Brake problem?

General General Lotus Talk
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Was out the other day and found that if I progressively brake (i.e. keep pressing the peddle slowly until the wheels lock up) the rear wheels seem to lock up before the front (clutch peddle depressed during braking, so no engine braking effect) I’ve done a track day since and its not been a problem, ie rear end not snapping away or anything untoward. I know this is something that you wouldn�t normally do, but I’m not sure if this a brake problem or not? Anyone any ideas.Cheers Dave

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Dave,The brake system was designed and balanced to USE the engine braking effect to reduce the rear wheel lock up. So no, its not a problem.Phil

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Cheers PhilThis puts my mind at rest.Dave

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How does engine braking reduce the rear wheel lock up? Doesn’t make sense to me!Cheers

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May have a brake bias fitted. should you be able to find it, should be able to adjust it.Thats my 1/10th Euro worth.Phil GT

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quote:Originally posted by MarkD:How does engine braking reduce the rear wheel lock up? Doesn’t make sense to me!CheersI guess that as the engine speed drops below tickover revs, the engine is trying to speed the car up to prevent stalling.

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Ideally you’d have things neutral but that just doesn’t happen as weight moves around, things get hot etc. So rear bias is fine and safer for regular use which is why I’m sure Lotus set things up that way.Turning-in with the rears locked up is easier than with the fronts locked up. In fact it creates more turn-in, the trick being to stop turn-in becoming rotation [image]http://www.exiges.com/ubb/NonCGI/images/icons/smile.gif[/image]A good simulation of why can be achieved by jumping on a bike, build up a bit of speed and then yanking the front back on hard.

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Having the back locking up before the fronts is bad. The only time I have had it happen to me ended up with me spinning at approx 100mph - fortunately missed everything and there was lots of tarmac runoff. No warning, no time to correct. At least if the fronts lock first you will carry on straight and can come off the brakes to correct the direction you are travelling in.How often do you see F1 cars locking the rears?Cheers

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Whoa! It seems to me that everybody is talking about different things!If Dave managed to lock the rear wheels in the street, but not in the track, he may have had a pavement surface problem (sand, oil, etc) or a surface/tyre temp problem.It didn’t happen in the track, which is OK.Phil’s comments then would make the rear even more prone to locking, or starting to slide.If braking hard (brakes only) the rear will not lock up. You could add braking force if you use the engine at the same time as the brakes, but you will break traction then. You can also see on the track that the rears never get as hot as the fronts, they just don’t have the power to do that (because of the bias and the caliper piston).Brendan’s comment was a joke, right? [image]http://www.exiges.com/ubb/NonCGI/images/icons/wink.gif[/image] good one mate !And Matthew, you’ve been watching lots or rallying, right? On a track, you can let the back loose using that technique, but it’s not optimum and then it takes big ones to get the tail sliding motion into a controlled power slide. [image]http://www.exiges.com/ubb/NonCGI/images/icons/shocked.gif[/image] If you can do that, cheers! but it’s not always desirable.Still, it doesn’t improve turn-in. For a quick turn in you need optimum adhesion. If you rotate sliding it just points you in the proper direction to start a powerslide. But it is rotation, and you don’t want that in an Exige or Elise (because of the rear weight bias mainly).And the Exige braking is NOT rear biased, I can safely tell you that.My 2 cents, happy sliding!Uldis [image]http://www.exiges.com/ubb/NonCGI/images/icons/wink.gif[/image]

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I agree with Mark & Uldis - you do not want rear biased braking, esp. on a mid engine car with rear biased weight distribution.[This message has been edited by RussT (edited 30 September 2002).]

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quote:Originally posted by Uldis:Brendan’s comment was a joke, right? [image]http://www.exiges.com/ubb/NonCGI/images/icons/wink.gif[/image] good one mate !Err thanks. But I was being serious. Perhaps I didn’t explain myself very well, or maybe I’m just plain wrong. But it seems logical to me that it would be slightly harder to lock the rear wheels with the clutch engaged and the engine still running. Please explain where I’m going wrong.

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[image]http://www.exiges.com/ubb/NonCGI/images/icons/shocked.gif[/image]Brendan, when you’re braking that hard you’re normally also changing gears, otherwise why brake that hard?If you’re changing gears you have the clutch pressed, so the engine will feel no effect of what’s happening at the wheels.If for some strange reason your your engine is feeling what is happening at the wheels, the wheels friction with the pavement is driving the engine, and the engine is acting because of internal friction, like a dynamic brake (your engine’s electrical power could be off then and it would still be spinning and applying negative torque).So, in short, the engine is not driving the wheels while braking (clutch in) and if you’d like to brake without using the clutch, the engine is braking, not accelerating.And if you’re braking without changing gears, and accelerating at the same time… that would be fun!Cheers! [image]http://www.exiges.com/ubb/NonCGI/images/icons/wink.gif[/image]Uldis

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Actually, after reading my post, I realized that I’m talking about track driving, very hard braking, like for a hairpin after a straight.On a road, normal driving, you brake and not use the clutch, that is normal, still then what I said earlier applies, the engine is being driven, it’s not driving.Rgds,Uldis [image]http://www.exiges.com/ubb/NonCGI/images/icons/smile.gif[/image]

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This is getting interesting. I totally agree with what you’re saying - up to a point. I was considering the case where you’re not pressing the clutch pedal. And I agree that when braking normally, the engine is acting as a brake. The engine is being driven by the wheels. But surely as the wheels begin to lock, the brakes have to not only kill the rotational inertia of the wheels, but also of the engine and gearbox. This alone would require a lot more energy than just stopping the wheels. At this point, you also have to consider the idle control trying to stop the engine from stalling.Feel free to tell me this is rubbish. As long as you can tell me why [image]http://www.exiges.com/ubb/NonCGI/images/icons/smile.gif[/image]

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I see what you say, but you’re supposing that the ECU is going to react and feedback the opening of the IAC valve (remember, it’s not throttle anymore, because the throttle is completely closed, so the idle circuit is taken over by the IAC).The IAC valve is very slow to react (this is why a rough idling engine sometimes stop), and its domain is only the 1000-1300 rpm.As you’re braking hard and going from the 7000 or 5000 rpm range, you never get to the 1000’s rpm range, would be insensible to do so, you want the newly chosen gear to be engaged by the 3000’s at least (depending), so you’ll never be in the IAC range.And if a rough idling engine stops sometimes, you can see that the slow reaction of the IAC is a part, and it also proves that at the 1000’s rpm range, the engine does not have enough torque to do any other job other than just barely keep the engine alive without load. Any extra load would stop it.Try this: drive the car in any gear at 1200 rpm (not touching the throttle, IAC only) and tell me if the car stalls or not.Don’t do it downhill, do it in flat or slightly uphill so that the engine is not driven by the wheels.Happy testing Uldis [image]http://www.exiges.com/ubb/NonCGI/images/icons/smile.gif[/image]

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Fair comment about the reaction speed of the idle control. I’m just trying to figure out why the rears would be more prone to locking with the clutch pedal pressed down. I do seem to remember Nick Adams or Dave Massey on the official Lotus BBS saying that the car is set up with better brake balance with the clutch engaged, and he said pressing the clutch would explain why somebody was suffering from rear brake lock-up.Maybe I was talking a load of arse when I suggested it was the idle control. Could it be what I was saying about the rotational inertia of the engine and gearbox?Maybe I should just give up now [image]http://www.exiges.com/ubb/NonCGI/images/icons/smile.gif[/image]

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Brendan,The simple answer to why the rear wheels have looked up when braking is,that your passenger has yanked on the handbrake,the thing is passengers do not realise what the car is capable of doing and panick,so the first thing they do is YANK THE HANDBRAKE!

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quote:Originally posted by Brendan: I’m just trying to figure out why the rears would be more prone to locking with the clutch pedal pressed down. I do seem to remember Nick Adams or Dave Massey on the official Lotus BBS saying that the car is set up with better brake balance with the clutch engaged, and he said pressing the clutch would explain why somebody was suffering from rear brake lock-up. [image]http://www.exiges.com/ubb/NonCGI/images/icons/smile.gif[/image]BrendanAin’t this a simple case of…, brake hard, weight transfer to the front of the car (assuming you ain’t turning at the same time), rear goes light, depressing the clutch removes engine loading on the rear wheels so the overall combination might result in locked rears? the front didn’t lock first because of the weight of the car over the front wheels??Have we got the same pads front and rear in the offending car?

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Brendan,"I do seem to remember Nick Adams or Dave Massey on the official Lotus BBS saying that the car is set up with better brake balance with the clutch engaged, and he said pressing the clutch would explain why somebody was suffering from rear brake lock-up."I find it very hard to believe that somebody said it like that, maybe on a particular car there was something fishy.But the Elise/Exige’s bias is well set. It will not lock up. You might initiate some looseness in the back but have to do it on purpose with the clutch/lower gear and inappropriate abuse of the engine.If without the effect of the engine (clutch or not in gear) the rear wheels in an Elise/Exige lock up, take it to the dealer for them to sort out. There’s something wrong there. (or you can just release the handbrake [image]http://www.exiges.com/ubb/NonCGI/images/icons/wink.gif[/image] )Cheers,Uldis

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Oh, I though you were supposed to use the handbrake [image]http://www.exiges.com/ubb/NonCGI/images/icons/wink.gif[/image]This reminds me of a club night I went to a couple of years ago at Curborough. Some numpty in a Maestro locked up and overshot the corner at the end of the main straight. He later said he wan’t able to slow down in time because he accidentally found 3rd gear instead of REVERSE [image]http://www.exiges.com/ubb/NonCGI/images/icons/shocked.gif[/image] I can’t believe this guy thought that using reverse was a good way to slow down.