Seamless Gearboxes (aka blowing in the shifts)

[xxChrisxx]

Kimi made no gearchanges from start to 100kmh, which shows his gearing was higher than usual for 1st gear.

Whilst a 100km/hr up-shift from 1st would be considered high for a normal car, A long (low ratio) 1st gear is typical for any car which has a high power to weight ratio and therefore is traction limited at low speeds, and therefore would not benefit

Further to this, doesn't the rule for 100kph in all gears effectively dictate your overall 1st ratio (inc. The FD). As you wouldn't want to go longer than that, and the rules mean you can't go any shorter?

[machin]

I agree; those rules pretty much force you into having a specific gear ratio for 1st these days...

but those rules only came in for 2012... http://argent.fia.com/web/fia-public.ns ... 2-2011.pdf

granted, that does mean you can't do much regarding the exhaust blowing (I'm trying to get this back on topic)

But (getting off topic again.... sorry!) those rules weren't in for the 2011 season: http://argent.fia.com/web/fia-public.ns ... 2-2010.pdf

Low and behold, as simple physics predicts; the cars even in 2011 were also shifting out of 1st gear above 100km/hr:- (about 120km/hr I reckon, but it is very quick!):-

[youtube]http://www.youtube.com/watch?v=m0ObtTfK4WU[/youtube]

[andylaurence]

http://www.competition-car-engineering. ... examp2.JPG

In reality I'd say go for a slightly shorter 1st than the first green curve shown, this would ensure that on the hot days that you visit a track with a really high grip tarmac you can make full use of that grip by having a little bit of motive force in hand on the "normal" days.

I suspect I agree, but Optimum Lap shows this for my car with gearing as it is today and that's just a little too long.

[autogyro]

Shame about the hashed area.
I suppose that is force 'available' not actual?
The use of the tyres and clutch as a constantly variable transmission through this faze is often overlooked.

[andylaurence]

I suspect that the graph from Optimum Lap is a little out. There are many simplifications in the code as they try to make it easier for people to use. That makes it a little less accurate. Perhaps the hashed area isn't nearly so large in reality. My launch times don't tend to vary whether I give it 10000rpm and side-step the clutch or give it 6000rpm and feed in the clutch more progressively.

[autogyro]

If you slip the clutch you are altering the gear ratio.
If you are slipping the tyres you are varying the tyre grip.

Either way the hatched area, (which is the major area from stationary to the first shift) is simply an area of acceleration where you have replaced further useable lower ratios with a difficult to control form of CVT.

[machin]

The hatched area represents the speeds where the the car can generate more tractive (accelerative) force at full throttle than the tyres can handle. The driver must modulate the throttle at these speeds to avoid wheelspin.

The graph is simplified because it doesn't include the effect of slipping the clutch at very low speeds. But either way; you cannot accelerate a (wheel driven) car faster than the traction limit allows which is why, as Andy points out, the car's low speed acceleration is similar regardless of launch technique used (and regardless of whether he had additional "shorter" (higher Ratio) gears or a "shorter" (higher ratio) final drive. This is exactly why people like Quaife only provide "long" ratio 1st gears on their motorsport boxes.

I sound like a broken record!

[machin]

This is a similar chart from my own program, which shows the full tractive effort curves in each gear, and also shows the maximum possible force that could be generated in first gear with clutch slip if there were sufficient grip to use it



There would be no point in adding any more additional "shorter" (higher ratio) gears to this application because the car can already generate more motive force at full throttle and with clutch slip than the tyres can handle at low speeds.

(Unlike Andy's car, the car featured in this graph creates lift (rather than downforce), which is why the grip curve gently curves downward as the speed increases).

[rjsa]

If you slip the clutch you are altering the gear ratio.

This statement is as wrong as one can get.

When you slip the clutch you are dissipating engine power as heat and limitig torque transfer to the gear box. Nothing happens with gear ratio.

[Saribro]

The effective ratio between engine speed and wheel speed -is- different though, else there would be no slipping. I think autogyro has a point.

[machin]

This one area where Auto is correct; slipping the clutch alters the effective gear ratio, and allows you to create more accelerative force at the tyre contact patches with which to move the car than you would otherwise have if you didn't have any clutch slip (provided there is enough grip to use that additional force); internal combustion engines produce no motive effort at 0 rpm, so you need to use clutch slip to use the torque available at a higher engine rpm to get the car moving; this also multiplies the force in the same way that a gear-stage does, but, as rjsa says it also wastes energy in the form of heat.

You may think that it would be better to have an additional "short" (high ratio) gear rather than rely on clutch slip at very low speeds as this means you'd have less heat loss, however, practice shows that it is better to have that additional gear ratio further up the road speed range (to allow use of a smaller engine rpm range), since the low speed acceleration is traction limited anyway (so you can't use the full motive effort that the "short" (high ratio) gear would provide). This can be seen in the gear selections used by f1 teams.

[autogyro]

At what road speed (wheel rpm) can the best traction be extracted from the tyres?

[rjsa]

It is all wrong. Clutch slip just sends excess energy from fuel out there as heat. And torque transmitted to the gearbox loses completely the connection with engine speed.

Just try it: If you have a stick around, just engage the clutch lightly and start pumping on the gas. Engine RPM will go up and down while you won't feel a hair of difference regarding car acceleration.

Clutch slip <> torque converter, if that's what you guys are thinking about. A torque converter actually increases torque to the gear box as the engine spins up. But there you have an hydraulic mechanism that is speed dependant while the clutch uses dry friction, which is not speed related when speed <> 0.

[machin]

Here's a good test to prove the torque multiplication effect... take your family hatch (I.e. low powered car) and gently get the car rolling, and fully engage the clutch. Now quickly open up the throttle... you will almost certainly not break traction (spin the wheels) because generally family hatchbacks don't create enough motive force, even in first gear, and they weigh a lot (compared to a race car).

Now get the car rolling gently again, rev the engine up high, feed in the clutch and open up the throttle to maintain high revs... wammo: spinning wheels, even in your family hatch. The spinning wheels occur because you are attempting to transmit more motive force than the tyres can handle, showing that you've increased the force by use of the clutch. Basically you've temporarily created a high ratio difference between engine speed and road speed, and thereby increased force multiplication. However, whereas a normal gear is about 98% efficient the use of the clutch in this way is a lot less efficient because of the heat generated....

[rjsa]

The wheels spin because when you engage the clutch there is a torque spike from absorbing the accumulated kinetic energy from the redlining crank shaft and flywheel.

No multiplication effect.

Zero, nada, zilch.

No one never driven a car with a busted clutch? You just play with the pedal while cruising and the engine keeps revving up and down with no effect to speed. You can do that, go to 80kph, press the clutch slightly and play with the gas.

Just try it before answering again, please.