Damper travel

[Jason Plato]

Dear all ,

Does anyone know of any methods of increasing damper travel - before hitting bumpstops . I was once told about removing sleeves ?? .

I dont wish to adjust the platforms , car is 97 1.6 superlight which sits lower than the snakes B...ocks .

 

Cheers

Dave

[Mike Bees]

Remove the spacer at the top (or bottom depending upon which way up your dampers are mounted) of the damper. It's a long solid-looking hexagonal thing...

 

Mike

[Jason Plato]

Cheers Mike ,

Is it the bumpstop thingy ? made of rubbery stuff ?

Dave

[Mike Bees]

No, it's a piece of metal that lengthens the damper.

 

Mike

[Nick]

Mike, I wish I'd got you to show me this at the weekend! Maybe it would be a good idea to get rid of it on mine, as my 7 was running on the bump stops until recently. I'll have a look when it's stopped raining! Why is it there?

 

Cheers, Nick

[Peter Carmichael]

[Rhetorical question] Which is better for grip: Running on your bumpstops or running on your sump?

 

The problem is that the low point on a Caterham is dead in the middle, so when you heave over bumps or pitch into heavy braking, you want the bumpstops to even out the end of suspension travel and avoid that nasty clanging scraping sound. However, in roll the bumpstops come into play far too early.

 

What is needed is a decoupling of the individual wheel travel into their synchronous (heave, pitch) and oppositional (roll, warp) components. You could then apply a bumpstop which would only come into play when the sump is in danger.

 

The metal spacers keep the bumpstops doing something useful in pitch and heave conditions. If you remove the spacers you must choose to avoid bumpy roads but will have better progressive suspension travel in roll.

 

If you are on the bumpstops all the time then you are *too low*, by definition. An alternative is to raise the engine on its mounts, but with tall DOHC engines you would then need a bonnet bulge and your luvverly Caterham would start to resemble a Westf%&ld - urgh!

[Graham Sewell]

IIRC running onbumpstops is a major no-no. If you hit them regularly, then one of three factors is wrong:

 

1. Suspension geometry - some idiot has lowered the car too much smile.gif

2. Damper design - not enough travel available before the bump stops prevent the damper internals doing the same job

3. Spring rates are too soft

 

Knowing PC's level of knowledge I assume that 1 and 3 can be ignored - this leaves number 2. In this case, maybe getting on to racing shock manufacturers would be an idea so that you can get some bespoke ones made.

 

For us mere mortals, I would suggest playing with numbers 1 and 3 so that the car either has further to travel before hitting the bump stops or you have to turn or brake harder.

 

Cheers,

 

Graham

[StewartG]

I reckon that to get the front shocks in the middle of their travel at reasonable ride height settings we need hslf length shock extenders.

[Peter Carmichael]

Graham, we seem to be agreeing but just picking up on semantic issues:

 

The dampers dissipate bump energy as heat. The bumpstops are a rising rate spring, storing energy and dissipating only a little through material hysteresis (heat). The bumpstops and dampers together form a system for coping with energy input into the chassis.

 

The bumpstops should take significant hits from time to time. They are the 20 portion of an 80:20 rule. If they aren't getting used they shouldn't be on the car.

 

The geometry of the Caterham, mostly forced by the dogma of a spring/damper at each corner, means that the bumpstops act prematurely in roll.

 

One of the ways to mitigate this is to stiffen up the car in roll, so that it never gets near the bumpstops.

 

If you drive a car that is set to a very low ride height down bumpy roads, the bumpstops are going to see far too much action. This isn't a fault. It is the consequence of having a specialist car set up for specialised conditions and using it in inappropriate conditions. Nobody expects a Range Rover to set fastest laptimes on the race track.

 

The spacers are not the problem. They are vital for everything to work on the road. On a smooth track you could remove them, but with a stiff setup it will make next to no difference, IME. The problem will only show itself if there is a mid-corner bump.

 

You could, alternatively, take the existing bumpstop and cut it in half, refitting only half of it. This will give a more abrupt bumpstop action which will come into play later in the travel. The consequences when this does come into play will be nasty although it will happen less frequently.

 

You take your chances...

[Mike Bees]

Agree that the bumpstops are there to save your sump. Not sure that the spacers are ideally lengthed though. Anyone done a test to see how much ground clearance they have with the springs removed, i.e. sitting on the bumpstops, with and without the spacers? Maybe a half-height spacer is the answer.

 

Nick - this is only relevant for wide-track, since only they have the spacer (and I can't remember if yours is wide or narrow).

 

Mike

[Nick]

Interesting stuff, thank you. My 7 was originally running pretty well on the bump stops - at rest! Hyperion raised the ride height so that, at rest, there was about 1 cm of travel before the bump stops.

 

At Peter's suggestion, I raised the rear ride height, initially by 2 turns, then 4. This (as far as I can tell on the road) has cured my understeer problem but has resulted in more tram-lining (but not badly) and a bumpier ride (definitely slower on some roads). I wonder if 4 turns was too much and has upset the castor, and may also have 'strecthed' the shocks too long so that they are 'topping out' - my term for the opposite of bottoming out.

 

Maybe I could drop the back by 2 turns, and then drop the front by 2 as well. To my thinking (and I stand to be corrected) this would give the same or similar effect as I have now, without changing the castor as much (I don't know how to adjust castor); one problem is that it would reduce suspension travel before the bump stops come into play.

 

I shall leave the spacers alone.

 

All good fun - I suppose!

 

Cheers, Nick

[Peter Carmichael]

If you do the trig, the height difference from the change in platform heights is irrelevant to the castor. The ride height at the rear tunes a rear steer effect restricted to the rear axle.

 

Nick, I get the feeling that you are going to try jumping around a lot until you chance on a good setup. Looking at the whole system and setting up a baseline which is theoretically close will be a more productive approach and get you there quicker. The baseline is something I can give you, but the closed loop feedback and testing, making small adjustments is something that you can then do yourself. The key bit is that the adjustments from the baseline should just be a matter of fine tuning.

 

You are now talking about bumpy road manners, topping out and all sorts of things. All we were setting out to do was to cure a bit of understeer.

 

Don't try anything too drastic at any one time. Don't be in so much of a rush that you change lots of things at once because you will lose sight of cause and effect.

 

I wouldn't change the front again. 10mm damper travel before the bumpstops is a good baseline figure and won't be badly wrong in any respect. Have you had the toe and camber checked after the ride height change? If you touch the front you have to check and realign everything.

[Nick]

Peter - ah, the voice of reason!

 

After Hyperion's flat floor setup, the car felt pretty good - but still prone to a bit of understeer. At this stage, you suggested stiffer front springs, raising the rear ride height by 2 turns, and changing the rear arb from 4 (stiffest) to 2. I haven't touched the springs, but did raise the rear ride height by 2 turns. I posted this in the original thread but cannot remember if you saw it. I went for a drive after the adjustment and decided to raise by another 2 turns, although the platforms were nearing the end of available adjustment. Another drive, and things felt better. I then adjusted the rear arb from 4 to 2, having adjusted the length of the links to suit, so that the arb was not under tension at rest; hopefully I have explained that well enough!

 

It all seemed much better: understeer had gone but it wasn't chronic oversteer. I was more aware of the rear of the car but wasn't sure if this was because it was too rear biased (in which case I could wind the rear down a bit) or because it was a new sensation that I simply wasn't used to. Also, there is only so much I can tell from the road rather than a track session.

 

Later on, I drove along a bumpy road I know quite well but hadn't driven on for a while. I couldn't go as fast as before, as the car didn't ride the bumps so well, and occasionally there was a clonk from the rear that had never happened before; the car also tramlined more, although not horribly so.

 

This led me to wonder if the adjustment to the rear ride height had affected the front castor, and also if perhaps I'd 'stretched' the rear shocks too much and the clonk was them over extending. Maybe you saw my reasoning, even if it was invalid!

 

I have not touched the front, and following your advice, will not do so.

 

I have not had the geometry checked since I raised the rear ride height.

 

I am keen to follow a reasoned route and do not want to jump around. Reading your post, I see what you mean and appreciate your advice rather than just giving up on me! I thought I was being logical but can see now that that was not so - a little bit of knowledge is a dangerous thing.

 

Do you think my perceived increased tramlining and lower speed on bumpy roads is actual? If so, could this, together with the clonking rear, be a result of my raising the rear ride height?

 

I guess I should revert to how things were to re-assess? The trouble is, my back's playing up and so I can't do that - and you know how it is when you want to play but can't!

 

Once again Peter, thank you for your help thus far, and for pointing out that I was straying. Any light you could shed on bumps and clonks would be much appreciated.

 

Cheers, Nick

 

PS - Good luck fine tuning that engine of yours - and then, no doubt, having to play around with your own suspension again!

 

Edited by - Nick on 3 Oct 2000 15:14:28

[Peter Carmichael]

FWIW, I have all the balance I need with the spring platforms close to the bottom of their travel, not the top. This makes it look like there is something profoundly different between our two cars. This is where taking stock of the whole set up is in order. I have emailed you a recently compiled description of my suspension set up which may help you spot something.

 

Peter

[dogvet]

Much as I admire the threads on this topic, surely the only way to even attempt to assess the tweaking is on the track with data logging. Otherwise it is all too subjective. I would suspect that only the very best and very intuitive drivers even on a track can replicate sequential laps that would give any meaning to the time differentials.

Having decided to leave my car alone for a season and just do a load of track days, my car handling, perception of what the car is doing, and consequently the lap times are much improved.

"Change the nut behind the wheel" seems to be a reasonable adage!!!

It also seems to be reaffirmed by an Elise driver with a standard car that has won a variety of novice drivers awards against a lot of tweaked kit

at sprints and hillclimbs by track practice.

 

Edited by - dogvet on 3 Oct 2000 16:18:53

 

Edited by - dogvet on 3 Oct 2000 16:22:36

[Nick]

I appreciate your comments, but my position is that I have done a number of trackdays over two years as you are now doing them, ie without changing setup. I have driven a 7 that felt superb, and am interested to see if i can get mine to feel the same; it is precisely the subjective feel I am currently interested in, rather than outright speed; I am aware that the feel I like may well be slower, and then I shall possibly revert to my known settings, or a compromise.

 

I am not a 'tinkerer' by nature but would like to improve the feel of my car; if it can be achieved relatively simply with a bit of help from the likes of Peter, I will be very pleased; if not, I know my original settings and so can easily revert and just carry on as before.

 

All the best, Nick

 

 

[Jason Plato]

Wow , a fair old response on this question !!

 

I decided to investigate if the bumpstops are comming into play during hard cornering / bumpy roads and if the bumpstops are saving my sump ( Good thought Mike )

Then I will know if there is any margin to play with in the removal of the rubbers or the spacer .

To determine this I will apply the tops of the dampers with chalk which will mark the bumpstop when contact is made .Then give it a damn good thrashing around my local trading estate / sprint circuit / test circuit as soon as we get a bit of dry weather .

 

What clearance beneath the sump do other owners have with a 1.6 S'Light running dry sump ?? . I've got 55mm with driver ( 90 KG ) . although this is incresing as the miles pile on ....... the sump has had 5mm of ally scraped off its base already whilst thrashng around Wales .

Cheers

Dave

[Nick]

I have heard that apart from the obvious nightmare of really clouting your sump, it is possible to knock the sump drain plug loose with the consequence of rapid and complete oil loss. sad.gif

[Jason Plato]

Nick ,

Yeah been there done that ,the hose connection which enters the lowest point at the rear of the dry sump pan was cracked over the kerbs ! at Oulton park .

thankfully one of the guys in work was able to weld and tap a new thread for the plug .

Caterham have since uprated this plug with a stronger mounting bracket .

dave

[Graham Sewell]

Doesn't this imply that you have the ride height set too low for a road going car! Councils are leaglly able to put in speed humps of upto 4 inches, and as the sump is behind the wheels they will not be protected.

 

This is where a decision on the use of the car needs to be made - either road or track because the optimum setup for both is an impossible compromise (even in a Seven).

 

As ever, YMMV

 

Cheers,

 

Graham

[Jason Plato]

I agree with your thoughts Graham , maybe it is too low ? .

This is why I would be interested to here of the ride heights of other owners disregarding the reccomendations in the build manual

The ride height has not been altered for 10,000 miles and speed bumps are not an issue where I live thankfully !.

The handling however is fantastic and well ballanced with excellent results in 8 sprints this year , so I dont want to alter it too much .

Dave

[Peter Carmichael]

The bumpstops do not come significantly "into play" until they are compressed a good deal. You can squeeze them between your fingers quite easily to achieve 5-7mm of travel. Therefore if you have 10mm of damper travel before touching the bumpstops, you have over 17mm of damper travel before anything significant happens. That is equivalent to 27mm of bump wheel travel on widetrack (ratio of 1.63:1), leaving you with 28mm clearance on the sump if this is happening in pitch/heave. Which means you have just over an inch of travel in which to accelerate the car's mass away from impending sump destruction, with the prospect of catseyes and road crown eating into that margin.

 

So for your 55mm clearance, half of it is essentially free travel controlled by the spring/damper rates and half of it has a progressive rising rate provided by the bumpstop, which I suspect the bumpstop will restrict you to using about 15mm. i.e. the sump will have a margin of 10mm to allow for road irregularities.

 

This appears to me to be ideal behaviour for a low-riding roadgoing Seven.

 

What is unfortunate is that these sort of wheel deflections are much more commonly encountered by individual wheels in cornering when they hit mid-corner bumps when the sump is not under threat at all. You can't fix that without pushrods and clever clever custom suspension or raising the engine in the chassis (think of it as lowering the chassis around the engine and it doesn't seem so bad).

 

If you remove the bumpstops or spacers you will have to avoid driving down bumpy roads, presuming that you do so at the moment.

 

And seeing as you ask, my Superlight has about the same clearance as yours.

 

Edited by - Peter Carmichael on 4 Oct 2000 16:38:44

[Nick]

I hope you all took notes, as there will be a short quiz after the interval...wink.gif

[Jason Plato]

So are we all agreed ........... mine & Peters cars are chuffing low , however they are not too low .

If I were to remove either part of the bumpstop or the top spacer I would be significantly decreasing the possible lifespan of my sump .. if I continue to drive flatout down bumpy roads and take the apex at Oulton Park on the inside of the kerb .

 

55mm between sump and road does'nt mean I'm fat , it is quite normal on a superlight ,or of course Peter is as fat as me ! .

 

So in conclusion , I had ned not write my original question in the first place ??!!?? and I'm not fat and my car is not abnormal !!

:-)

Cheers guys I feel much better now , think I'll have another donut before I write my next pointless question .

Yours Happily

Dave .

 

 

[Jason Plato]

Finished my donut .

 

Just a thought ..............why then can everyone else at the Bristol club meet drive over the mini sleeping policeman , but I can't ????.(12-14 cars EVERY month !!!! )

You sure our cars are not abnormal Peter ???

 

Dave