Never heard of this before... In a FWD car for the quickest launch, pull on the handbrake and launch against that as you ease it off over the first few metres.
Sounds counter intuitive but it works (first car with launch control, had to give it a whirl to see what i was missing!). The theory goes something like the resistance of the rear tyres as the car pulls forward makes the car try to rotate about the rear wheels, forcing the front wheels into the ground. The extra grip given allows you to put down more torque sooner, significantly outweighing any effects of the drag of the handbrake over the first few metres.
I guess i should have twigged this earlier, i already knew about using the handbrake for extra traction in a FWD in snow. You learn something new every day. It's a service, honestly :-) About as much use as launch control mind ;-)
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Surely that's the wrong way round? Every car I have used the handbrake on while moving squats the rear onto the ground which would indicate the front going lighter.
To see the effect travel at about 20 mph and pull on the handbrake, and the back squats.
Keep the handbrake on and try and reverse, then the back kicks up forcing the front down.
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I presume what actually ahppens is the 'butt-dynamometer' feels it is accelerating faster as the brakes are released.
On the actual road/track I'd be surprised if it makes any positive difference.
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Zero - not sure, the back end definitely goes down slightly in reaction to the handbrake, but so do the times. Can't say whether the front end didn't lift as much, i'm not sure i felt much change at the front really.
Been using a Vbox mini and I made sure it's mounted correctly / securely.
Launch control with nothing else, i.e. foot planted, revs bouncing off the 3200rpm limiter = 7.6 secs to 100km/h. Lots of wheel spin and scrabbling sideways before it sets off. Felt slow to begin.
Launch control with revs manually held at 2400rpm then after releasing the brake, applying as much throttle as you think you can get away with until it's on the floor = 7.1 secs, 7.3 secs, 7.2 secs, 7.2 secs
Launch control against the handbrake = 7.0 secs, 6.9 sec, 7.1 secs, 6.9 secs
There's a +/- 0.1 sec error on the Vbox, but subjectively both the 6.9 secs ones felt good and the 7.1 felt fluffed a bit so that kind of ties in with the bum dyno. Consecutive runs were in opposite directions and there were a few cool off periods between some but they don't reflect in the timings, i.e. 7.3 was after a cool off but that was probably more just because i lost my feel a bit.
Can't get what autocar got (6.8) but below 7 secs is good enough for me.
The vbox is a cool bit of kit, it's not mine but i'm tempted not to give it back :-) Cheaper than a full data logging setup in a track car, does g forces to improve your cornering on a track and lap timing plus a few other things, data logging on screen and to an SD card.
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Can you try Launch control against the footbrake? Some left foot braking should take the nose down a bit, and negate power spinning out on a wheel with less traction
(assume its not limited slip diff, and the launch control turns off the traction control)
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I can try applying foot brake during the launch, when launch control's enabled it doesn't engage the clutch until you've fully released the foot brake to begin.
If you try pressing the accelerator against the foot brake outside of launch control, like for a fast start in a slushbox, it only goes to ~1k rpm before mechanical sympathy and a wee whiff of clutch smell stops you.
It's not got a mechanical LSD but XDS (acronym overload?) - braking a single spinning front wheel - is still active with TCS switched off, although i think i can disable it too by holding the TCS off button for a second or something.
Routine is -
Switch TCS off
Aircon off
Foot on foot brake
Engage either manual or sport mode on the gear stick
Give as much throttle as you want (clutch is now disengaged, it revs freely up to 3200rpm)
Release footbrake, millisecond pause then the clutch engages
Last edited by: Skoda on Fri 18 Mar 11 at 23:45
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What fun it all sounds Skoda.
Don't break yr jalopy though.
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Note to self...avoid anything SKoda's driven like the plague.
:-)
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Ahh GB + AC, i've one eye on upgraded clutches anyways in a couple of years and these things are well proven by others who feed 450+ nm of torque through them. I will be doing a gearbox oil change well ahead of manufacturer schedule though.
Eventually when this car's no longer the daily driver it'll be upgraded ready to go as a track car. It's already got it's first track session booked (with an instructor again).
What's the point of buying something that pretends to be sporty and never using it :-) Plus features are there to be tried out!
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>> What's the point of buying something that pretends to be sporty and never using it :-) Plus features are there to be tried out!
Correct and correct, Skoda. Sound man. Be careful though.
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I don't understand the vbox figures, but, the theory as presented is definitely wrong.
You always lose much more than you gain if you work out the forces at the driven wheels using the moment argument in the OP. A back of the envelope calc shows that you lose at least 6 times more traction than you might gain.
However, it's entirely possible that there is some marginal benefit in damping the response of the vehicle's traction and transmission control functions.
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>>To see the effect travel at about 20 mph and pull on the handbrake, and the back squats.
>>Keep the handbrake on and try and reverse, then the back kicks up forcing the front down.
Z - Those effects are absolutely nothing to do with weight transfer. All you are seeing is the handbrake torque reaction on the suspension.
Weight transfer and the movement of the body are two seperate things.
Even if I made the suspension of a car completely solid, and there was no body movement at all, weight transfer under acceleration, cornering, and braking would still happen, and their magnitude would be unchanged (except for some extremely pedantic and trifling second order effects)
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>> Z - Those effects are absolutely nothing to do with weight transfer. All you are
>> seeing is the handbrake torque reaction on the suspension.
Indeed but its still moving the body about relative to the ground, and if the back goes down the front is going up. If the front is going up you will have less traction.
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At the end of the day, the only way to a really quick start is to be brutal to the transmission, tyres and clutch. But if the suspension design permits axle tramp, you'll soon be walking...
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Interesting debate but it won't work in the Xantia (handbrake on front wheels)!!!
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>>If the front is going up you will have less traction.
The front won't necessarily be going up.
The back going down because the handbrake torque does not mean there is any weight transfer from the front.
Last edited by: Number_Cruncher on Sat 19 Mar 11 at 09:37
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I used to like the Hillman Hunter configuration of the handbrake on the right-hand side of the driver. You could simultaneously put the car in gear with the left hand, start releasing the handbrake with the right hand and start releasing the clutch. It avoided the time-consuming procedure of using the left hand for both gear lever and hand brake.
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Here's the outline of the calc which shows the theory as presented in the OP can't work;
I've assumed the handbrake is applied enough to cause the rear wheel to lock - but, the argument holds for lower values of brake drag too.
The extra vertical load on the front axle is R2*mu*Rr/E
Where
R2 is the vertical load on the rear axle
mu is the coefficient of friction between tyre and road
Rr is the rolling radius of the wheel
E is the wheelbase
This extra vertical load on the front axle allows this extra traction
R2*mu^2*Rr/E
But, the cost is the drag from the applied rear brake
R2*mu
So, the question is, which is the greater - the added traction, or the brake drag?
Taking out the common factor R2*mu, we are left with mu*Rr/E
Rr/E is usually about 0.1, and mu can be be taken as 0.7, their product is much less than one, meaning that the extra drag far outweighs any traction advantage.
If my assumption of brake locking offends you, then simply replace R2*mu wherever you see it in the snippets of my calc with a symbol of you choice which stands for the braking force from the rear axle. The logic still stands.
Last edited by: Number_Cruncher on Sat 19 Mar 11 at 09:59
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Tend to agree with NC even if a bit baffled by his calc.
Reckon the benefit of the hand brake is prior to the launch and the clock starting - it stops the car rolling away ;-) er, I mean the engine can be pulling against it and the turbo can spool up, as I say, before the car moves forwards and the clock starts.
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If the car is stationary while you are revving it up against the handbrake, there is no weight transfer going on. If the car is accelerating in any way, surely weight transfer is going backwards, towards the rear wheels?
And if you have compressed the rear springs with torque effect, and weight is going backwards, surely the nose has to come up?
Last edited by: Zero on Sat 19 Mar 11 at 10:31
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Is the object of all this faffing around and tyre smoke to get you to the next set of traffic lights a nano second quicker?
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If I want to get there that quickly, I use full throttle upchanges. Quite easy in a diesel, more alarming in a petrol.
How brave are you?
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A school chum's father used to enjoy impressing us with clutchless gearchanges in his (company) mk3 Cavalier.
He could go both up and down the box completely smoothly without his foot touching the clutch pedal. Absolutely perfect matching of revs on downchanges, with a near imperceptible shift from the passenger seat. Never heard so much as a clonk from the transmission.
They purchased the car off the company and ran it to over 200k without any ill effects, too.
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The guy who owns the vbox has probably forgotten more about all different types of racing in his 40 odd years than i'll ever learn in my lifetime. Went up this morning to say thanks and asked him what he thought.
He's not into drag racing but he's done it a good few times over the years. He's heard of all sorts of techniques like lining up with your front wheels pointing slightly to one side, adding weight to the front of a FWD car (relocating weight's fine but adding weight's not) and a couple of others i didn't completely follow.
Says they're all flawed in some way or another, drag racing's basically pretty simple. The only valid use he's aware of for a handbrake is to preload the drive train, normally in a RWD car, he doesn't recommend that technique either though for the best time.
We did 2 runs with him driving although not as good a road there. Proper technique is slightly higher revs to begin, just over 2,500 but he thinks you could go slightly higher, and much harder on the throttle sooner. The car protests more, lots of noise from the tyres and for longer but it didn't feel like there was wheelspin. We didn't time it (car was 2 up, and had stuff in the boot) but now i know what to do. It's quite impressive from the passenger seat when someone knows what they're doing.
Curiosity satisfied, doubt i'll ever use it again though, that's plenty abuse for a cars lifetime :-)
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>> Is the object of all this faffing around and tyre smoke to get you to
>> the next set of traffic lights a nano second quicker?
No purpose to it at all ON, was more enjoyable that sitting in front of the telly though! Now i can say been there, done that.
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>>If the car is accelerating in any way, surely weight transfer is going backwards, towards the rear wheels?
Yes, if the car accelerates forwards, you absolutely must get rearwards weight transfer.
Compressing the rear springs with the handbrake, however, makes absolutely no difference to this weight transfer - the suspension's reaction to the handbrake torque is purely an internal force system which doesn't affect the contact forces between the tyre and road.
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>> >> And if you have compressed the rear springs with torque effect, and weight is going
>> backwards, surely the nose has to come up?
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Yes, but when you release the handbrake you unlease all the energy in the rear springs, which is transferred to the front wheels?
As I understood the brutal fast-getaway technique, you get the wheels spinning, straining against the wound-up suspension. Then on releasing the handbrake the car is catapaulted forwards, also transferring weight to the front wheels as the rear springs unwind.
You get a similar effect in a rear-wheel drive car, but you can't spin the wheels, you burn the clutch.
Needless to say the Popemobiles don't get driven in this way. I like to engage the clutch at idling speed, and accelerate only at a rate that doesn't spill the glass of water on the bonnet.
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>>Yes, but when you release the handbrake you unlease all the energy in the rear springs, which is transferred to the front wheels?
No, I think you might be making the same mistake that Zero was - body movement and weight transfer are not one and the same.
To take an extreme example, consider a car like a 2CV, with leading arm front suspension and trailing arm rear suspension. Under braking, the nose might tend to rise as the rear of the car would squat. If you follow the body motion = weight transfer school, you might think load was being transferred to the rear axle - it isn't!, the weight is being transferred forward, just as per usual.
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>> No, I think you might be making the same mistake that Zero was - body
>> movement and weight transfer are not one and the same.
No I didn't make a mistake. I wasn't suggesting that weight transfer was making the back or front go down. I agree that weight transfer only accompanies acceleration or deceleration.
Merely saying that if one end is moving in one direction, Up or Down, there is a chance the other end will move the other way. In fact I know it does, I have seen it happen.
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>>In fact I know it does, I have seen it happen.
Ah!, we are straying into a complicated area.
The weasel answer is to say it can happen as you say, but, not automatically, or necessarily.
A more complete answer, however,...
The pitch behaviour of a car, among other things, depends on a ratio called K^2/ab, or sometimes, dynamic index. Yes!, vehicle dynamicists do actually call it the k^2/ab ratio.
In this context, k^2 is the radius of gyration squared, a is the distance from the front axle to the c of g, and b, the distance from the c of g to the rear axle (so, a+b=E)
If k^2/ab = 1, then, a sudden change in load at one axle produces NO change in load at the the other axle - i.e., complete independence.
Interestingly, this aspect of dynamics also applies in the roll and yaw planes.
This means that if your car has, say a roll axis k^2/ab ratio which isn't equal to one, although you might have a suspension system which looks like an independent suspension, and might be listed in books as being independent, it won't actually behave like an independent suspension under dynamic loading!
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Thanks for that NC, wont pretend i follow it all but i guess the inputs seem to include everything that would be relevant.
I can't make the Rr/E = 0.1 work for me, Rr = ~2 metres, E = ~4 metres == 0.5. Not that it makes any difference to the eventual outcome but i'm thinking i've misunderstood this bit because most cars i can think of would be about ~0.5 here? Where have i gone wrong?
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Rr is the rolling radius of the tyre - not the circumference.
A typical rolling radius might be 0.35 m, and a typical wheelbase ~ 3m, hence, Rr/E of about 0.1. What ever the exact numbers are for your car, it's clear that the wheelbase is going to be much longer than a rolling radius.
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>> I used to like the Hillman Hunter configuration of the handbrake on the right-hand side of the driver
>> It avoided the time-consuming procedure of using the left hand for both gear lever and hand brake
The 05 Fiat Ducato minibus I sometimes got sent out in was set up like that. Agree it saves a second or so of faff when pulling away, but it wasn't much good if I was holding a Mcdonalds cup / fag / phone in my right hand at the time :)
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Well I don't begin to understand the science but what I do know is that on my car which is a diesel auto 4x4 if you haul the handbrake on tight, momentarily build the revs up in "D" and then knock the handbrake off it takes off like a determined whippet after a squirrel...
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>>momentarily build the revs up in "D"
Yes, that will defiinitely work - the drivetrain will be producing lots of tractive effort from the moment you let the brake off, rather than waiting for revs to build.
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>> >>momentarily build the revs up in "D"
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>> Yes, that will defiinitely work - the drivetrain will be producing lots of tractive effort
>> from the moment you let the brake off, rather than waiting for revs to build.
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>>
>>
As per my post 10:05 yesterday.
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Handbrake for a quick start? - Zero
