Heel and toe compulsary in rwd with lsd?

[bramley]

I seem to remember hearing that there is a chance of causing the rear wheels to lock up if driving flat out (on a track) and changing down without using H&T. Can anyone elaborate on that? I think I get the principle, but have no idea if this is likely to apply to my lowly E30 track car or if it's only 600bhp race cars.

I always H&T but this question is being asked because I'm going to let a mate drive my track car tomorrow, and I doubt he'll be comfortable with H&T.

Thanks

[PetrolTed]

It is relavent. One of the few times I spun my old TVR S4 was changing down a gear and locking up the rear wheels. That car had a massive 168bhp

[bramley]

Thanks Ted, I'll ask my mate to practice in his car asap!

[GreenV8S]

Rev matching on downshifts is what matters. H&T is one way to do that, while braking. But you don't need to do it while braking; just brake, change gear (with rev match) and turn in works fine.

H&T can be quite tricky to learn, is only required if you're going to brake right up to the corner in an effort to save fractions of a second, and under those conditions if you don't H&T accurately you can end up in a lot of trouble. So don't try using it in anger unless you are confident that you can do it accurately and reliably.

[chris7676]

Haven't noticed that and I simply can't heel and toe so I don't. In fact I often use the engine to aid braking especially in the Elise which is underbraked on the rear.

[Shaun_E]

I'm not very good at H&T so don't bother. I sprint, hilllimb and track a 226bhp Caterham with a plate type LSD. You have to be a bit sympathetic on the down changes - if you bring the clutch up too sharply or change down in a corner then you do risk locking the rears but given you are braking at the same time the effect isn't catastrophic.

[Don]

H&T is certainly a good idea. Normally not doing so won't bite unless you have some lock on (are turning) and let the clutch up with a bang. But it can certainly cause a spin alright...

[PetrolTed]

Shaun_E said:

I'm not very good at H&T so don't bother. I sprint, hilllimb and track a 226bhp Caterham with a plate type LSD. You have to be a bit sympathetic on the down changes - if you bring the clutch up too sharply or change down in a corner then you do risk locking the rears but given you are braking at the same time the effect isn't catastrophic.

That's a fair point. I usually only heel and toe in cars I'm familiar with. Rev matching after braking is the other option.

[sa_20v]

Stupid question, but why would the wheels lock - what's are the mechanics behind this? I'm assuming revs being too low on down-shift would cause the locking, as opposed to too high?

[Don]

sa_20v said:

Stupid question, but why would the wheels lock - what's are the mechanics behind this? I'm assuming revs being too low on down-shift would cause the locking, as opposed to too high?

That's right. The effect is "clutch drag". Basically the engine is spinning at too low a speed for the gear you are changing to and the road speed. Normally what happens is that the clutch gives way a bit whilst engaging and spins a bit whilst the engine speeds up...once the two are spinning at the same speed all is well and the clutch is fully engaged. Some bikes are designed to do this with a "slipper clutch".

The downside is that if the clutch does not slip the speeding up of the engine creates a braking effect on the drive wheels. If the rev difference is great enough you'll either destroy the clutch or "lock" (brake heavily) the wheels.

Since it's only the drive wheels that are affected this can unbalance the car.

Make sense?

[Chris71]

You don't need to heel'n'toe to balance engine speeds though. I often double de-clutch even in the daily driver, but rarely heel'n'toe on the road.

ETA Just to clairfy it's the mismatch in engine and transmission (wheel) speeds that causes the problem. You can brake, bring your foot off the brake pedal as you bring the clutch up with the other, then blip the throttle, re-engage the clutch and get back onto the brake. It'll lose a few yards over constant braking, yes, but it's more than addequate for a non-competitive track day.

(And, no, that's not full double de-clutching, there you need to put the gearbox into neutral before matching the speeds with your foot off the clutch, so it spins up or slows down the lay shaft, but the above will still cure excessive engine braking and let the synchromesh do the hard bit.)

PS Ah... The S-Series, that took me back! You didn't even need to fluff a gearchange, I think I came close to locking up just by lifting off at high revs in second gear on one occasion. Wouldn't want to have done it on entry to a corner!

Edited by Chris71 on Friday 4th September 12:00

[Don]

Chris71 said:

You don't need to heel'n'toe to balance engine speeds though. I often double de-clutch even in the daily driver, but rarely heel'n'toe on the road.

You don't need to rev-match - generally the braking effect of clutch drag on a downshift is quite slight. On the road and well away from the limit of adhesion it's not going to matter much.

Double-de-clutch is irrelevant to heel and toe. You can combine them or simply not bother with double-de-clutch. Synchromesh obviates the need in most modern cars: although you do get more of the gearbox spinning at the "right" speed before engaging the new gear.

I don't often heel and toe on the road because I don't overlap braking and gearchanging very often. When I do then I H&T: especially if I'm doing something like a really hard brake coupled with fourth to second for example.

[Chris71]

Don said:

Chris71 said:

You don't need to heel'n'toe to balance engine speeds though. I often double de-clutch even in the daily driver, but rarely heel'n'toe on the road.

You don't need to rev-match - generally the braking effect of clutch drag on a downshift is quite slight. On the road and well away from the limit of adhesion it's not going to matter much.

Double-de-clutch is irrelevant to heel and toe. You can combine them or simply not bother with double-de-clutch. Synchromesh obviates the need in most modern cars: although you do get more of the gearbox spinning at the "right" speed before engaging the new gear.

I don't often heel and toe on the road because I don't overlap braking and gearchanging very often. When I do then I H&T: especially if I'm doing something like a really hard brake coupled with fourth to second for example.

I was busy editing the previous post when you replied! Yes, heel'n'toe is seperate from rev-matching, but I'd question the idea that you don't need to do it. Some cars with poorly setup engine management have truly ferrocious engine braking (as mentioned above) and it can be more comfortable to match the engine and transmission speeds, even if you leave the synchro to do its own thing.

[mr_fibuli]

I had this problem with my old 200SX on track. It shouldn't be too dangerous as the lock up is very brief while the engine spins up to the right speed, and you should be braking in a straight line.

It only happens if you are braking right to the limit of grip, the extra engine braking from the non rev matched down change is then just enough to push the rears too far and lock them up.

I think I got round it just by letting the clutch out more smoothly, not being particularly skilled at heel and toe.

[Munter]

I dont have an LSD in the 5 (or the MR2) but I'm just gentle bringing the clutch up as I can't H&T due to being a piss poor track driver.

I do however find myself being oone of the last of the late brakers...so I can't be going too far wrong.

Talking of the MR2 I'd get away with a downshift to 2nd in the middle of Tower...just

[sa_20v]

Don said:

sa_20v said:

Stupid question, but why would the wheels lock - what's are the mechanics behind this? I'm assuming revs being too low on down-shift would cause the locking, as opposed to too high?

That's right. The effect is "clutch drag". Basically the engine is spinning at too low a speed for the gear you are changing to and the road speed. Normally what happens is that the clutch gives way a bit whilst engaging and spins a bit whilst the engine speeds up...once the two are spinning at the same speed all is well and the clutch is fully engaged. Some bikes are designed to do this with a "slipper clutch".

The downside is that if the clutch does not slip the speeding up of the engine creates a braking effect on the drive wheels. If the rev difference is great enough you'll either destroy the clutch or "lock" (brake heavily) the wheels.

Since it's only the drive wheels that are affected this can unbalance the car.

Make sense?

Yup, ta muchly!

[Alex]

As long as you brake to an appropriate speed for the next gear, there should be no danger of locking the wheels. This is what a race instructor would recommend.

[varsas]

I heel and toe on track (but not often on the road) partly because I am worried about locking wheels but more for mechanical sympathy. I just can't bring myself to bring the clutch up when there is 4k+ rpm difference between the road speed and engine speed. I have tried but even in my old Vectra, which I didn't care about I couldn't make myself do it. Odd but true....I also can't 'do' burnouts for the same reason; something in my brain won't let me.

Edited by varsas on Friday 4th September 12:41

[Don]

varsas said:

I heel and toe on track (but not often on the road) partly because I am worried about locking wheels but more for mechanical sympathy. I just can't bring myself to bring the clutch up when there is 4k+ difference between the road speed and engine speed. I have tried but even in my old Vectra, which I didn't care about I couldn't make myself do it. Odd but true....I also can't 'do' burnouts for the same reason; something in my brain won't let me.

+1.

[boxsey]

Shaun_E said:

I'm not very good at H&T so don't bother. I sprint, hilllimb and track a 226bhp Caterham with a plate type LSD. You have to be a bit sympathetic on the down changes - if you bring the clutch up too sharply or change down in a corner then you do risk locking the rears but given you are braking at the same time the effect isn't catastrophic.

I also bring the clutch up smoothly when changing up the box as it gives me just enough feel to detect and save me from one of those 'wrong slot' moments