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MG Midget and Sprite Technical - Rear dampers

The rear dampers on my A35 (similar nits to Sprite) dampen nicely for half their stroke but then get softer towards full bump. No leaks,fresh oil and no lost movement. Is this simply because the bores are worn out or is it something else? I assume the damping rate should be linear. The net effect is that the rear suspension tends to reach full compression and crash out if I hit a big bump.


f pollock

If you've had the fluid out of them and refilled they 'might' have an airlock
disconnect the link and work the lever up and down through full travel a few times and see if it comes right, make sure you go right to the end of travel in both directions, that's where it will burp if it's going to
William Revit

I am having a ponder on this one too... *strokes chin*

I know this is subjective, but how stiff is the arm in general? If they are functioning correctly they should need quite a lot of force to move by hand.
Malcolm Le Chevalier

Sounds like air bubble to me. The air is compressible, so when travel gets to that bit they will suddenly go soft. Purge the air as Willy describes.
GuyW

Thanks boys. The dampers were fully stroked over a couple of days when filled and even had their viscosities tweaked in the same way you adjust dashpot oils to get a matching pair. They aren't especially stiff to move (using 30 weight oil from memory) but it's this easing off at the top of the stroke that has me puzzled. Ideally I'd want it to get progressively stiffer towards full bump. But the car is lowered and it's worse with 4 people in it so maybe the standard spring rate has too much compliance.
f pollock

Fergus,
my understanding is that they should be hard to move, perhaps if you have a long arm on them you get more leverage but yours don't sound right to me.

I can't find the video I want but this of MGA front gives the general idea and the dampers don't vary that much.

Whether the A35 (saloon?) dampers were softer I don't know, I've lost my link to the A30 Austin Service Journal.

https://www.youtube.com/watch?v=ZKrQAi4V2Iw
Nigel Atkins

Hi Fergus, I should have realised if it was you that you would have dealt with the proper engineering aspects and filled and installed them properly!
Quite apart from the softening of the damping, if the car is lowered are they just bottoming out too soon? Is it possible to move the lever arm round on its spindle? Or to replace the second arm with a shorter link?
GuyW

Guy - the dampers are getting full movement, but I think the suspension is articulating too close to the bump stops - or to describe it more accurately, there seems to be a tipping point where too much load causes the spring to say **** it, and the spring then goes through a disproportionate amount of deflection. My theory about the dampers was that this deflection seems to tie in with the progressive softening, and had I a linear damper rate, the car might ride clear of the bump stops in all conditions. I suppose I could elect to fit van springs but the ride is already stiff enough.
Nigel - I guess the factory damper rate may exist somewhere but there's no mention in the workshop manual. But maybe mine should be set up stiffer. The Frogeye has adjustable Armstrongs on the back and can be set with virtually no resistance at one end to completely solid at the other. I think mine are about 4 clicks from solid and from memory gave a similar degree of stiffness.
f pollock

Fergus,

if you're considering adjustables for your A35 have you seen Richard b's thread 'Adjustable Valves for Armstrong lever Arm Dampers'?
Jeremy MkIII

Fergus,
I hope I have some stuff here that might help, or provide information.

I found the weblink I was looking for, my belly had blocked the view. If you haven't already got this it's full of very useful info that covers the Austin range up to Healey 100 (things like the very unmentionables and how to waterproof and dustproof the A30).

400 pages, so I'll leave it with you to look. - http://www.acmefluid.com.au/larry/austinservicejournal.pdf

From my previous post quoting Peter Cadwell in the Archives here -

Peter Cadwell on filling the dampers -
" ... That is the filler hole. Of course you can't see how much to add as you are filling the air reservoir. Unlike a differential or transmission, the oil should NOT be at the level at the threads, but a half inch below, which you can't see. So, just add some, if you must.
On the rear, the same size wrench opens the plug on the side of the shock just below the lid gasket. This is easier to not overfill.
The danger of overfilling is you will cause a future leak by displacing the air space necessary for the expansion of hot oil and force the oil out of the shaft packings.
The only reason you would want to add or top up the oil is if you are certain oil has leaked from the shock. Oil otherwise does not get consumed. Adding oil thusly is only a remedial repair.
Peter c".

- And. -

"Peter Caldwell of World Wide Auto Parts (USA) who used to post on here and is very well respected for his better than new exchange service on LA dampers cautions about using thick oil, 30w max (I know you put 15w).

"Stick with the 20W oil recommended (AW68 spec). At most use 30W (AW ISO 100) Best are synthetics like Silkolene or Redline suspension oils. They hold up under heat waaaaay better than standard hydraulic oils.... and these shocks get hot."

AW I think means anti wear, AW68 would be AW ISO 68 and 15w is shown as ISO 46.

IIRC Peter said there were several ways of mechanical uprating (not oil) but IIRC for DIY heavier valve springs (but as always don't go on my memory)."

Malc is the man here for valving.

HTH
Nigel Atkins

Lever arms are a bit like the much derided Spax telescopic damper conversion. If the arm is horizontal with normal ride height and when normally weighted then axle deflection would result in equal up and down movement of the arm hence the same degree of damping. If you lower the car then the lever arm would be higher than horizontal so giving less damping on compression. So should you not shorten the link between axle and arm? to restore equal up and down deflection.
As deflection increases the damping effect reduces as the angle of the arm increases. It's not linear.
Rob
MG Moneypit

Thanks for all the comments. I've decided to pull the dampers and see if anything obvious stares back. Meanwhile here's a picture of the damper arm in its current ( kerb) condition. This shows the arm at 20 degrees from the horizontal - presumably if the car was returned to standard ride height this angle would increase. Rob would you still suggest the arm should be horizontal? Sprite arms are horizontal but presumably the point you're making was that the pistons should be nominally at mid point in kerb condition for equal up and down deflection. As an aside Sprite drop links are very much longer than A35.



f pollock

Fergus,
swapping subject slightly and going back a bit, you put your dampers aren't especially stiff, in the JT vid he says (referring to the front MGA damper) " ...it should be nigh but impossible to push on this and get this to move ... it should have an enormous amount of restriction".

And I noted JT said "The amount of restriction changes between the compression stroke and the rebound stroke"
- but I don't know what differences there will be with MGA front to A35 rear dampers.
Nigel Atkins

"The amount of restriction changes between the compression stroke and the rebound stroke"

Yep. IIRC the rule of thumb is that rebound is about 1/3 the resistance of compression (I think I have that the right way round, I don't have my notes to refer to right now!).

That is because in one direction you are controlling the sprung mass, the other you are controlling the unsprung mass. The sprung mass is always greater, so you need greater damping.

Thanks for the name check Nigel ;-) It's been a while since I messed with shocks, too busy squirting petrol! But I have plans to get back to it soon.
Malcolm Le Chevalier

So here I've removed the damper and put it in a jig to measure the arm position relative to the total swept angle. The horizontal datum is the thin yellow line and the upper, lower and central positions as thick yellow lines. Doing the maths the theoretical mid point should be 21 degrees down - my measured kerb mid point is 20 degrees. So I suspect I'm pretty much on the money here.
The next thing will be to try and establish weighted full stroke deflections (timed) for bump and rebound. This will show where I am now and where I might want to be to make it, say 20% stiffer. I've got some Silkolene 30 weight oil coming Monday so will see how it goes.



f pollock

Have a look at pages 356-7 and 348 of the pdf from the weblink I put up of the Austin Service Journal. - http://www.acmefluid.com.au/larry/austinservicejournal.pdf
Nigel Atkins

I used to fly model aeroplanes and the aileron cranks were arranged so that at rest the ailerons were in line with the wing but on deflection it was arranged so that there was more down than up deflection for the same movement of the operating rod.
It should be the same with lever arm dampers. i.e. if at normal kerb weight the arm is below horizontal (whether the lever is at midpoint or not) it will mean you have more damping effect when the spring is compressed and less when the axle is drooping. In droop you have the straps to stop the dampers bottoming.
I'm not an expert on this but I do remember my model flying days which are all of 30+ years ago and to me the ideas are very similar.
Rob
MG Moneypit

So as a follow up from last week here are the refreshed dampers ready to be fitted up today. The drop links have been shortened 12mm to bring the ride height to the stroke mid point, and the units flushed and refilled with Silkolene 30. This has increased the weighted full stroke drop time from 39 to 53 seconds. As an aside there is now a very obvious difference in the resistance between compression and rebound, and the dampers no longer soften towards the end of the stroke. Case closed I think, and thanks to all who contributed.



f pollock

Well done.

Another case of clean and lubricate (with a little engineering alteration to links) and use of the appropriate good quality oil.
Nigel Atkins

Rob - I used to fly RC planes too.

The ailerons are arranged so that there was more UP than down. This was to prevent adverse yaw.

Adverse yaw is the condition where the drag from the aileron causes the aircraft to yaw in the opposite direction to where you want to turn when you bank the wings.

So by having more drag on the up aileron than the down aileron the aircraft banks and the extra drag on the up aileron assists in the turn by creating some yaw.



But yes, similar idea to the operation of the dampers.
Chris Madge

I think there is still some single channel RC equipment in the loft!! along with a few engines. I do need to declutter.
Bob Beaumont

Adverse yaw - didn't you counteract this with a bit of opposite rudder? Did on flying T21b Slingsby Sedbergh gilders IIRC.

Cheers
Mike
M Wood

Yes you can, or better still design it out.
Chris Madge

Chris Madge. I used to fly mostly electric, I even designed and built an electronic speed control for one of my planes which was very successful. My most successful was a Cessna type single geared electric which I flew whenever I could. I gave it all up eventually. I couldn't stand the frustration of working all week then having it wet or windy or both for the weekends. That was 30+ years ago which is my excuse for forgetting about the details.
I hope F Pollock is all sorted now.
I noticed the arms on the dampers are incredibly long compared to midget ones, the back end of an A35 must be quite heavy.
Rob
MG Moneypit

Rob - I was into IC, initially small two strokes, then larger 4 strokes up to 20cc and eventually larger 50cc to 75cc petrol powered 1/3 scale aerobatic planes.

Yes I know what you mean, very frustrating when you spent all week looking forward to flying and it either rained or was too windy! But when the weather was nice, it was a great hobby. These days I think it’s all drones.

Sorry for the thread drift.
Chris Madge

Thread drift is great!

Look what is available these days:
https://www.dbsportandscale.com
&
https://lasercutsailplanes.co.uk/product/slingsby-t-21-radio-controlled-gliders-short-kit-plan/

Cheers
Mike

M Wood

Traditional ‘balsa bashing’ - where you build something from a traditional kit from the likes of DB or from scratch with a plan (as I used to do) is on the decline. It’s very expensive and time consuming to build when compared to ARTF (almost ready to fly) models where you start with a prebuilt model that just needs your radio gear installing.


The ARTF stuff is built very cheaply in the Far East at a much lower cost than you can build yourself and the quality and finish is better than most modellers can achieve in their workshop. For example you can buy yourself a Spitfire ARTF, assemble on Saturday and be flying it on sunday. Crash it the next weekend, and buy another.

The guys that are into scale modelling, ie build a particular instance of an aircraft build from scratch but these are a minority now.

Chris Madge

It made me go in the loft! 6 ic engines mainly glowplugs from 0.30cc up to 1.5cc. I used to model in the mid 60's so the RC was simple rudder only with fixed travel driven by a mechanical escapement. Loads of plans for free flight sport aircraft. Took me back!!
Bob Beaumont

Bob - you wouldnt recognise the hobby now, its very different!

I started out with free flight, building competition rubber powered models. There is probably only a handful of people still flying free flight or control line now.

RC Models, and their engines are bigger now. For IC, a beginner would learn to fly on a high wing trainer powered by 0.40 CU or 6cc IC engine with 4 channel radio gear, 60 inch span. Once they have learnt to fly they often move onto larger models.

My largest petrol powered models were around 100 inch wing span and 16+ lbs in weight. Bigger models fly better and more scale like.

But electric power is really taking over from IC now as battery technology is much more advanced. Electric is much more reliable, and clean. No more cleaning down the remains of two stroke oil from your model at the end of your flying session.

Battery technology is so good that it is preferred in competition flying because of its superior power to weight, and torque. Modern charging systems mean you can fly your electric model, land, plug in a charger run off your car battery, and be flying again in 15 mins.
Chris Madge

Most of mine were free flight around 36 inch wingspan. The RC aircraft were larger high wing around 55 inch powered by 1.5cc They had a high lift wing capacity due to the weight of the receiver and batteries. They rather floated along. I quite liked building semi-scale as they were less fragile and performed well. I recall building a Concorde with a 25 inch wingspan powered by a .3 cc engine as a 'pusher'. Was really fast but glided in a straight line once the engine cut so made retrieval a long walk!
Just could not imagine electric powered models then!
Bob Beaumont

Bob I remember as a young lad eagerly waiting for my monthly Aero-Modeller magazine through the post. In the middle would be a free plan. I built nearly everything in those magazines - gliders, rubber powered, a few small IC powered ones, and CO2 powered little motors too.

I always wrote my name and address on my models as occasionally they would get taken by a strong thermal and rather than glide back down to earth would go higher and higher and disappear ! Invariably some kind sole would return my model a few days later :)




Sorry, thread has been completely hijacked now :(
Chris Madge

I still have an unfinished balsa 'sailplane' glider on the top of a bookshelf. It has made quite a few house moves so far! Never did quite finish it and occasionally think I should. It still has its box with a price of 27/6d on it, so that dates it!
GuyW

Guy, just a bit of balsa glue, some tissue and dope to finish it off.

Dope smelt of pear drops, it stank my bedroom and the house out, and made me light headed!
Chris Madge

Yep Aeromodeller was my monthly read too. I also occasionally got Radio Control Model and electronics to have a drool over the equipment.
Only lost one aircraft, at Bembridge aerodrome on the Isle of Wight. A 24" wingspan biplane called Grasshopper it was a free plan in Aeromodeller. It flew off into the long grass and disappeared. I still use my X Acto modelling knife when cutting out things like gaskets etc. Its got to be 55 years old now.
Bob Beaumont

I thought dope had more of an earthy, herbal smell. But the light headed bit is right ;-)
Malcolm Le Chevalier

This is fascinating the crossover between Spridget fixing/driving and aero modelling. I built free flight models in the1980s, from rubber band powered to CO2 powered and a Mills Diesel. Stopped when I realised I could fly full-sized gliders - lovely old wood and fabric open cockpit - on what upgrading to RC would have cost. Then on to rusty old Minis and Minivans for transport. And fixing them.

Vernon was a balsa kit supplier I remember.

Vintage aerolomelling 'SAM 35' had a UK branch, still going I think.

Anyone tried electric round the pole flying, Ballards of Tunbridge Wells?

Cheers
Mike

M Wood

Home built slot cars anyone? Not just the cars but a large 3 dimensional rubber-painted hardboard track with gradients, bends with slots following the racing line and banked corners. All the scenery and terrain of course, including a Monaco style semi open sided tunnel.
GuyW

Mike, I think you mean Veron, I had a Veron Vespa Minibipe kit powered by a Cox Pee Wee made by them.
Guy, not home made track but home made cars using Revell/Monogram /MRRC chassis. I used Airfix track and a range of Airfix bodies. They made their own cars but were very slow, the Revell equipment was much faster. An early conversion was an Airfix Frogeye which led to other things..........!
Bob Beaumont

I built a Veron Impala, my first foray into RC.

https://www.vintagemodelcompany.com/52-veron-impala-radio-controlled-beginner-slope-soaring-glider.html

Mike I had forgot about electric RTP (round the pole), we had a setup at air cadets (no. 1F squadron, Leicester). We would knock up a couple of models from sheet balsa, sand a rough aero foil, and rubber band an electric motor on the front. Then proceed to thrash and trash them around the pole. :D


In my late teens I got into free flight competition (duration) rubber powered models. The propellor blades were made from laminated balsa sheets around a former to create the twist and curve of the blade. A jig was used to drill a hole at the inner ends and the blades were mounted on the hub so that they could pivot. During the powered phase of the flight centrepetal force kept the blades at right angles to the fuselage giving thrust, and then when the rubber motor stopped the blades would fold back against the fuselage during the glide stage to reduce drag. With out thermal assistance you could get around two minutes of flight duration which is a lot for a rubber powered model. They were trimmed to fly in large circles, but even with just a breeze they could cover long distances. A miniature clockwork timer from a child’s toy was used to trigger a ‘de-thermaliser’. This tilted the tail plane to around 45 degrees and brought the model down vertically in a controlled and safe manner to prevent it flying away and loss. You could set the timer between say 30 seconds to several minutes. The one in my hand was one you could buy commercially, but the home made ones could be made lighter.

Some competition models were really quite sophisticated, with people experimenting with carbon fibre in the early 80s and miniature clockwork devices to change the trim of the aircraft during several different phases of flight - climb, cruise, glide.

Yep my other geeky-ness was model aeroplanes!



Chris Madge

Amazing how many of us were into flying models. When I gave up I kept a couple of kits in the loft intending to keep them till I retired. But then I got into Midgets and decided to sell them many years ago now. One was a DB Piper Cub, the other was a Guillows P38 Lightening rubber powered twin.
I got more than my money back on both.
I even drew up scale plans for an AVRO York (commercial Lancaster) in Dan Air livery (commonly known as Dan Dare) with 10 foot wingspan. The only part I built was the tail plane and rudders before giving up for good. I may still have the plans somewhere drawn on wallpaper rolls.
Rob
MG Moneypit

Gosh this thread (drift) really takes me back to my youth

I built a Guillows Arrow, it was a good quality kit. Red and Yellow tissue paper. I remember seeing the picture of the Lightening twin and thinking that looked amazing.

I think genuine old kits like that are worth quite a lot of money now?

Shame I never kept any of my old models.

Chris Madge

I had a quick scan on the web and prices for some original kits are suprising. I guess its more mature people reliving their youth! Yes all mine went years ago, I doubt if they would have survived in a flyable condition anyway The effects of castor oil from the engines rather ruined the balsa!
Bob Beaumont

Guillows kits are still available (ebay), I'm tempted to buy one.
Chris Madge

This thread was discussed between 27/05/2020 and 04/06/2020

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.