a 24x3" and different to a 24x3" with 170mm cranks …etc, etc

I must admit I haven’t done a lot of riding with 140mm cranks - my impressions were formed after having very brief ( <5min ) rides on borrowed trials unis.

The 150’s were on a 24x2.175, and the 170’s on the 24x3 Gazz- not an equal comparison. It may sound silly, but my legs are a fixed length, which I think makes universal comparisons difficult; the same forces we consider with wheel size-vs-crank length are applicable, as well as others…

Because I have gangly long legs with relatively low amounts of muscle, the extra distance my feet move through the sweet spot are important regardless of wheel/crank ratios, when sucking energy out of drops/roll outs and in resisting spin-up on the way down.

There are important points to be made for the posture caused by long cranks and the effect on stability in a drop, too; again, a positive or negative depending on your physiagomy, or a word spelled like that.

Nothing will substitute for a weeks-worth of agressive riding for coming to grips with the characteristics of new set up.

-Christopher