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Damper Sizing On Lowered Gt6

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Could someone provide some info on min/max length on a race lowered gt6. Working on rear right now but need front info too. I am using the chassis mount points, not the wheel well points. Thanks. If the standard length spit rear koni/spax will work that is all the info I need.

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The only damper that works on the rear of a GT6 (or Vitesse with the chassis brackets,as per my book) is the Koni 80/1717.

 

There is a reason for this.

The Koni damper is the only one (within reason, unless you go for something exotic), that can get to an almost 50/50 compression/extension ratio.

 

This is due to the position of the valve right in the very bottom of the unit, so if you have 3" of bump and 3" of rebound from what should be roughly mid travel you can have a damper approx 9" between eyes at static.

 

That would be the basis of any calculation, and what is very very frequently wrong on those triumph cars, including TR6 & GT6.

 

(eg. The Spax dampers I checked 2 week ago, fitted to TR6 rear are 15.5" fully open between fixing points.

This is too short, but if you were to lengthen this to the right length to 16.75" you would get a fully closed setting of only 11.75" so it would then hit the bump stop, way before the spring is in sufficient compression for proper suspension travel.

 

If you use a Koni instead you could that 16.75" damper to close down to approx 8.5-9" which is exactly what is needed...)

 

quote of last week "I wouldn't even fit Spax dampers to a shopping basket".

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GT - So those are spec'ed as front shocks for a mini. Just making sure you meant 80/1717.

 

It doesn't matter what they are originally spec'ced for.

It's no suprise a lowered cooper S 1275 has absolutely minimal suspension travel as lots of British stuff has similar situations/constraints.

That engine even has the same conrod length as a TR6 (!)

 

All you have to do is to change the damper bush because it's rather small.

 

I would have it revalved, but that is just what I do.

 

If I ever need Koni lengths I either call my specialist Koni sport specialist or send a mail to Koni France.

They are both very helpful.

 

TBH if you want a Koni made with reduced stroke for the front,- (absolutely neccessary if you need to make a high rate spring sit right in the platform fully extended rather than flopping about), then you need to do a proper racing dual adjustable damper.

 

It's heavy money, but everything that works properly is.

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If you aim for new dampers you should be aware that it is helpfull

to have a proper damping built in first as this afflects necessary lengths.

The "Cheap Charlys" offered for the Triumph are mostly not the best

but a compromise between costs and roadholding.

 

I wonder why never a yellow Koni was invented what fully opened would be

much better than a closed red one what is only suitable on perfect street.

 

The bounce/rebound rate of a good street damper should be in the range of 1:3

Normal red konis have only a poor bounce rate and by turning in the bottom valve

only the rebound is increased beyond all limits.

 

The rebound length depends on spring and damper rebound rate and anti roll bar and must be tested

not let the car under hard driving conditions go into the rebound stop.

Not popular to say but tuned cars need some individual tests.

It is easy to understand that if you inhibit the damper to rebound by closing the valve

it takes a long time to travel to the rebound stop.

Therefore not that much rebound way is required.

This guides to the rebound length depends on closing of bottom valve, what is individual!

 

If somebody might ask why not make bounce and rebound length longer than necessary

the answer is that in practice the space is limited.

 

The bounce stop is easier to test, only springs and dampers need to be taken off and axle has to be lifted against the stops.

Than measure the distance between the connecting points for the damper and reduce what you got for the estimated amount

of the rubber bump stops to be compressed.

Never ever a damper is allowed to collapse fully! Something will break! Only at the front of TR6 the bump stop is integrated in the damper.

But carefull! So often I could see the lowered car sitting on the bump stop with bad influence on spring rate and bounce way.

Under normal driving conditions the bump stops both bounce and rebound are not allowed to hit!!!!!!!!

 

 

Do not believe in other data!

Our cars are individual and never had good shock absorbers

or proper setup with springs and anti rollbars due to the money requiered.

It is essential to go through the process of developing and have proper dampers.

The result is impressive and gives road holding like a modern sports car.

 

And at last the complicated process of setup might be the reason that many people claim

to get other/better solution on other ways. They simply run into a one way street and stuck in!

Than even the horrible manipulation on lever dampers might give better results but are never good ones!

 

Just could see a video of the 44 Bob Tulius car with bad road holding.

You can see that the car his tending to bounce on the rear especially when cornering.

Today such a behaviour need and should not be tolerated.

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The bounce/rebound rate of a good street damper should be in the range of 1:3

Normal red konis have only a poor bounce rate and by turning in the bottom valve

only the rebound is increased beyond all limits.

 

Our cars ....never had good shock absorbers

or proper setup with springs and anti rollbars

 

It is essential to go through the process of developing and have proper dampers.

The result is impressive and gives road holding like a modern sports car.

 

I pretty much agree with most of above, but it's doctrinaire to state "a bump/rebound" rate as being right for the car.

This is variable with many other factors.

You are also correct that STD Koni dampers are far from perfect & have been made wrong for the best part of 35 years for all those Triumphs.

(The koni specialist was somewhat astonished when I told him that).

+

I found a 15-20% variation in what what was supposed to be a brand new pair of konis when we measured them.

What chance on earth, do you get from any other Avo/Spax etc marque?? About zilch!

 

It may interest you to know, I had to make quite a lot of changes because of the improved behaviour with prog springs.

 

Here we really are in non-linear territory, which when you start taking weight transfers + right ride height into the equation on a heavy front/rear engined car, it's a non trivial calculation to get right, inevitably needing highly revised damping.

 

Here is a double adjustable I made for the Countach front with prog rated spring.

 

You have a raging animal behind you going to wild weight transfer & light front end, with 500bhp of 48v V12 power, then goes to big compression on heavy braking.

 

In short 2 totally different extremes of the damper/spring spectrum.

 

koni_lambo.jpg

 

The original kit at the bottom washed out madly on roundabouts and was horrible, because it would then try to swop ends as soon as it hit that dump of torque at 4000-5000rpm.

This cured most of the trouble, but we didn't use 3:1 damper ratio (far from it).

 

I'll try to post some of the other weird stuff.

Here is what I did for another dual wishbone set-up on the Jag.

This is a single adjustable and we have whopping weight transfer and load problem in this place.

 

The Koni damper is overloaded in OEM application, there's no doubt, which is why they specced Bilstein for the "sports pack"

Jaguar of course were just messing about, not doing things evenly remotely properly as usual.

 

It needs MASSIVELY more bump setting at low spring rate & about 30% more rebound from high suspension travel back, or we can't control the huge amounts of energy being thrown around.

All Jaguar did was fit heavier dampers and leave it that, making the whole system open to shock loading and axle tramp thanks to the rear anti-dive playing havoc under braking

 

This would be the classic place for a double or quadruple adjustable damper.

It also happens to be supporting easily 1000kg at times on that spot on occasions!

 

We can "get away" with the single adjustable with a higher rebound setting, but due to the construction of the damper, can't be reconditioned so has to be made with revised settings from new....

 

The antidive aggro can't easily be dialled out, but what can you do when half the suspension is bodged to a subframe and the other bit is bolted to the body?

:woot:

 

koni_strut1a.jpg

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I pretty much agree with most of above, but it's doctrinaire to state "a bump/rebound" rate as being right for the car.

This is variable with many other factors.

 

I found a 15-20% variation in what what was supposed to be a brand new pair of konis when we measured them.

What chance on earth, do you get from any other Avo/Spax etc marque?? About zilch!

 

You have a raging animal behind you going to wild weight transfer & light front end, with 500bhp of 48v V12 power, then goes to big compression on heavy braking.

Misunderstanding: The rebound rate itself may be choosen to the needs of the car and differs widely.

But the damper is specified normally by four data, the low- and highspeed rate for bounce and rebound.

I wanted to say that if you specify the bounce rate you are not free to take any rebound rate

but it is recommended to take the rebound about three times higher.

 

That is exactely ignored if you have a damper that only changes rebound

and maybe end up to 20 times higher than bounce. So one opinion must be wrong.

 

I had the chance to visit dampers worksite years ago and watch my dampers being modified.

After that each damper is put into the testmachine and tested individually and they are all the same.

 

The raging animal is purely the same if you put a V8 into a TR6,

a bit less power but much less weight than the Countach.

I am happy all worked out the last 30 years and will not drive that fast anymore

but there was also the need to do something on suspension to keep on the street.

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Misunderstanding: The rebound rate itself may be choosen to the needs of the car and differs widely.

 

But the damper is specified normally by four data, the low- and highspeed rate for bounce and rebound.

 

I wanted to say that if you specify the bounce rate you are not free to take any rebound rate

but it is recommended to take the rebound about three times higher.

 

That is exactly ignored if you have a damper that only changes rebound

and maybe end up to 20 times higher than bounce. So one opinion must be wrong.

 

I was lucky to meet the bloke that designed pretty much all the race dampers that have been any good for the last 30yrs in the world.

(Slinzac)

He worked for and designed lots of them inc Penske, some Koni stuff and lots of others.

 

Spending an hr chatting with him made your head go round.

Another of those lucky people to "bump into", on the rebound (duh sorry chaps, but couldn't resist!)

 

Funnily enough, we were talking about the Lambo stuff.

He said it sounded about right at 60/40 split* after we had taken them off to measure what worked best by feel,-

The original dampers were all crap on Italian exotica.....all spring preload, no bump, loads of rebound & thick rollbars!

 

(*That was very close to what was used on some quite famous Ford rally cars they worked on.....)

 

A friend of mine used Proflex on the Challenger race cars which won just about everything.

Reiger and Proflex are both Dutch (again).

 

http://www.reigersuspension.com/

http://www.proflex-shockabsorbers.com/

 

They are among the world leaders.

Don't forget, in a competitive formula a banal DAMPER can get you 1-1.5s a lap, MUCH more in the wet when power is no use but control is.

Handling and braking have always been the ways to beat much faster more powerful cars

 

(Makes me think of the Williams engineered 6R4 Group B cars again).

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He said it sounded about right at 60/40 split* after we had taken them off to measure what worked best by feel,-

 

Not too many new details I think....

And the crude data are difficult to understand.

What ist "at 60/40 split" ???

And more difficult to understand in a foreign language

even for me although I am right now working on that project.

 

"to measure what worked best by feel"

hopefully does not mean you want to pull out the dampers by hand

and decide if they have the proper damping force???????????

 

Mine simply said have three dampers built in:

One is the Bypass Valve

One is the lowspeed spring loaded valve system

One is the highspeed spring loaded valve system.

 

If you feel on the damper by hand you only fiddle around in the bypass area.

No one can continously open a damper in the lowspeed and not at all in the highspeed area.

 

To come close to the forces that apply at the front axle one should be aware

that the damper has a moving ratio of 1:2 to the wheel what makes it necessary

to a have damping forces in the range beyond 3000 Newton!!!!!!!!!!!

Not even a Gorilla is nearly able to feel that power.....

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"to measure what worked best by feel"

hopefully does not mean you want to pull out the dampers by hand

and decide if they have the proper damping force

No you are quite correct, it's not my normal method to measure dampers.

 

Most people use a dynomometer by SPA, (which I occasionally borrow) but I won't publish my graphs because it would make it dead easy to copy the settings & modify them wouldn't it?

 

The "feel" bit is usually coming from the funny looking thing you sit on located roughly half way down the human body.

It's one of the best "measuring instruments" it's said, and is OK for measuring G forces, in all sorts of strange directions.

(I won't go into what certain people do with it, never mind the "G" stuff).

 

The next best bit attached vaguely by some muscle and bone to the bit you sit on,- the long one on the RIGHT.

This enables the humanoid to work out if he has actually managed to squeeze the RH pedal firmly enough to the metal of the floor from its initial position of rest, so as to get the central "dynometer" working in the correct dynamic conditions.

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On 4/9/2013 at 6:49 AM, GT said:

All you have to do is to change the damper bush because it's rather small.

Evening, firstly apologies for delving into history for this post.

I actually ordered a set of the mini 1275gt dampers, for my mk2 rotoflex GT6, and as you say the holes in the eyes are too narrow to fit the car.

Im running the chassis mount extensions, currently with old spax.

 

GT,  I dont suppose you (or anyone) can remember what size and where you got the wider bushes from please?

Again, apologies for dragging this back out into the open!!

Martin

 

 

On 4/9/2013 at 6:49 AM, GT said:

GT - So those are spec'ed as front shocks for a mini. Just making sure you meant 80/1717

 

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Martin, what possessed you to order Mini dampers?

Suggest you contact Jigsaw.    I buy dampers from Mark for my race Vitesse and they have always been good. Range with the extensions is only about 80mm.

John

Edited by JohnD

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Martin

I have the 80/1717 Konis on my GT6 Mk2 with NJ CV conversion. (GT is correct)

The tube in the damper bush has a thick wall and can be drilled out to suit a 1/2" bolt that should suit the top and bottom bolt fixings. 

Ian

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3 hours ago, JohnD said:

Martin, what possessed you to order Mini dampers?

Because they are the right length and (bottom bush aside) fitting a GT6 mk2 and early mk 3.

Koni don't do a damper for the roto GT6 with inner wing pick-up points.  They do do one for the swing axle cars which can be used either with the CV conversion on the chassis mounts (may not line up that well) or with the chassis extension brackets (not quite the right length but near enough).

Nick

 

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2 hours ago, Nick Jones said:

Because they are the right length and (bottom bush aside) fitting a GT6 mk2 and early mk 3.

Koni don't do a damper for the roto GT6 with inner wing pick-up points.  They do do one for the swing axle cars which can be used either with the CV conversion on the chassis mounts (may not line up that well) or with the chassis extension brackets (not quite the right length but near enough).

Nick

 

Yes JohnD as per Nick.

Thanks everyone for replying (much appreciated)
I'm still running the rotos for now with spax on the chassis extensions.

GT64ever -   I can indeed drill out the bottom hole to 1/2". However..

 my top bolt & hole in the chassis bracket is 5/8" (15/16mm), in fact the new damper outer diameter of the sleeve is 17mm, so drilling that leaves me with no sleeve at all.

Thanks again

Martin

 

 

 

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ok on closer investigation, my spax dont have a sleeve. they have a 1 piece poly bush that narrows in the middle like this =><= my top bolt is just shoved through.

The bottom one is the same because the lower mount has a sleeve on it.

i am going to see if i can knock out these sleeves from the new koni's

 

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Martin

I was forgetting that we fabricated our own chassis extension bracket and in doing so used a 1/2" bolt. (also used it to 'correct' the fore/aft alignment issue).

You might be able to have a couple of shouldered washers made up to allow you to use a 1/2" bolt, which would avoid having to change the bushes in the Konis.

Ian

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Could any / all advise on rear dampers for a GT6 with rotoflex - and chassis extension brackets.

I'd decided to go for Koni, normal would be 80/1389, but in removing the old leaking SPAX units, I realised the top mount sleeve was designed for assembly only, disassembly was a complete arse and needed considerable force and bodgery to get an only slightly rusty sleeve out (there is nowhere to push / drift the sleeve from the 'blank' end - left hand side in image that I'll try and attach....).

So questions:

  >>  Is there a recognised mod to make disassembly easier?  I was going to file a small notch in the left hand retaining part of the bracket to allow a small punch to access the end of the sleeve.

  >>  GT in the messages above promotes using the Koni 80/1717.  This looks to be from a Mini.  Is the selection of this damper to replace the upper mount with a conventional bush (easier to dismantle)? Or...

  >>  Is the 80/1717 selected for it's damping characteristic?  I've Emailed Koni and received the following reply:

The size of the two dampers is fairly similar, as are the mounting points, my concern with your proposal of fitting them on the GT6 is that they are much softer in both bump and rebound force, Rebound can be adjusted up, but the bump force is less than half of what is specified for the GT6, and I doubt they would give the right amount of damping control. 

Part No             Open       Closed      Bump      Rebound

80-1389            316mm    246mm    450nm    950nm

80-1717            296mm    218mm    200nm    700nm 

 

IMG_20181001_163358511.thumb.jpg.64ca4643ed059c002af76598d2e6c702.jpg

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Personally I would not buy any dampers until I had measured the length of the operating  range needed on the car.

10 minute job. Jack the chassis /body up until the wheel dangles and measure the maximum length needed. The measure the minimum length if the suspension bottoms out on the bump stop. With the operating range you can look at the offerings from the manufacturers. My stock shockers (TR6) topped out and the car lifted a wheel - very dangerous on corners. I ended up speaking to Gaz who were pleased to make a set to the operating range I needed which they did within three weeks and they charged me less than the price from the suppliers.

And they work beautifully.

Alan

 

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I have a lowered GT6 MK2. 1/2 Inch plate at diff. Bracket conversion. CV conversion.

I use Spax TT3611. Works fine street(12 klicks from soft)  and trackday (17 klicks from soft).

 

Martin

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