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Been asked: Braking and high speed shimmy. - Suspension/Brakes/Wheels/Hubs/Steering/Geo - The Lotus Forums Jump to content
Roberto Esprit

Been asked: Braking and high speed shimmy.

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As the title says it feels like I got a midget caught on my brake caliper at high speed. It is enough to pulse through the steering wheel. The thing of it is I know wheel weights can cause a shimmy but its amplified under braking but there is no low speed indicator and the suspension is all new and tight. Also the wheels are nice and tight too. No bearing slop... I will get the wheels balanced when I get the summer tires put on but I dont " Think" That its the weights. I could be wrong as the rears have a dramatic amount on them but the shimmy seems to be in the front end.

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Check that the calipers and pads are all nice and free, slight binding there can mean the calipers don't move as well as they should to the minute distortion on the discs. If that doesn't solve it then, check the discs for run-out using a Dial test Indicator. Also worth checking the rears, because a variation on the rear discs can cause pulsing in in the hydraulic system, and as the front brakes are the more effective it can manifest there.

If you are lucky, it's just resistance in the calipers, but could mean new discs required.

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I have a bad feeling its the discs. I dont have a indicator anymore gotta buy that too. It really seems the only headaches in this car have been stuff that wasnt serviced when it should have. The P/o babied it but that dont mean he didnt ride a set of pads off and neglected to do the discs. I can live with it until the temp warms up. It keeps me off the pedal. In other words I think your right and I am just wishfully thinking its wheel weights.

Another question to anyone who sees this. Should I scrape the weights off? Remember I couldnt balance the tires due to a lack of a machine.

 

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Generally, if weight are slightly out, they are out by less than it would be in the absence of any weights, so no weights likely to be much worse than imperfect weights.

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I am imagining theres something up with the rim. Not that I think its bent noticeably because I checked that but they do get dinged ect and thats usually when 2oz get put on. I think ill keep the weights. just one month to go and I can put the summer tires on and maybe change the brakes.

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I've had a high speed shimmy on mine since I got it. New discs take it a way for a couple of thousand miles then its back. According to the service history my car's previous owners had warranty work and several sets of new discs/calipers going back to when it was brand new.

I thought mine was either a rogue or that Brembos were crap, then got a ride in a Sport 350 which did exactly the same (on AP brakes as you know).

You could spend fortunes chasing this down. I'm thinking its down to the front suspension bushings now, but won't know until that nightmare job crops up.

I just call it a "feature" now, but if you ever do find a soltion that lasts more than 6 months please Jesus come back on here and tell us.

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Never had the problem on my Esprit, which is the only one i have driven in anger...   But I did get it a lot on my rally cars.. It was bloody annoying as it was not consistent..  Different set ups for different events it came and went.   In the end it came down to ARB bushing and castor angles..   On the tarmac event we ran the set up lower/stiffer with higher castor , no problems. but on the forest set up we got the shimmy.. Increasing the castor helped but made the front end too aggressive , the answer was stiffer ARB bushing with compression strut..  Bloody good after that..    So if the symptoms are comparable then the solutions should follow..  I would start by fitting new front wishbone bushes and re-check the GEO set up..   Not as expensive as buying discs all the time and will isolate that area.. worth a try.... ( On the rally cars the ARB acted as the front tie link on the Esprit's lower wishbone.)   

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Great info Dave. As you say it is not consistent.

On my car the 2 ARB to chassis bushes don't fit the u clamps! There is a couple of mm gap around the circumference on both. An MOT tester pointed this out once so I bought some new bushes (plain rubber, pre-LOTAC) and they were just as slack! I wondered if someone had jacked on the u clamps and put them out of round but they are undamaged, so I just put it down as a "feature"!

The back of the car needs bushes and dampers pretty sharpish, in for a penny/pound I should do the front as well, but not looking forward to it!

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1 hour ago, Loose Cannon said:

so I just put it down as a "feature"

It certainly isn't. Maybe your ARB bushings brackets are not the right ones? On my car they are tight (S4s, rubber), and I would be surprised it's normal on a GT3 too (or any car from any manufacturer actually). You should dig further that question in my own opinion.

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As I recall it on mine, the ARB-lower arm bushes aren't tight to the arm, they do have the odd mm of movement or so. That coud look odd to an MOT tester if they compare it with many other cars that perhaps have metal-encased bushes.

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They are not very tight to the lower wishbones indeed — at least when the wheels are hanging, not sure when the car is on the floor.

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Ps - in addition to @CHANGES above - please also check the upper arm bushes -mine were yellow poly and totally floppy and unsuitable for the application.

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This is a very good point.. Not just any polly bush will do... Lotus spent a fortune in time and research to come up with the LOTAC red polly alternative.. They are a real step up.  Not cheap but worth it..   Unfortunately there are cheap alternative out there some yellow some purple , some even as split bushes. The split bushes are the worst and totally crap , they are contradictory to the suspension design format. If any of these are fitted your Esprit it will not perform or handle as designed .. unfortunately most owners only wake up to this when a spirited drive end up in the scenery.... ouch.!!!    

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Plenty of folks actually making their own poly bushes now Dave,  you buy the mixture and pour you own !  Do not hold up much hope for the quality of them though !

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Not saying they are any good lol,  but folks are doing it :P

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Nice and lopsided as well. Look great them

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14 hours ago, CHANGES said:

 Unfortunately there are cheap alternative out there some yellow some purple , some even as split bushes. The split bushes are the worst and totally crap ,

Just received a nice set of purple split bushes from SJ, no alternative was asked for or given, I had just presumed that I would receive a good replacement part.....I presumed wrong. Good advice, I'll get them changed.

Who supplies the Lotac bushes?

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8 hours ago, Steve V8 said:

Just received a nice set of purple split bushes from SJ, no alternative was asked for or given, I had just presumed that I would receive a good replacement part.....I presumed wrong. Good advice, I'll get them changed.

Who supplies the Lotac bushes?

 

8 hours ago, Giniw said:

I suppose any official Lotus dealer could provide them. But, you won't need to go very far: https://www.thelotusforums.com/shop/87-poly-bushes

(not exactly cheap, though :cry:)

The price although not cheap should not be the consideration... Most are buying the split bushes because they are cheap and easy to fit, (no press or tooling needed)....    Most have no comprehension on the difference and how it effects the handling, those that do are just silly...     There will be those out there who have the splits fitted and say they are fine, but they have no idea how they can effect the outcome of aggressive or emergency manoeuvre . 

Reason.....  The Lotac or original bushes are friction fit to the arms and the center steel collar is bonded in..  When fitted to the chassis the center steel collar is pinched to the chassis or snub washers preventing any movement..  As a result any movement of the arms will result in a flex or twisting within the bush itself..   This acts as part of the suspension ,holding the arm in a set position when un-sprung...  ..  Hence the reason you only torque up the bushes when the Esprit is set at its correct or chosen ride height... Lotus and pollybush spent a lot of time testing and retesting till the polly compound used gave the suspension response required for the Esprit..   

The split bush has a loose center collar and no heavy friction fit to the arm.. As a result the arm will slip on the bush.....  Taking into account that the center steel collar will be pinched by the bolts , means the two split ends will slip either on the center collar or the arm , which ever offers least resistance.. This means the bush is not acting as designed,   more worrying will ware very quickly as the polly becomes a soft friction surface...  As a result the GEO will quickly go out of alignment , angular ride heights will become inconsistent effecting the dynamic balance of the Esprit..    Sounds dramatic I know ,  but safety should always be a consideration....It may not be immediately apparent in steady day to day driving , but when you need it to perform and it does not , who do you blame..   you would not fit cheap brakes pads to an Esprit ,  would you.!!!!

Good choice in returning them Steve... always safety first.. 

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Very well described and about time it got out to the public. Thanks Dave.

The Lotac ones can be bought from just about every single 3'rd party supplier I have spoken to (and got them from).

Regarding the front arb middle bushes and their holding bracket, I bought a set from a well known supplier, and they ratteled inside the u-shaped brackets. I tripple checked and rang the supplier WHO said they were supposed to fit, but they didn't.

I then called Steve at SJ's instead and told him I specifically wanted the purple set, which is meant for another car, the Sport350, rather than my 1990 SE. They are not supposed to fit, but they fit tight and neat. They don't Rattle and Work as they are meant to. Mind you, I am referring to the two middle front arb bushes. The SE from 1990 and thereabout have same size thicker front arb as the much later Sport350. Inbetween Lotus used thinner from arb's. These two middle front arb bushes are not part of the Lotac programme, but seems to come from Powerflex.

Kind regards,

jacques

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Several professional mechanics of my acquaintance have informed me that brake related shimmies are no longer thought related to warped rotors, in general, as this does not typically occur. Instead modern pad formulations are designed to transfer a residue onto the rotor surface which increases the friction coefficient, thus increasing force or reducing effort at the pedal. This can be problematic when the residue is deposited unevenly, leaving a patch on one segment of the rotor. This can occur when the car is brought to a halt from high speed with fierce braking and the brakes remain clamped for a time after the car is stationary. Rather than machine turn the rotors the mechanics will machine sand the surfaces back to an even condition. FWIW.

Cheers

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Interesting. Indeed I already heard about those deposits which transfer to the disc when it's warmed when you are supposed to bed them in (I am referring to the EBC procedure here, but it's most probably the same for every pad).

On my good old daily/rainy weather car I have a similar vibration and although I still need to investigate more (when the Lotus is sorted :blush:), I had already checked the discs with a comparator and they are still very flat, well within the manufacturer tolerances. So as the bushings and track rod ends looked OK, my assumption so far is that it could well be the wheel bearings (no play but 200 000 km and a bit noisy when hard cornering). Or maybe that pads deposits then, which I will check too.

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