Problem with Gibson sprung suspension

[David Thorpe]

I'm building a Gibson E4 kit and have constructed the chassis as per the the kit, using the supplied Gibson suspension system of spring hornblocks.

I'm very happy with the chassis, which runs beautifully. What is causing me great problems, however, is setting an accurate ride height. As I understand it, the Gibson system requires that as the loco stands properly on the track, there should be no upward movement of the axles which are restrained by the screw in the hornblock, with the spring providing for downward movement when required. Unfortunately, the springs seem much too strong and when (after much fiddling) the screws are set at the correct position in the hornblocks to achieve an accurate ride height, the springs insist on pushing the loco up beyond the ride height so that the loco is in effect being supported by the springs, not the screws. There's not an awful lot of room to weight the loco, and such lead as I have applied doesn't do much to cure the problem.

As I see it, all I can do is either to weaken the springs, presumably by cutting something off their length and then expanding what remains, or (my reluctant inclination) dis-assembling my otherwise perfect chassis to incorporate a different system, probably simple beam compensation.

Any guidance would be gratefully received.

David

[steamraiser]

As your chassis runs well riding on the springs, I would leave well alone. This is better than riding on stops using the springs just to keep pressure on the wheels.

You could trying different springs such as buffer springs or springs used for carbon brushes in motors (Graham Farish N ).

Can limiters to the amount of downwards travel of the bearings be fitted to the bottom of the hornguides?
This would reduce the ride ride hight.

Gordon A
Bristol

[grovenor-2685]

This would reduce the ride ride hight.

Well yes but then it wouldn't be sprung at all!
The problem here is that the design is meant for springs which are to weak to take the weight and the loco should sit normally on the bump stops, ie the screws. Short of modifying the chassis the only solution is to use weaker springs.
The simplest rebuild may be to measure the excess ride height then move the hornguides upward in the frame by that amount, but it does mean unsoldering all the hornguide, deepening the frame slots then soldering it all back. Not much fun.
Somewhere in the depths of my bits boxes I have some packets of the original Studiolith springs which came in different weight ratings, if you weigh the loco we can work out which ones should work. I can post you a packet but it might take a while to find them as my storage has been disorganised by some roof repairs.
Regards

[David Thorpe]

Thanks Keith, that's a very decent offer. Before I put you to the trouble of trying to find the springs, however, I thought I might remove the existing springs,snip, say, a third off them (the amount will probably be a matter of trial and error), stretch the remaining two-thirds until the length is a bit longer than the screw, and then replace the stretched sections in the hornblocks. I'm no engineer, but I feel that that should give a weaker spring, although whether it will still do the job it's meant to I don't know.

David

[Will L]

... I'm very happy with the chassis, which runs beautifully. What is causing me great problems, however, is setting an accurate ride height. As I understand it, the Gibson system requires that as the loco stands properly on the track, there should be no upward movement of the axles which are restrained by the screw in the hornblock, with the spring providing for downward movement when required. Unfortunately, the springs seem much too strong and when (after much fiddling) the screws are set at the correct position in the hornblocks to achieve an accurate ride height, the springs insist on pushing the loco up beyond the ride height so that the loco is in effect being supported by the springs, not the screws. There's not an awful lot of room to weight the loco, and such lead as I have applied doesn't do much to cure the problem.

Yes tricky little blighter the E4, which for those that don't know, is a 2-4-0. The prototype had more weight on the front carrying axle than either of the driving axles and a model tends to share this problem. Your question has had me pulling my E4 kit out of the to do pile and wondering how I'd do it.

In my book the right answer would be CSBs (Continuous Springy Beams) rather than individual springs, as it is practical, even easy, to change the springs and adjust the ride hight. But for you that might not be very helpful advice, as you have the chassis built and putting in CSBs at this stage might itself prove a bit tricky. Though fitting compensation with the drive on the middle axle could prove interesting too.

Sticking with what you've got, you won't make springs softer by shortening them, precisely the reverse, so either you find something softer or think harder about where extra weight may go. Unfortunately the obvious added weight answerers are all at the front which is why the E4 is a tricky..... However there is room for weight in the big square splashes in the cab, under the cab floor, under the cab roof, particularly if you have built a loco with the later cab roof profile, and don't forget between the chassis frames. Also, from the drawings and looking at the kit, it looks to me as if the boiler back head is going in too far forward in the cab, and you could, with advantage, stick a sheet of lead behind it so that it is in right place further back in the cab.

There is one other rather more radical option. A Highlevel Loadhauler + gearbox could be used with the motor on the leading axle and the motor mounted forward down the boiler toward the firebox. That would leave about an inch of clear space in the firebox that could be solid lead. Bet your springs would knuckle under to that.

Will

[Andy W]

If, as Will points out, shortening the springs you have will make matters worse then do you have room to solder some brass wire to the frames and have the end rest on the hornblocks? It should be relatively easier to then alter the strength of the springs by re-positioning/bending them. The thought of you undoing any of your work on the chassis hurts.

[David Thorpe]

In my book the right answer would be CSBs (Continuous Springy Beams) rather than individual springs, as it is practical, even easy, to change the springs and adjust the ride hight.

I did consider that when I started on the chassis. I didn't do it for two reasons; first, I'd never tried CSBs before and was apprehensive about them, particularly as while i could find plenty of articles on the theory, there were precious few about putting them into practice. I'm sure that there was once an article on the CLAG site which covered the practical aspects, eg type of wire, how far above the hornblocks the fulcrum points should be positioned, and so on, but I can't for the life of me find it now. I should of course have bookmarked it when I first saw it..... Secondly, it seemed that I was going to have little choice but to drive the leading axle and I didn't really fancy introducing myself to CSBs in that situation.

Also, from the drawings and looking at the kit, it looks to me as if the boiler back head is going in too far forward in the cab, and you could, with advantage, stick a sheet of lead behind it so that it is in right place further back in the cab.

Already fitted and firmly stuck, I'm afraid, so too late for that!

There is one other rather more radical option. A Highlevel Loadhauler + gearbox could be used with the motor on the leading axle and the motor mounted forward down the boiler toward the firebox. That would leave about an inch of clear space in the firebox that could be solid lead.

That's what I've actually done, albeit with a RoadRunner+ rather than the Loadhauler. The Mashima 1220 fits very neatly down the boiler and it all seems to work, although extracting the chassis from the loco body is a trifle more difficult than would otherwise be the case. It all feels wrong, however, and I'd much rather drive the rear axle - if in the end I do take the chassis to bits I reckon I might be able to do that (drive the rear axle) by adding a High Level DriveStretcher to the RoadRunner. Simple beam compensation would then be relatively easy, with the driven rear axle fixed.

Bet your springs would knuckle under to that.

Unfortunately, they don't, or at least not consistently. Unfortunately, the hornblock cutouts in the chassis seem a little too low so that when proper ride height is set, the springs are almost fully compressed. Following your advice I have abandoned my idea of shortening and then expanding the springs, but I feel it might be practical to merely shorten the springs so that they are only a little longer than the protruding screws and will not therefore be so heavily compressed. As I've got several spare springs on which to experiment, I feel that that's worth a shot at this stage before going on, if necessary, to Keith's suggestion of deepening the frame slots.

If, as Will points out, shortening the springs you have will make matters worse then do you have room to solder some brass wire to the frames and have the end rest on the hornblocks? It should be relatively easier to then alter the strength of the springs by re-positioning/bending them. The thought of you undoing any of your work on the chassis hurts.

I'm quite used to having to take chassis to pieces As to your suggestion, which i very much like in principle, the Gibson hornblocks are semi enclosed in frames and, as I see it, these would get in the way of any springy wire. Mind you, it seems that most hornblock systems these days are semi enclosed in this way.

Thanks everyone for your suggestions and comments - much appreciated!

David

[Tim V]

Isn't it a tender engine?

In which case, move the motor to the tender (where it's the best place anyway) and fill the vacated space with lead. Improve the haulage powers as well.

[David Thorpe]

Thanks Tim, but that would involve cardan shafts, universal joints and other complications which, whatever the advantages, are currently beyond me!

David

[Russ Elliott]

David - do you have 1mm clearance between the top of the hornblock and the hornguide?

[David Thorpe]

David - do you have 1mm clearance between the top of the hornblock and the hornguide?

I can't measure it precisely, but it's about 1mm, possibly slightly more.

David.

[Russ Elliott]

I can't measure it precisely, but it's about 1mm, possibly slightly more.

Ok, we might be in with a chance. If you can make a small 'open trap' on top of the blocks, the following twin springy equaliser (one on each side of the chassis) is a bit of a bodge for your situation, but might be a way forward. I don't know what weight you can get on top of your drivers, but the spring will need to be in the 20 or 22 thou diameter region. I'll do some sums if you've got an idea of what that weight might be.

davey-block.png

[David Thorpe]

Ok, we might be in with a chance. If you can make a small 'open trap' on top of the blocks, the following twin springy equaliser (one on each side of the chassis) is a bit of a bodge for your situation, but might be a way forward. I don't know what weight you can get on top of your drivers, but the spring will need to be in the 20 or 22 thou diameter region. I'll do some sums if you've got an idea of what that weight might be.

Thanks very much for this, Russ. The open trap on top of the blocks already exists - I filed that to help keep the springs in place. As far as weight is concerned, I reckon that the loco will work out at a maximum of about 110g after I've added lead, most of which will be over the driving wheels. Front axle is currently the driven one, although i'm confident that I could if necessary drive the rear axle instead by fitting a High Level DriveStretcher to the existing RoadRunner+ gearbox.

This evening I have removed the springs, snipped about a third off them, and replaced them. This has improved the ride height situation but it still isn't perfect. This was my first real experience of the Gibson hornblocks and I really don't want to use them again - apart from the problem with the springs being too strong I find them very difficult to set up accurately.

David

[Russ Elliott]

I reckon that the loco will work out at a maximum of about 110g after I've added lead, most of which will be over the driving wheels.

Assuming about 80g over the drivers, i.e. 20g each, 20 thou steel will give about 0.5mm deflection if you go for the springy equalisers. With that kind of light weight, you'll probably need to alter the longitudinal position of the support to take into account a proportion of the motor weight on the driven axle, in order to better equalise the axle loads.

[steamraiser]

You could have added a packing piece to the keeper plate on each hornguide assembly, reducing the amount of downward travel of the bearing in the horguides, which in turn would limit the upward travel of the body relative to the track.

Gordon A
Bristol

[grovenor-2685]

You could have added a packing piece to the keeper plate on each hornguide assembly, reducing the amount of downward travel of the bearing in the horguides, which in turn would limit the upward travel of the body relative to the track.

Which you said earlier in the topic. But think about it, the ride height is set by the screw in the top of the hornguides before adding the springs. This being the classic MRSG design where the springs are not intended to carry the full weight but just push the wheels down into track hollows. The springs are stronger so are lifting the whole loco up off the stops.
If you now restrict the downward movement of the bearings to the point where the ride height is correct there will be no springing!
Slackening off the screws at the top has no effect as the springs are not bearing on the screws but on the fixed top of the hornguide.
Regards

[steamraiser]

Apologises for the double posting. I thought I had made a posting earlier but missed it when I read through the topic.
Gordon A

[David Thorpe]

I'm afraid that Russ's maths are beyond me, so instead I chickened out and merely snipped about a third off each of the springs. Ride height and performance on the test track is now fine although how the loco performs on a real layout remains to be seen.

David

[nigelcliffe]

Is there a reason why you can't wind your own lighter springs ? I've made many from phos-bronze wire (for buffers, not suspension) using a drill bit held backwards (flutes inside chuck) in a pin chuck. Trap the end of the wire in the chuck alongside the drill flutes, hold it under tension and wind the chuck slowly.


- Nigel