59 replies to this topic

[Scuffers]

What do you think it was?

how long is a piece of string?

 

without being there or having data to look at, it's worse than guessing.

 

running different pads front/rear is OK, so long as you know what your doing, as said, pad/disk coefficients of friction are not all the same and not linear either, so you have to have some knowledge of the operating temps as well as the basic pressures/balance/pad area's/disk characteristics.

 

then you have to consider that different pads and different initial bite, so for example, if you have a rear pad that's initial bite is much more aggressive than the fronts, you can guess what's coming next, even if both pads have the same coefficient of friction.

 

Braking is all about confidence on the pedal, if you have a disk/pad setup that's too aggressive on application, then your going to start braking earlier and lighter to counter this, not good.

 

example of this is Padgid's RS15's, compared to RS14's, they are on paper a better pad with flatter friction curve and higher levels of friction, however, in reality their initial bite is massively aggressive compared to RS14's so on a light car, there very likely to lock on initial (hard) application, put these on the rear and you can see where this is going.

[SteveA]

Sorry Scuffs it wasn't a genuine question, more a matter of how could they possible know without being there and knowing my setup, conditions etc. I was there the poster wasn't, they don't know my car, I do. Bottom line is there was too much rear bias, the root cause of that be it temps (and therefore reduced retardation), friction efficient disparity or mechanical issues is something that will never be known.

[Nev]

If you modify with brake bias in any way and are unsure as to the effects, IMO it is a good idea to switch the ABS on and do some lock out tests on smooth flat tarmac. If the rears are consistently locking up first at least the ABS will keep you from pirouetting like a ballerina and you'll know you have a serious issue (prior to switching the ABS off).

 

[SteveA]

My ABS was turned on Nev and had worked successfully on previous slow speed tests, so don't assume it will save you.

 

Also, how would you know if the rears were locking up if you have the anti lock system switched on?

Edited by SteveA, 20 January 2016 - 12:00 PM.

[Scuffers]

My ABS was turned on Nev and had worked successfully on previous slow speed tests, so don't assume it will save you.

 

Also, how would you know if the rears were locking up if you have the anti lock system switched on?

really?

 

Bad as the std ABS is, it's not that bad!

 

no way with it working will you be able to lock the rears, it simply can't happen.

 

That's not to say your rears were not under-rotating (what a great phrase that is!), and this may will have contributed to the spin, but it's extremely unlikely it was the primary cause.

 

Like I was trying to get at above, most brake issues are down to poor matching of brake hardware combined with questionable suspension setup, the latter being the bit most people ignore.

 

What I am getting at here is that these cars are very susceptible to brake-induced geometry issues, ie, any slack in the top wishbones/toe links (specifically the rears) will be exacerbated with brake application as the load forces on them change dramatically, this then has the effect of brake-induced rear-steer, and IMHO most of the cars I jump into with 'interesting' brakes have this going on.

[SteveA]

Yup, ABS enabled and light on the dash off (Clearly the ABS didn't trigger) and you can hear the tyres screech under braking on the video before the car starts to rotate.

 

The suspension was a fully matched set of components (shocks, springs, arb's, and bushes) from Cornering force. I had done the best part of 15 trackdays in this configuration and the only thing that had change on that day was the pads. Not saying your point isn't valid that the extra braking force may have loaded the components more and highlighted a weak point but the suspension had been decently considered and the brakes were all standard except for the pads.

[Scuffers]

video link?

 

edit - cornering forces setup... making more sense now.

 

remember what I (and others) have told you about rear ARB's...

Edited by Scuffers, 20 January 2016 - 03:39 PM.

[SteveA]

Their setup is maybe a little bit soft but I'm not sure how a rear arb would made any difference when braking in a straight line.

 

 

Fast forward to about 4:40 for the 'good stuff'

[Scuffers]

Looks to me like ABS was not working...

 

that said, the root cause looks more like a combination of grabby brakes + uneven surface + questionable setup

 

what disks/pads are we talking, with what tyres etc?

 

 

[Scuffers]

Other question, why did you not just get out of the brakes when it started?

 

better to bury the car in the gravel straight ahead than do what you did.

[Nev]

Steve, I've watched that video 4 times, I hear no evidence of ABS working (ie the stacatto of rapid on/off traction limit). Just based on that video (only) I'd say your ABS was not working for some reason on that straight.

 

My ABS works quite well as I have carefully matched my rotating circumferences, and it has saved me on a couple of occasions, which is why I made my above recommendation.

 

[The Batman]

That's the first time I have seen that vid, that's well odd! One wheel locked up causing the spin?

[Scuffers]

That's the first time I have seen that vid, that's well odd! One wheel locked up causing the spin?

easy to do when the tarmac under you is not 100% flat (which it's not there).

 

My initial thoughts are considering the lack of speed and braking point, he could/should have been able to get out of the brakes and still make the corner.

 

My feeling is that apart from the locking, the rears are steering to some extent, not sure if that's brake related or bump-steer, but you can see the same effect before this point (watch the steering wheel - it's subtle, but there)

[SteveA]

Other question, why did you not just get out of the brakes when it started?

 

better to bury the car in the gravel straight ahead than do what you did.

 

The car has never locked up on me before and It caught me by surprise, by the time I reacted it was too late.

 

Steve, I've watched that video 4 times, I hear no evidence of ABS working (ie the stacatto of rapid on/off traction limit). Just based on that video (only) I'd say your ABS was not working for some reason on that straight.

 

My ABS works quite well as I have carefully matched my rotating circumferences, and it has saved me on a couple of occasions, which is why I made my above recommendation.

 

 

Mine worked in testing 100% of the time but not when I needed it, maybe the bumpy surface confused it?

 

 

what disks/pads are we talking, with what tyres etc?

 

 

 

Std VXL disks, PF1's on front and RC6's on rear, Yoko AD08's

[Scuffers]

Humm... I'm thinking it's more of a combination issue, I dislike RC anything, but 6 are a bit on the grabby side.

 

combine this with too much damper rebound, soft spring rates, and some bumpsteer issues, and bingo, you're there.

 

It's not unusual to have momenty under-rotation into braking zones like that as the rears unload with weight transfer and the effects of too much rebound on the rear preventing the tyre staying in stable contact over track undulations, the extreme bit in your case was the effect this had more than the initial under-rotate.

 

this may sound odd, but higher front spring rates + stiffer front ARB and slightly more mid-speed front bump would likely have stopped this happening in the first place.

 

 

 

[Exmantaa]

Humm... I'm thinking it's more of a combination issue, I dislike RC anything, but 6 are a bit on the grabby side.

 

combine this with too much damper rebound, soft spring rates, and some bumpsteer issues, and bingo, you're there.

 

It's not unusual to have momenty under-rotation into braking zones like that as the rears unload with weight transfer and the effects of too much rebound on the rear preventing the tyre staying in stable contact over track undulations, the extreme bit in your case was the effect this had more than the initial under-rotate.

 

this may sound odd, but higher front spring rates + stiffer front ARB and slightly more mid-speed front bump would likely have stopped this happening in the first place.

 

 

 

 

Interesting..

 

I had some serious rear stabilty issues last summer when braking hard after the Zandvoort straight and had to take it easy there... (stock brakes with RC5+ on 888's; 400/550 springs. Quantum blacks) 

First I re-connected my rear ABS sensor to get a functional dynamic rear proportioning again, but still an unstable rear at that braking point. Then backed of the rear dampers a tad and it became more stable, but not enough time left to test more.

 

Will now look into a combination of all your points above, as that should not happen with the stock calliper set-up.

[Scuffers]

 

Humm... I'm thinking it's more of a combination issue, I dislike RC anything, but 6 are a bit on the grabby side.

 

combine this with too much damper rebound, soft spring rates, and some bumpsteer issues, and bingo, you're there.

 

It's not unusual to have momenty under-rotation into braking zones like that as the rears unload with weight transfer and the effects of too much rebound on the rear preventing the tyre staying in stable contact over track undulations, the extreme bit in your case was the effect this had more than the initial under-rotate.

 

this may sound odd, but higher front spring rates + stiffer front ARB and slightly more mid-speed front bump would likely have stopped this happening in the first place.

 

 

 

 

Interesting..

 

I had some serious rear stabilty issues last summer when braking hard after the Zandvoort straight and had to take it easy there... (stock brakes with RC5+ on 888's; 400/550 springs. Quantum blacks) 

First I re-connected my rear ABS sensor to get a functional dynamic rear proportioning again, but still an unstable rear at that braking point. Then backed of the rear dampers a tad and it became more stable, but not enough time left to test more.

 

Will now look into a combination of all your points above, as that should not happen with the stock calliper set-up.

 

no, it should not.

 

my guess is you have way too much toe-in bump steer at the rear, thus under heavy braking, it's transitioning into toe-out as the rears unload.

 

this needs to be sorted, and make sure that your wishbone bushes are 100% NOT moving laterally.

 

next thing, you need higher spring rates, for track I would consider anything lower than 550/650 significantly compromised.

 

Also, what static toe have you on the rear?

 

Back to brakes, I have run AP 2 pots all round on my own car for years (with bias valve) and yes it's a little OTT, but works perfectly well and safely, although I would NOT use RC anything pads.

 

only time I have had similar issues was with Padgid RS15 rears in the wet, won;t be doing that again (stick to RS14/RS29)

Edited by Scuffers, 20 January 2016 - 06:48 PM.

[Exmantaa]

Rear toe is 0°15' each side, what should be -2mm on a 17" rim. (with ca. -2° camber)

 

Funny, as I suspect to have an issue with my rear bumpsteer...

My rear always felt a bit "wobbly" and not really planted on track, so I tried to adress all possible underlying issues: tried some Quantum Blacks (had Hoffman NTR's), new ball joints all round, had some knackered rubber bushes, but have now fitted Elise-shop sperical bushes. And while apart I even changed out the old rear subframe for a stiffer Turbo version.   Ow, and changed-out the brake callipers for some known good ones. (1 rear dustcap was worn)

 

Still Wobbly and not really planted, while my mate had a perfect day running with my old Hoffmann NTR's and the same basic geo and brake set-up...

Discussing this all some time ago; I remembered that it used to behave pretty OK on track when I only had a pair of Exige 111R Bilsteins fitted. But I'm unsure if that was before or after I fitted a pair of ES toe links at the rear and exactly that bumpsteer setting is now my main suspect...   

 

Shame that good bumpsteer settings seem one of the most guarded secrets on the Lotus forums.    

 

[Exmantaa]

Edit to the above; I most likely already fitted my ES toe-links end of 2012, while it went first on track (with the Bilsteins) in 2013.

Never checked the bumpsteer since fitting the kit in 2012 though. Will check for balljoint play and heigt settings...

 

 

(sorry for going off topic... )

[SteveA]

Interesting stuff, thanks Simon. I have a full blown race car now so no issues associated with not being setup correctly