P-38 build

DIscuss anything relating to non-standard cycles and their equipment.
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n0ct0
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Re: P-38 build

Post by n0ct0 »

I’ve taken crank length into account going from 170 to 155 and adjusted the gearing by the same percentage. Of course your cadence has to go up to compensate also. When I’ve changed by just 5 mm on bents and uprights I’ve just left the gearing, although I did definitively feel the difference, both in the gearing and the amount my knees had to go through rather than my hips.
UpWrong
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Re: P-38 build

Post by UpWrong »

n0ct0 wrote: 28 Apr 2022, 7:53am I’ve taken crank length into account going from 170 to 155 and adjusted the gearing by the same percentage. Of course your cadence has to go up to compensate also. When I’ve changed by just 5 mm on bents and uprights I’ve just left the gearing, although I did definitively feel the difference, both in the gearing and the amount my knees had to go through rather than my hips.
Yes, I think I'll need to adjust the gearing since the current lowest gear is barely low enough. Hmm, and the RD won't handle bigger than the current 34T sprocket so I'll need to drop the granny ring from 24T down to 22T. I have a 9-speed Deore m591 with 22/32/44 rings which will do the trick. This currently has 170mm cranks. I might get them shortened to 140mm which apparently is about the shortest most people can go without it feeling weird and needing acclimatisation. Also swapping from 160mm to 140mm is more likely to make a noticeable change to the hip angle.

EDIT: trigonometry suggests the change in the hip angle of 20mm drop at the top of the pedal stroke is about 1.3 degrees!!
UpWrong
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Re: P-38 build

Post by UpWrong »

Darn, the Deore cranks are too fluted for shortening. I could buy some Spa cranks and get them shortened but they are 74/110 BCD so I won't be able to fit a smaller granny on them than the one I currently have. Looks like I'll have to adapt to the closed position, or lay the seat back more. I think I'd struggle with reaching the ground comfortably if I do the latter. I could pinch the 150mm cranks from the Paseo but again those are 74/110 so it raises the gain ratio. That's the problem with a 700c drive wheel, high gears.

I could get a hanger extender with a view to fitting a 9-speed 12-36 cassette, though that will lose 9% at the top, though if I'm spinning quicker that will make up for it.

So, get a cheap right hand triple crank from Spa and have it shortened to 140mm along with a LH crank from my spares box. Get a hanger extender, and fit a 12-36 cassette from my spares box. I may need to add a chain link
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n0ct0
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Re: P-38 build

Post by n0ct0 »

Welcome to the crazy world of recumbent gearing. 🙂

Although to be honest most of my bikes are undergeared.

Every now and again I look at a Rohloff, right up to the point where I see the price….. 😮
UpWrong
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Re: P-38 build

Post by UpWrong »

I'm hesitating to change the cranks and gearing now. I rode 40 miles on non-flat terrain on Monday, my knees were fine and I appreciated having the 160mm cranks on the steep bits. By the end of the ride I wasn't noticing the closed position either. OTH, the £30 crankset I ordered from Spa has arrived so I might as well have them shortened and give it a go.

I also lost a spoke on the ride, so I'm junking the DT-Swiss rim and replacing it with a Mavic Open Sport one. Just hoping the spokes I bought for the job are the right length. There's always confusion about the ERD of Mavic rims since they measure it differently.
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Tigerbiten
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Re: P-38 build

Post by Tigerbiten »

It's probably to late now but SJS Cycles do 104/64 cranks from 150mm upwards.

If you think recumbent bike gearing is bad.
Try recumbent trike gears as you have no minimum speed on one, hence no minimum gear.
Just over a 6x range is about as wide as you can get with a pure derailleur setup.
Add a hub gear (Rohloff and triple or 3x9+ SA hub) and you can get just over a 10x range.
I love the 18x range of my twin hub gear setup.

Luck ........... :D
UpWrong
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Re: P-38 build

Post by UpWrong »

It's pretty much taken me the weekend but I now have the rear wheel rebuilt with a different rim, the Mavic Open Sport. And i've used a Schwalbe plasticised rim tape for the first time. I've been getting problems with Jante Velox cloth tape moving around the wheel well. Not tested yet. My wheel building confidence has been eroded. I'm never sure how tight to tension the spokes. I thought I had the tension too low previously but I heard the spoke break on this occasion and it sounded a bit explosive so I wonder if it was too tight. The DT Swiss rim didn't have eyelets so maybe that contributed.
UpWrong
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Re: P-38 build

Post by UpWrong »

So I did 180 miles of loaded touring in NL and the new rear wheel held up fine. Back to back comparisons with the XP convinces me that i'm generating less power on the P-38, so I've decided to open up the hip angle by:

1) moving to shorter cranks (160mm to 140mm)
2) reclining the seat to its maximum (another 2cm on the seat stays).

Each change opens the angle up by about 1.15 degrees, so around 2.3 degrees total. That should be noticeable. And a prerequisite for doing this is going to a 559/349 wheel configuration. That should mean I can reach the ground ok, and will reduce the gearing to compensate for the shorter cranks. I'm having the front wheel built since it saves time and smaller wheels are more difficult.

I'll go into more details when I'm putting it together. The end-product should be lower lighter and more aero.
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Tigerbiten
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Re: P-38 build

Post by Tigerbiten »

UpWrong wrote: 28 Jun 2022, 8:07pm So I did 180 miles of loaded touring in NL and the new rear wheel held up fine. Back to back comparisons with the XP convinces me that i'm generating less power on the P-38, so I've decided to open up the hip angle
...... snip .............
I'll go into more details when I'm putting it together. The end-product should be lower lighter and more aero.
It may not work out as you expect.
All my reading on bents leads me to the conclusion that opening the hip angle decreases power output.
The reason is as the angle increases you engage your glutes less.
This leaves your quads to generate a higher percentage of your total power.
How much power you loose I've never been able to work out.
But this reliance on your quads is the reason you need to build up "bent legs".
This is also one reason why bents tend not to climb as well as upwrongs.
Hopefully the decrease in weight and aero drag do give you an increase in overall speed.

Luck .......... :D
UpWrong
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Re: P-38 build

Post by UpWrong »

Tigerbiten wrote: 2 Jul 2022, 2:54pm It may not work out as you expect.
All my reading on bents leads me to the conclusion that opening the hip angle decreases power output.
The reason is as the angle increases you engage your glutes less.
This leaves your quads to generate a higher percentage of your total power.
How much power you loose I've never been able to work out.
But this reliance on your quads is the reason you need to build up "bent legs".
This is also one reason why bents tend not to climb as well as upwrongs.
Hopefully the decrease in weight and aero drag do give you an increase in overall speed.

Luck .......... :D
I've recently read that power decreases when the hip angle gets less than 130 degrees, regardless of whether it's an upright or a bent being ridden. Between 130 and 140 degrees is optimal.
KM2
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Re: P-38 build

Post by KM2 »

Would the hip angle of 130 make the gluteal muscles work in their optimal range to produce the power?
UpWrong
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Re: P-38 build

Post by UpWrong »

KM2 wrote: 4 Jul 2022, 3:14pm Would the hip angle of 130 make the gluteal muscles work in their optimal range to produce the power?
Have to admit I've lost the source which quantified loss of power wiith the angle. It's accepted that power diminishes with a TT tuck, and likewise when sat upright on standard bike, so a range between 130 and 140 degrees seems reasonable. I think the P-38 might be a bit less than that. Other experienced 'bent riders have commented that the P-38 closed position is a hindrance and not a help when it comes to hill climbing. Like I said, I feel that I generate more power on the XP with its more open angle. My quads are pathetic so maximising my glutes is the key.
UpWrong
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Re: P-38 build

Post by UpWrong »

My conversion to 559/349 is determined by which 16" tyre! I've gone for the TruBlu 40-349 120 tpi tyre which was developed for Aussie pedal prix racing and came out well in Trisled's recent multiple tyre RR tests. RR reduces with a wide rim so the rim I bought has a 20mm internal width, and happens to be single-wall, 28 spoke. Since I'm using 6-bolt rotor disks, the hub is a 28-hole Hope Pro 4. I realise now I might have saved money using a centre-loc hub with an adaptor. Anyway, I reckon the built wheel with tyre will be very light.
UpWrong
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Re: P-38 build

Post by UpWrong »

The new wheel came quicky. The hub is colour co-ordinated with the frame :) . Weight is 570g, existing 20" wheel was 800g so 230g lighter. It's been built as 1x cross. I'm a bit surpised it's not 2x for a disc wheel. I didn't specify and wasn't asked,
IMG_20220706_103506_HDR_75_resize.jpg
UpWrong
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Re: P-38 build

Post by UpWrong »

I guess the 1x cross spoke pattern improves the angle of the spoke with the rim. The rim holes have a slight offset which must help the build too.
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