Dropper for kid (I know, I know...)

FrankZappa6 · Oct 19, 2016

I'm using this one on my Karate Monkey to aid with getting on an off the bike with a Thule Ride Along Mini front mounted kiddie seat. Others have pointed out that it bears a strong resemblance to the PNW model.

Using a bathroom scale as a force gauge, it takes 40-50 lbs to get it moving then it drops to 30-40 lbs for the first 1-2 inches. The resistance then builds for the final travel and peaks at about 60 lbs near full compression.

Obviously, YMMV but the price was right. It took about a week to show up.
Best,
FZ6

Feldybikes · Oct 19, 2016

FZ6 -

Thanks for pointing out the Tranz X post and especially for measuring the activtation force. I did a bit of searching and found the manual for that post

and it seems it has a preload adjust on the bottom. I take this to mean it's probably a coil spring inside?? Have you messed with this at all? Your force measurement puts it right on the line of not being able to work.

FrankZappa6 · Oct 23, 2016

Well, I took my TranzX out and put a socket on that nut to see what would happen. It basically unscrewed the bottom cap out of the outer shell. I think the only thing that nut does is hold the bottom cap onto the inner shaft. Here's a pic.

To disassemble further, one would have to remove the nut. The problem is, everything is heavily lubed and the shaft turns with it. There's no squared off flange for a wrench and I couldn't get the nut loose. I didn't want to put a set of vise grips on it.

I could be wrong, but my impression is that a 50 lb kid could probably get about half the travel out of the post if they had a good bounce going in. There is an initial resistance to overcome and the force definitely stacks up as the hidden spring compresses.

Thanks for posting the manual. I don't recall seeing an English version. Good luck in your quest!
FZ6

lixxfe · Oct 23, 2016

Lighter springs for GD

I have 2 Gravity Dropper posts, in 27.2". One is 2" stroke, 325mm Length, the other is 3" stroke, 300mm Length (yes, shorter length on the longer stroke unit).

The spring in the 325mm unit has dimensions:
Free Length = 9.25" length
Wire Dia.= 0.073"
Spring Outer Dia. = 0.85"
#active coils = 24

Using the spring rate calculator at https://www.thespringstore.com/spring-calculator/spring-rate-calculator.html and assuming music wire for the material:
Spring Constant, k = 3.6 Lbs/in.

Minimum Preload when installed = 4.0"
Spring Force at top of stroke = 3.6*4.0 = 14.4 Lb.

For the spring in the 300mm unit:
Free Length = 8.00"
Wire Dia. = 0.072"
Spring Outer Dia. = 0.85"
#active coils = 21
Spring Constant, k = 3.9 Lbs/in.
Minimum Preload when installed = 3.2"
Spring Force at top of stroke = 3.9*3.2 = 12.5 Lb.

I ordered two of these to stack together to reduce the spring force: https://www.leespring.com//product_spec.asp?partnum=LC067K10S&springType=C&subType=

Specs:
Free Length: 4.00"
Wire Dia. = 0.067"
Spring Outer Dia. = 0.845"
Spring Constant, k = 4.5 Lb/in.
(#active coils not specified, but I counted 12/each)

Two of these stacked together halves the spring constant, so:
k= 2.25 Lb/in.

When installed in the 325mm length GD:
Min. Preload = 2.5"
Spring Force at top of stroke = 2.25*2.5 = 5.6 Lbs.

So about 2.6x less Spring Force (top of stroke).

My calibrated hand says this is about right

.

The reduction in effort to drop the seat is very noticeable.

The only issue is the springs cost about $10/each, and there is $50 minimum from Lee Springs (unless you pay $20 handling fee).

Feldybikes · Oct 23, 2016

FZ6 - thanks for the disassembly. I may still give that a short since the post is relatively cheap and it has a remote. I"m not highly confident it'll work for the kid, but maybe still useful as a backup for me.

lixxfe - good idea on the aftermarket springs. Looks like I could something similar from from MSC, though there's a 10 spring minimum which is 5x more than I need.

lixxfe · Oct 24, 2016

The MSC spring would offer some benefit. Spring constant for two is higher at 2.85 Lb/in, vs 2.25 Lb/in for the Lee springs. Even at qty. 10, cheap enough to take a chance and try, I reckon.

iamkeith · Nov 8, 2016

lixxfe said:
I have 2 Gravity Dropper posts, in 27.2". One is 2" stroke, 325mm Length, the other is 3" stroke, 300mm Length (yes, shorter length on the longer stroke unit).

...

I ordered two of these to stack together to reduce the spring force: https://www.leespring.com//product_spec.asp?partnum=LC067K10S&springType=C&subType=

Thanks for posting this! And thanks to the OP for asking the question, too.

Dropper for kid (I know, I know...)

FrankZappa6

Feldybikes

FrankZappa6

lixxfe

Feldybikes

lixxfe

iamkeith

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