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Mono Rubber block height

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rparigoris

Joined: 24 Nov 2009
Posts: 804

Posted: Tue Mar 18, 2025 10:22 am Post subject: Mono Rubber block height

Hi Group

What is the acceptable distance between the upper and lower compression plates for the black rubber shock absorber before replacement is necessary sitting on main gear with wings on?

I have a XS Mono with 914.

Mine looks a little bulgy at 2.3"

Thx.

Ron P.

Area-51

Joined: 03 May 2021
Posts: 429

Posted: Wed Mar 19, 2025 12:07 am Post subject: Re: Mono Rubber block height

After 800 landings 181 undercarriage measures 3.0" between LG-05 plates, under load, tail down, BEW 325kg.

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erichdtrombley(at)juno.co
Guest

Posted: Wed Mar 19, 2025 7:03 am Post subject: Mono Rubber block height

Ron,



The build manual specifies a distance of 79 mm (3.1 “) between the plates.  This is a critical dimension as it can affect the operation of the landing gear.  It also dictates the amount of over-center on the LG08 legs and the corresponding horizontal load transferred to the stops on the frame, especially durning landing.  Finally, the dimension also affects prop clearance, which isn’t all that great with the block set to spec. 



As my rubber block compressed over time I made up the difference with spacers made from black ABS.   I started with a 0.25” spacers some years ago.  I now have three spacers installed making up a gap of approximately 0.73”.   The new spacers are made from 6mm thick material, although it was sold as 1/4” thick sheet.



So, my rubber block is close to the same dimension as yours.  



Erich Trombley

N28ET 

Classic Monowheel 914

peterz(at)zutrasoft.com
Guest

Posted: Wed Mar 19, 2025 8:16 am Post subject: Mono Rubber block height

Great info Erich!



I have been thinking of doing the same.



Do you disassemble? Or do you use half plates around the shocks?



Any pics?thx!

PeteZ



 	  Quote: 	
		   On Mar 19, 2025, at 11:11 AM, Erich Trombley <erichdtrombley(at)juno.com> wrote:

 

 

 

 Ron,

 

 The build manual specifies a distance of 79 mm (3.1 “) between the plates.  This is a critical dimension as it can affect the operation of the landing gear.  It also dictates the amount of over-center on the LG08 legs and the corresponding horizontal load transferred to the stops on the frame, especially durning landing.  Finally, the dimension also affects prop clearance, which isn’t all that great with the block set to spec.

 

 As my rubber block compressed over time I made up the difference with spacers made from black ABS.   I started with a 0.25” spacers some years ago.  I now have three spacers installed making up a gap of approximately 0.73”.   The new spacers are made from 6mm thick material, although it was sold as 1/4” thick sheet.

 

 So, my rubber block is close to the same dimension as yours.  

 

 Erich Trombley

 N28ET

 Classic Monowheel 914

rparigoris

Joined: 24 Nov 2009
Posts: 804

Posted: Wed Mar 19, 2025 8:58 am Post subject: Re: Mono Rubber block height

Hi Erich

Thank you for reply.

Did you shoot for 3" gap when adding spacers or 3.1"?

Is your shock Red or Black?

Ron P.

dmac7

Joined: 05 Apr 2019
Posts: 54
Location: Canada

Posted: Wed Mar 19, 2025 5:12 pm Post subject: Re: Mono Rubber block height

I measured my XS Mono 914/Airmaster suspension block in the garage no wings on it and it is 2 11/16" between plates. This includes a 3/16" plywood shim.  Yes it supposed to be 3.1" but that is no load.  If one goes back and reads the old Factory Flyer's, Ivan mentioned putting a 1/4" shim in one of the factory Europa's, G-ELSA if I recall.  I did purchase a new suspension block from Swift but it's on standby for now, I'll put it in later. 

 I thought the block was made from some type of poly?

dmac7

Joined: 05 Apr 2019
Posts: 54
Location: Canada

Posted: Thu Mar 20, 2025 9:52 am Post subject: Re: Mono Rubber block height

This is a old post from Andy Draper ( before XS ) regarding the two types of rubber blocks including the material type and pre-load.    



       The original shock absorber required a pre-compression to bring the

dimension betrween the centres of the top and bottom plate bearings down to

5".  The weight of the aircraft then compressed the rubber further, bringing

the top and bottom plates quite close to the limit stops.  To allow further

compression of rubber to take place and not compromise propeller clearance

we added a spacer.  Following the drop tests we decided to reduce the amount

of pre-compression such that the distance between the plate centres would be



The materials of the red and black blocks are the same: polyurethane grade

RHS/204/all weather 75=B0 Shore hardness to standard commercial tolerances.

The black block is 3/8" deeper than the red to accommodate the extra 1/4"

required between the top and bottom plate mounting centres.



The reason for the colour change of the block is that we have changed the

manufacturer.  The first gave no choice of colour, the second allowed a



        (a)     to ease identification of the changeover,



        (b)     as it is in a muddy environment a light colour may be

considered inappropriate.

We don't consider that the rubber block has any ability to dissipate energy

(although of ocurse it will to a degree) which is why we have introduced a

damper system.  The red block plus spacer will be slightly stiffer than a

black block without, but when you try to quantify the difference in the real

world i.e. on an aeroplane during a landing, you'd need a very sensitive bum

to be able to feel it.



Since the modification which calls out the requirement to cut the Vees in

the rubber blocks (Mod no 16  dated 7 November 1995) was issued, the tooling

for these blocks was altered so that those later recipients of fuselage kits

had the mod done for them.



The Vee shapes and the dampers are not related.  All blocks must comply to

Mod 16, with or without dampers, as otherwise there is the possibility of

over-stressing the reaction plates.



When the shock absorber has been set up correctly and the aircraft is sat on

the ground ready for flight, the fuselage will be in a nose up attitude of

around 9=B0.  Fuel, people, baggage, etc. will all alter this figure.  Use it

as a double check if you have an inclinometer handy.

Andy

erichdtrombley(at)juno.co
Guest

Posted: Fri Mar 21, 2025 9:00 am Post subject: Mono Rubber block height

Pete,



Per the manual the 3.1” distance between the plates is set prior to installing the assembly in the aircraft. 



Erich Trombley 

N28ET

Classic Mono 914

Area-51

Joined: 03 May 2021
Posts: 429

Posted: Sun Mar 23, 2025 1:45 am Post subject: Re: Mono Rubber block height

This has been a great discussion... After a routine inspection of 181's undercarriage up on the hard stand today, measuring up the block at the same time. In the last three years the block has sagged from 78mm to 60mm... The numbers say each digit at the rubber block is approx doubled for propeller tip clearance changes... Will have to swap it out and get it all right before the big prop gets bolted on 👍👨‍✈️

dmac7

Joined: 05 Apr 2019
Posts: 54
Location: Canada

Posted: Sun Mar 23, 2025 10:23 am Post subject: Re: Mono Rubber block height

It seemed to me that the suspension block slowly compresses over time under load but when I removed it for a few years then put it back in, it expanded back ( from less than 2 1/2" ) to 3". Since then, 3 years of being loaded has it almost where it was again ( 2 9/16" )

  If one were to have a spare block maybe it would make sense to rotate them every two years and let them uncompress ?  Did anyone else notice they come back after being unloaded a few years?

Area-51

Joined: 03 May 2021
Posts: 429

Posted: Sun Mar 23, 2025 2:34 pm Post subject: Re: Mono Rubber block height

This is a characteristic of polyurethane.. I use 100SH skateboard wheels on my forward hangar cradle; after a day or more the wheels get flat spots and expand true again in about 20 seconds after rolling; the rear cradle uses 70SH wheels and they compress more if used under the main gear but also expand true almost immediately.

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