Lotus Elan

Oversteer on corners

PostPost by: JonB » Wed Feb 07, 2018 8:42 am

Ah so that is why you bumped it! If only I had a lathe and my welding skills were up to it.

When I measured the rear toe in and reported the result on another thread the comments were that it was bang on. Since then the bent n/s wishbone has been straightened and this may have made a slight difference. Enough to throw it out? We will see, but Occam's Razor suggests to me that my front alignment is the real problem. I have to say that faffing about under the car on the floor, trying to set the front alignment was not easy and I undoubtedly got it wrong.
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PostPost by: Orsom Weels » Wed Feb 07, 2018 9:36 am

JonB wrote:Occam's Razor suggests to me that my front alignment is the real problem. I have to say that faffing about under the car on the floor, trying to set the front alignment was not easy and I undoubtedly got it wrong.


Jon, one of these makes life a lot easier, http://www.trackace.co.uk/ & the price makes it just about viable for occasional DIY use. They are good, & easy to use. I've used one in the trade for some time, along side my old faithful Dunlop opticals, which are now about ten years older than god :D

Regards, Tim
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PostPost by: rgh0 » Wed Feb 07, 2018 9:47 am

Jon
Earlier in this thread I gave you a list of things to check or confirm - have you worked through all of those ? You appear to be focused on wheel alignment which is certainly important but there are other issues to consider

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PostPost by: JonB » Wed Feb 07, 2018 12:24 pm

rgh0 wrote:Your car should also have aluminium spacers that fit on top of the Chapman struts to bring the Aeon rubber springs into play earlier, do you have those?


Not sure. The Aeon rubber is present on at least one side (the nearside), looks healthy. Can't generally see it as there is a sleeve covering it, but noticed it when the car was on stands.

What is the front and rear toe- in?


Not measured since refitting the wishbones. Will find out when the alignment check is done, and report.

However, my rears were supposedly spot on when I reported on them a few months ago - but since then I have new bushes and the bent arm was straightened. Given I'm going to the alignment shop soon, I don't see the point of trying to measure it now. But like I said, I am pretty sure I have got the front wrong. I spent a lot of time trying to do this and ended up with the steering wheel out of line.

Do you have any other non standard front or rear components in the suspension?


Not as far as I can see, apart from the just-fitted CV joined driveshafts. The bushes are standard rubber. I think the rear shock absorbers might be Konis, there is an invoice in the history file that includes "machining Lotocones to accept Koni dampers" or some such (away from home now, can't confirm the wording but it did include "Koni" and "machining"). Rings a bell?

A properly setup Plus 2 should be nice and neutral unless a corner is really stuffed up and then it should be gentle under steer.

Can you describe more the situation when you experience over-steer e.g. speed, type of corner, load in car throttle or braking situation etc


At the point of testing: load was three adults, speed was "reasonably quick but not excessive", type of corner was fairly tight left hand - single apex, throttle on, no braking. We are talking about fairly enthusiastic driving down a UK B road that is in good condition - fast and windy, the sort that is fun to drive down.

Sorry if this is a little subjective. It's obviously not properly set up! I'm just hoping I do not need to fit adjustable rear wishbones, as I have had enough expense for now.
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PostPost by: Donels » Wed Feb 07, 2018 2:08 pm

In my experience I would suggest excessive front toe in. As the outer wheel becomes loaded during cornering the toe in on that side has a stronger effect causing the car to turn in more, making the car feel 'darty' and unstable under hard cornering. Excessive play in the front bushes could also have the same effect.

If it is the bushes just setting the tracking, no matter how accurate, won't fix it. Need the check the bushes first. Also check the steering rack bushes to check the rack is not moving and that it's correctly shimmed.
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PostPost by: ecamiel » Wed Feb 07, 2018 3:44 pm

People seem to missunderstand the term "Bump Steer"

Bump steer is a change in wheel direction (toe in or out) which happens with suspension travel, regardless of whether that travel and change of "toe" is caused by the car leaning in a turn or by a wheel hitting a bump.

Oversteer (another misused term) is when the rate of change of the slip angle is greater at the rear than at the front.

The condition you describe is the car's non-linear response to steering input.

I don't want to start a rave here but the correct terminology will help folks help you solve the problem.
And no, I'm not an engineer and will happily be corrected.

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PostPost by: JonB » Wed Feb 07, 2018 9:09 pm

Oversteer - I think you are describing "real wheel slip", but I agree with your definition. I used it as short hand for the car turning in more then the steering input would allow for. So, I turn the wheel a certain number of degrees to go round a single apex bend and it adds more degrees (maybe as the suspension loads up). I do not know what the correct term for that is, but I think I described it clearly enough.

I do know what bump steer is, as it's been discussed here many times and I've read the threads.

I don't want to start a rave here but the correct terminology will help folks help you solve the problem.


OK, so what is the correct terminology for "non-linear response to steering input"? I think you will find that the cause has a name (read: jargon like term), but perhaps not the symptom.

Re: starting a rave - best not to, eh..? :D
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PostPost by: MarkDa » Wed Feb 07, 2018 10:28 pm

Why not?
You did quite well with the zetec suggestion!
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PostPost by: JonB » Thu Feb 08, 2018 8:54 am

Yeah Mark, that was a bit of a holy war wasn't it?
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PostPost by: rgh0 » Thu Feb 08, 2018 10:45 am

If you have standard springs and dont have the aluminium spacer on top of the rear strut and/ or the Aeon rubber springs are in poor condition you will get significant roll overseer in the situation you describe especially 3 up.

Rear alignment issues may also contribute but that tends to be a more gentle effect unless it is wrong in a major way which is unlikely with standard A frames and bushes in good condition.

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PostPost by: JonB » Thu Feb 08, 2018 1:48 pm

Thanks Rohan. I will check to see if I can see the alloy spacers and Aeons. I assume the spacer lives right at the top of the strut, against the Lotocones. However, it is not shown in the parts diagram itself:

http://www.lotuselan.net/wiki/DA_-_Rear_Suspension_%2B2

..though it does say in the table, in the row below item 21: "Spacer - bump limiting" and "From chassis No. 1531".

Since my chassis is 1115 I expect not to find the spacers fitted unless they were the subject of a recall or service bulletin.
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PostPost by: Orsom Weels » Thu Feb 08, 2018 4:46 pm

JonB wrote:Thanks Rohan.I assume the spacer lives right at the top of the strut, against the Lotocones.


No Jon, they sit on the top of the damper tube, over the shock retainer nut. The aeon rubber sits right at the top of the shaft abutting the spring cap.

This is what they look like

+2 bump stop spacers.jpg and


The recess locates them on the top of the tube, they should be a tight fit over the nut, not rattling around

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PostPost by: mark030358 » Thu Feb 08, 2018 10:35 pm

Trust me you will have toe out at the rear of the car.
See my thread, I had exactly the same and other issues....

lotus-suspension-f42/stability-speed-t25650.html?hilit=Mark030358#p167784

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PostPost by: nmauduit » Fri Feb 09, 2018 9:13 am

mark030358 wrote:Trust me you will have toe out at the rear of the car.
See my thread, I had exactly the same and other issues....

lotus-suspension-f42/stability-speed-t25650.html?hilit=Mark030358#p167784

Cheers
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I respectfully disagree with what is expressed in the thread referenced regarding what would be "rear bump steer"

"I had a quite a conversation with the Guru from Kelvedon earlier this year about rear-end steering. He believes, iihic, that, as the rear suspension is compressed, the toe-in/out varies considerably, this can be either as "bump-steer" where both sides are affected equally, or whilst in a cornering body-roll situation, where one side is in compression and the other going the opposite way. "

I believe that recollection would be a distortion due shortening a conversion or possibly some misunderstanding : since A-frames are fixed (once adjusted if applicable) there is no steering per se at the rear (except from play in case of degraded bushes etc., or if the chassis is sufficiently bent so that the A-frame sweep angle is no longer perpendicular to the chassis, as mentioned above), so there cannot be any "rear steering" hence no "rear bump steer".

What I believe might have been discussed on the phone was classical bump steer, i.e. from the front wheels, as induced by loading that affects the front when the rear is affected. If the rear is compressed without any compression change on the front, I trust there would not be any perceptible "bump steer".

For memory here is a drawing found on net ( http://www.fiero.nl/forum/Archives/Arch ... 60592.html ) about the geometry leading to bump steer :
bumpedt.jpg and
bump steer: neutral when parallel

3 typical situations are described, zero bump steer being when the 2 circles are "parallel" (this may not be obtainable with all suspensions configurations when one cannot alter tie rods axis or length to suit, but can be minimized so that the 2 circles are locally parallel around the point of operation; stiffer suspensions, with reduced travel, would also minimize bump steer as there would be less travel for a given bump).
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PostPost by: JonB » Fri Feb 09, 2018 11:54 am

Here are the results from the laser alignment check.

front-alignment.jpeg and


Lots of red there, although the tester said he was happy with it and that there's no evidence of chassis damage. He also remarked that many new cars are worse. On a test drive, I could tell it was still diving in a bit on the corners, but overall feel is improved. It is still vague at speed in a straight line, which he predicted as the front castor angles are too low. As I have a spider chassis with long bolts passing through both top wishbone arms, I wondered if it was adjustable. Either way, the bushes will need to be done. One thing I spotted was that one of the nuts had been welded onto the shaft..

@Mark, I will read the thread before further commenting.

@nmaudit, thanks for the diagrams. I don't think anyone is suggesting I am experiencing "rear bump steer", and I do not see how it is possible with fixed suspension mounts and new bushes (there may be some bush flex but I seriously doubt it is enough to influence the steering. However you must be aware that these cars are sensitive to rear toe in angles, and as we can see, mine is out of kilter.
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