Lotus Elan

Chapman Strut Droop Limit Straps Pictures

PostPost by: type26owner » Mon Jan 23, 2006 12:30 am

Can't find the thread were I was asked for the pictures of my droop limit strap design so here it is. This the Dave Bean Engineering rear strut 2.5" ID spring modification kit. The picture of just the threaded collar shows what's left of the old spring perch flange sticking out of the bottom.
http://briefcase.yahoo.com/[email protected]

Look carefully and you can see the Rohan extra bumpstop in place. Man, does this make the rear stable at 10/10s. :lol:
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PostPost by: gjz30075 » Tue Jan 24, 2006 1:56 pm

Hi Keith,
Get a 'Page cannot be displayed' on the Chapman droop limit design folder. Love those track shots, though.

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PostPost by: type26owner » Tue Jan 24, 2006 2:17 pm

Greg,
It's the Yahoo end being cranky. Click the reload button a few times and it should finally appear. There are 5 pictures there which are 500kb each. Send me your email address offline and I'll attach the pictures in my reply.

I will try to record a movie with audio of a hot lap of Laguna Seca with my digital camera this time and place it in a folder here. Everytime I get to the apex of Turn 5 I realize I'm going at least 20 mph too slow. That turn has my number for some reason and I really suck there. :x :x Turn 5 is the one after going under the Bosch bridge at the end of the second longest straightaway. This is Windows Media Player file.
http://www.skipbarber.com/videoFiles/laguna_seca.wmv
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PostPost by: gjz30075 » Tue Jan 24, 2006 5:34 pm

Thanks Keith. You're right; a couple of hits on the reload button did it. I don't know if this has been mentioned before but what is the length of the limiting strap?

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PostPost by: type26owner » Tue Jan 24, 2006 5:50 pm

The droop is about the same as it was with the donuts still installed which is about 2 inches from the end of travel of the Spax inserts. It's really determined by whether or not the inside front wheel lifts off the road when cornering at 10/10s. It won't lift off with the stock swaybar because the bar is too soft. It will with a 13/16" one or thicker. If it lifts up then I shorten up the strap until it stops doing that.

Setting up the car to behave extremely well so driving to the limit is not physically hard is not all that expensive or difficult. It's the mental concentration level that kicks my butt after a little while. How those guys do it for hours at a time amazes me.
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PostPost by: jkolb » Sat Jan 28, 2006 3:43 pm

Keith, did you fabricate the lower mount, or is this something that is available? Is the interior surface machined to match the threaded collar?

Jerry
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PostPost by: type26owner » Sat Jan 28, 2006 4:25 pm

Made the aluminum lower clamps up from scratch with no drawing. Both sides are identical and they must be oriented like I have it to keep the nylon strap away from the tire/wheel and a hot brake rotor. The top steel angle-iron anchor on the chassis are different side to side. Only because I made two up to fit the right-hand side. When I went to install the left-hand one it didn't fit. Should have made two up to fit the LH side and it would have worked on both. Oh poo poo, it's a Lotus so getting it the wrong way around is fully expected. :(

The lower clamp is a location clearance, class one fit onto the OD of the acme threads of the DBE epoxied sleeve. The droop impact force is taken up by it butting up against the adjusting nut that is the spring perch. To adjust the strap length I actually make up the appropriate length I need and replace the strap rather than move the clamp. You could move the clamp and add captured split spacers though. I'd just loose the spacers eventually so that's why I did not do it that way. The straps are easy to make. The holes are melted through the nylon webbing and the ends are melted to stop it from unravelling. It's from http://www.mcmaster.com/ and it's p/n 3510T12.
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PostPost by: paros » Tue Feb 07, 2006 5:01 pm

Keith

Your suspension droop idea raised questions!
I am able to use slicks in Greece - only 6" wide 21" diameter and I certainly could lift the inside front wheel even with the standard front roll bar!

On corner exit I partly lift the inside rear as the car goes diagonal and the front inside lifts. Thus loss of traction, although for this year a plate type lsd should help as aginst my favourite torque biasing diff. I am also going from 170 lb rear springs to 185 to try and stop the rear squat on corner exit, and in doing so maybe reduce corner entry understeer.

I saw your silasto extra rear bump stops - I used to use these at the front of a wild Cossie powered Elan in the UK, but I would install them solid against the damper at ride height so there was no shock load and bump understeer as the springs suddenly went stiff - do you not get bump oversteer as the extra bump stop hits the daper under load? I tried an extra bump stop at first when I raced the current Elan in Greece and although it may worked at the steady state condition, I was always fighting so much oversteer I reluctantly removed the extra stop [ maybe they were too stiff? ]. I used zero droop in the Uk by getting the shocks tweaked with a tube inside to limit droop - many Elans years ago used at the front a leather strap around the lower wishbone to limit droop.
Richard
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PostPost by: type26owner » Tue Feb 07, 2006 5:24 pm

Richard,
With enough grip level like it sounds you might have the narrow wheel track of the Ealn comes into play and the car will try to bicycle up onto the two outside wheels. Bet you're there. Dave Bean told me he sold his 26R because he'd gotten up on the bicycle too many times and was not keen to continue to push his luck. Be careful.

My crummy Potenzas don't provide me with enough side bite to make the car want to bicycle on over except on the off-camber right-hand turn at the bottom of the Cyclone at Thunderhill. That corner I have had it happen but recovered in the weeds. I coast through that turn with lots of fear now.

Yes, when the outside suspension is supported on the bumpstops one has to avoid any bumps or rumblestrips. Carroll Smith has quite a bit of advice on this subject. You can zing right over the tops of the curbing or the rumblers with the inside wheels though, no problem! Looking forward to rumbling over the ones at the top of the Corkscrew. :D
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PostPost by: rgh0 » Wed Feb 08, 2006 10:40 am

Richard

A few comments on your suspension issues from my experience for what its worth.


> I am able to use slicks in Greece - only 6" wide 21" diameter and I >certainly could lift the inside front wheel even with the standard front roll >bar!

I could do the same when I ran formula ford slicks many years ago. Front wheel lifting has more to do with grip and squat on the diagonal opposite rear wheel than front roll bar


>On corner exit I partly lift the inside rear as the car goes diagonal and >the front inside lifts. Thus loss of traction, although for this year a plate >type lsd should help as aginst my favourite torque biasing diff.

I used to do the same particularly in LH corners in my RHD car. My Quaiffe diff solves this problem provided I dont 2 wheel the car. I dont recommend trying acceleration on 2 wheels if a plate type diff enables it unless you are an experienced bike rider and have a good roll cage!


>I am also going from 170 lb rear springs to 185 to try and stop the rear >squat on corner exit, and in doing so maybe reduce corner entry >understeer.

170 to 185 pounds is to small a step to make any real difference do it in 50% increments of spring stiffness if you want to feel any real difference. 170 lb rears should be more than enough and going up to 250lb will make the rear end very twitchy. Softer springs of 100 to 150 lb and spaced bump stops adding around 50 to 100 lb when in play gives the same rate on the limit to stop excessive squat while preventing excessive nervousness during the initial transition. Buying proper progressive rate springs for the rear gives the same at a higher cost


>I saw your silasto extra rear bump stops - I used to use these at the >front of a wild Cossie powered Elan in the UK, but I would install them >solid against the damper at ride height so there was no shock load and >bump understeer as the springs suddenly went stiff - do you not get >bump oversteer as the extra bump stop hits the daper under load?

No, the spaced out bump stop comes into play before you are fully loaded in the corner so no sudden transition to understeer.


>I tried an extra bump stop at first when I raced the current Elan in >Greece and although it may worked at the steady state condition, I was >always fighting so much oversteer I reluctantly removed the extra stop [ >maybe they were too stiff? ].

The standard bump stop spaced out is about the right spring rate in my experience. Without knowing your total setup hard to comment on why the oversteer but running high grip slicks and standard front roll bar would tend to generate oversteer with most other spring combinations


>I used zero droop in the Uk by getting the shocks tweaked with a tube >inside to limit droop - many Elans years ago used at the front a leather >strap around the lower wishbone to limit droop.


Limiting rear droop is all about keeping the drive shaft angles acceptable for me at least. By the time the droop limiter comes into play the inside rear is not doing much anyhow in terms of the handling as actual load on the wheel is only around 30 kg at this time.

I dont understand the comment about limiting droop at the front or why you would do it


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PostPost by: paros » Wed Feb 08, 2006 1:40 pm

Rohan
Thanks for the comments - I will sit and digest them.

As to why the front with zero droop. This was a Lotus Elan with a Cossie engine racing in GT with some aerodynamic download BUT the advice to use zero droop came from a then F3000 team owner - so I did! The result, and I have to say as far as I remember it was done with no other cahnges was half as second a lap.
Friend Carroll Smith advocates zero droop somewhere BUT I have to be honest and say when I have spoken about this to other people I cannot tell them why it should work. Car did not pick up inside front and had a very stiff fabricated chassis with substantial roll bars and 650 lb front springs.

The main track in Greece is bumpy and tight corners and two of them I loose traction as the inside wheel comes airborne - this confused the Quaife. Both corners were onto important straights, so I am hoping that I no longer get 7000 airbone revs with attendant shock loads when the inside wheel lands once I use the plate diff. I have gone for 90/45 ramp angles to try and get the benign cahracteristics of the Quaife at corner entry.
Having to do all this as although I won races last year I now have a genuine GT40 in my class!

I agree that 170 to 185 is a minute change but the car handles well [ eats tyres ] and I want to keep the balance, thus I aim to try and reduce the dipping at the rear on corner exit and full throttle as I should have more power which would potentially make the problem even more noticable.
First race is in a month so then it is theory into practice time!
Will see where I put the old yellow bump stops and maybe think again - thanks.
Too cold to work on the car this week as it is 3 degrees and very strong wind so gives a bit of thinking time!
Richard
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PostPost by: type26owner » Wed Feb 08, 2006 3:46 pm

Richard,
IIRC, the problem I was having was the front outside tire was being overloaded and the car would plow or understeer and as a result it was dreadfully slow but safe for the general public. By adding the bumpstop on the rear strut when the car leans over it bears down more on the rear outside tire taking some of that load off the front. This made the Elan go to neutral handing when at the limit of the traction circle and a bit faster and a whole lot more fun to drive since it will respond to steering inputs at anytime now. The transformation was quite remarkable.

Everyone, If you think you're fast now let me assure you the car can be tweaked to extract even more. :lol:

When Rohan suggested the cure I had my doubts but trying it was so easy it would have been silly to ignore his advice. All I can say is Rohan is the handling guru on this list. :wink:
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PostPost by: rgh0 » Thu Feb 09, 2006 9:53 am

Richard

Thanks I now understand what your front zero droop was all about. Its the equivalent of an infinitely stiff front roll bar which probably makes sense given the type of car and other suspension mods you were running.

It has some problem in transitions as it comes in instantly but provided it occurs before the car is fully loaded in the corner that is no problem just like the rear bump stops.

Softer rear springs and the spaced bump stops would help accomodate the bumpy track more than the plate diff?

But against a GT40 the only thing that will help is a really tight track.

Keith
I come from having struggled for many years on my own to understand how to get my Elan to perform well in competiton at low cost before the advent of the internet. Not a guru, but just someone who wants to share his experiences and I try to emphsise where its a personal situation rather than something that can be applied more generally by others. Handling is very personal in a lot of ways and depends on how your seat of the pants sensors react to car dynamics.

About 5 years ago I started tuning my corner entry and exit speeds to stop 2 wheeling in LH corners and at that point I new I was approaching the limits of a standard width Elan.

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PostPost by: type26owner » Thu Feb 09, 2006 2:03 pm

Rohan,
You saved me from having to do at least 100 hours of hard labor building a rear swaybar. I was seriously considering selling the car after my first horrible experience on the racetrack because it handled so poorly, was slow as a snail and not much fun to drive. You and I obviously have similiar opinions for the desired handling. You well deserve the praise I'm giving as feedback. :wink:

What level of G-force are you obtaining just as a guess? IIRC, the TireRack did a skidpad test years ago of all the performance cars and the highest cornering roadcar ever was the Super7 at 1.3 Gs. The only car to top it at 1.6 Gs was a winged sprint car with gobs of stagger. I'm approaching 1G I think with the RE92s.
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PostPost by: rgh0 » Fri Feb 10, 2006 8:40 am

Keith
To 2 wheel it I have to be getting over 1 G at least in transients. I am not far behind a friend of mine in his S4 Seven on similar tyres so if a Seven can do 1.3G I cant be to far behind that.

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