So what happens from a 40mph rolling start? Car B has shorter ratios than Car A so it must be faster, right!?

to simplify things,
1) what if we assume the no of gear changes is removed from argument by fitting only one gear to each car, one being twice as high as the other - ignoring clutch slippage delay - which will hit a speed within the range of the lower geared car first...? -- hmm, that brings powerbands into the equation...it was always at the route of the mystery anyway.

2)Now take a constant speed transmission where the engine will always run at optimum power at full throttle, no gear changes and no driver finess in pulling away and the gearing is infinitely changing, but with a twist - car twos same variable transmission goes through a final drive twice as high.............- in practice the variable transmission will detect hinderance of higher final drive in accelerating and remain longer at the shorter end of the infinite gearing for longer ----from this, what can be deduced...?

Do I hear the sound of goalposts moving?

Nah, I'm sure it was a penny dropping

Lanci, if you mean by my apparent siding with shorter gearing having first asked about taller....yes....err, and no!

I too have always believed that 5 close ratio gears, with fifth being direct and an interglactic 6th like the Holden Manaro / Chevy would be the way to go to combine relaxed economical off boost cruising with a sprint box set up - after all, dont all competition cars alter their gearing when a hi top speed is not required - why bother if there isnt a trade off in accceleration

Because it allows the driver to use the best part of the powerband more often.

This particular penny dropped for me back in 'O' level physics....

Physics dictates that the higher acceleration would create higher g loading (Newtons third in action?) - the harder the car is forced forward the harder the occupant is pushed back - floor a Scooby in first compared to fourth to experience difference in acceleration - first would put you through the back window if it wasnt for the seat resisting the counterforce and fourth would barely raise a smile - difference? - Gearing?

of course it would be faster, if a car accelerates at a faster rate then it is going to reach the desired speed in a shorter time therefore making it faster, am i missing something here, that is surely very simple

unless you are saying that the desired speed is something like 150mph then yes the shorter gearing wont reach that speed but in the example of our cars then i'm sorry but making the gears shorter definately makes is more more accelerative, i know this for a fact being one of only a handful of people to have actually done this on the coupe

i made no other changes to the car other than the gearing and when the new box was fitted the car accelerated faster because the intermediate gear ratios were shorter, sorry but that's just fact

so how does that perfectly acceptable explanation of yours side with the constant engine speed transmission comparison sited above - the gearbox seeks out lower gearing when held back by the higher final drive, thus higher gearing was impeding progress

I have to say, I am warming to the powerband reasoning, but not certain either way..

....floor a Scooby in first compared to fourth to experience difference in acceleration - first would put you through the back window if it wasnt for the seat resisting the counterforce and fourth would barely raise a smile - difference? - Gearing?....

Do you have any proof? Or does it just 'definitely feel faster'?

Getting back to Isaac Newton, in a=F/m, what are you using for F?

..is this keyboard worki..........

no i don't have proof as i didn't go out and strap timing gear to my car or any such other thing but it definately accelerates faster

i would refer you to my earlier comment when i suggested that you accelerate faster in 3rd than you do it 4th between 2 speed points, which you agreed with, lowering the gear ratos merely make the 4th gear closer to the 3rd gear therefore making the car more accelerative, so can you tell me why you still insist given this is a fact that altering gear ratios makes no difference to acceleration

car manufactures pay huge attention to gear ratios, do you think they would do this if it made no difference to acceleration

Interesting discussion....

If two cars of the same power but different gear ratios accelerated ONLY in one gear, the car with the lower gear ratio would accelerate faster than the car with the higher gear ratio and the car with the higher gear ratio would end up travelling faster by the end of the exercise.

However, if ALL the gears are taken into account, the end result will far less different (although probably not quite identical)

The problem with shorter gear rations is that they run out quicker and you have t change up (to a higher ratio)

Let's simplify this a bit....

You have two bicycles. One has a six-speed rear deraillieur with a HUGE first gear and the other one has a five-speed rear cog, with the same ratios as 2nd, 3rd, 4th 5th and 6th on the 6-speeder.

If the test was how quick you can accelerate in 1st gear, the 6-speeder will win, as it has the lowest ratio.

However, if the test is how quickly you can reach top speed (which of course will be indentical, because the top gear ratios are the same), then the difference between the two bikes will be negligible

The reason for gear ratios is to make best use of the range of engine revs (whether it's for efficiency or power, depending on application)

Some good posts here, carry on. . .

In trappy's two cars above, if it was standard VT power and weight and assuming a 'zero-shift' box, car B would be about 3.9 metres ahead at 28 MPH - and stay there all the way to top speed (because of the identical gearing thereafter).

If you switched from a 3.11 final drive to a 4.0 final drive, thus changing all the gears in proportion, the advantage in all gears would give the car with the 4.0 final drive better acceleration in all gears, upto it's maximum speed of course, which would only be 0.7775 of the speed attainable with the 3.11 final drive (given that the standard car actually has enough power to pull 5750 RPM in top gear, which I doubt it has)

I started thread but it seems to have been hijacked and offerings ignored... so off for a sulk..

Right, let's have another stab at this.



I wasn't sure what you were getting at in your earlier post, but it's fairly obvious that a car with a gear ratio that's twice as long as another will not accelerate as quick...



Yes, gearing, and also grip (let's not go there though).



Now I'm home, I'm looking at the formula on my spreadsheet(I made it so long agao now I can't remember how I measured force).

I've got the formula 375*(bhp/speed) to measure the force
AND
the forumla 9.80665*(Force/weight) to measure acceleration.

The rest of the spreadsheet applies these to the current speed and works out how far the car travels over time. It's a big old beast and not easy to read if I'm honest. It kind of grew as a bit of a hobby so didn't have a clear direction that would be easy to understand later...



OK, so in your argument, what do you think happens when you switch into 5th gear in the 'low ratio car'? You're now at the same speed as the other car but with a higher as he's still in 4th... You're gonna have that situation where each car pulls a length between changes... Ever race a SEAT Leon Cupra R in a standard 20vT??



What you fail to mention, is that the car with the shorter ratios will only have the advantage while both cars are at the same speed in the same gear. I'll try to explain with a demonstration;

Car 1 3.11
1st: 41mph
2nd: 70mph
3rd: 103mph
4th: 135mph
5th: 171mph

Car 2 4.0
1st: 32mph
2nd: 54mph
3rd: 80mph
4th: 105mph
5th: 133mph

At certain speeds, both cars will have more 'leverage' due to gearing.
0-28mph Car 2
28-41mph Car 1
41-54mph Car 2
54-70mph Car 1
70-80mph Car 2
80-103mph Car 1
103-105mph Car 2
105-133mph Car 2
133mph- Car 1

Here we have a whole 'swings and roundabouts' thing. While this is happening of course, you also have to consider the powerband, which of course is the same for both cars in this example. Does this make sense?

Just to add, I used this website a LOT when putting my spreadsheet together.

Well, that's where you are going wrong...

I suggest you start again from fundamentals.

p.s. apologies to griffster - I am just trying to make sure what you read here is not misleading.

I have proof, that changing overall final gear ratio (in my case by changing wheel diameter) affects acceleration a lot..

On my tipo 2l 8V I used standart 185/60/14 (182cm circumference) when I swapped to smaller ones(195/45/14 and 164cm circumference) the acceleration through the gears improwed a lot.

On standart wheels it was impossible to catch standart 16v tipo in good condition.On smaller wheels, there was no difference in acceleration between my 8v and mates 16v.., and after few tweaks done to engine (intake, exhaust and fuel delivery/ingnition mods) the acceleration was impressive (15,5s for 1/4mile...)

On my heavily modified coupe, the time for 1/4mile with ordinary tires 195/50/15 (181cm cirumference) was 14,8s and top speed 153km/h.

because of the (in some term) high top speed indicates heavy traction problems I borrowed slick tires from mates 20VT - In the same day.
The slicks were 200/605/16 -these had circmference 190cm... after this swap I was unable to get under 15,2s and best top speed in finish was 148km/h.. so swapping to "longer" gear ratio meant in my case notable 0,4s on 1/4 track.... On smaller slicks wich I have currently (200/580/15) the time went under 14,5 territory with top speed unchanged (152-153km/h)

this change of tire circumference is comparable as change from standart 16v final gear 3,733 to 3,56 (wich is for example in tipo..)

Trappy you issue here is you confused as to what a ratio is. The ratios in the gears you mention are the same except for 1st on the 6 speed example thus there is no difference they are not lower they are just assigned to a different number on the cluster, hence starting in the same ratio gear from 40 mph to say 100 there won't be a difference. But this thread was about changing a final drive which if you don't know will lower the gearing on all gears such that the max achievable speed in each gear will be lower. The car with lower FD will accelerate faster to its maximum speed (which will be lower than before). We are totally ignoring the power curve and % torque drop in each gear; thus for example 2 std 20vt's one with say 3.5:1 FD and the other 4.2:1, the 4.2 will out accelerate the 3.5 everywhere because it develops more axle torque to accelerate the vehicle (ignoring the factors I mentioned).

This chap actually write it up well http://www.rubydist.com/Family/Power.html

I am well of aware of what the final drive is... in fact changing it makes no more difference than changing the wheel/tyre circumference. You can change ANY of the transmission variables, ratios, the final drive, the tyres, the wheels etc, etc, it simply won't make any difference to the time taken to reach a certain point if the car's driven in a straight line.

For me, the frustrating thing is that I didn't document or set up the spreadsheet so I could later see what I did... For information, there are 183,464 cells containing formulae that work the stats out... Here's a 'biggy' from one of the cells...

=IF(AND(V16=1,W16<VLOOKUP(MAX($B$27:$B$389),$B$34:$C$389,2,0)),375*((MAX($B$27:$B$389)-U16)/T16),375*(X17/T17))

From what I can tell, it's seeing if the max force is greater than the max tyre grip and limiting it if so but, if not, it's working out the force available for acceleration (If the car's in first gear, it allows it to use maximum power up until it reaches max bhp rpm to represent slipping the clutch). A different column has already reduced the power to allow for wind resistance. More columns then work out how much the car accelerates for that tenth of a second, how far the car travels in that tenth, and then tots up the accumulated distance and speed. Simples.

All I can remember is that the 375, hp and mph formula was the key to making the whole thing work by somehow circumventing the neccessity of sodding around with torque calculations... and it does work. I add the following values along with wbhp across the rev band and it works out the performance to within fractions of actual road test data for ANY car.
Vehicle Name:
Driven Wheels:
Weight (kgs):
Weight Distribution (% on front):
Wheelbase (inches):
Gear Ratios
Final Drive:
1:
2:
3:
4:
5:
6:
7:
Rev Limit (rpm):
Tyre Width (mm):
Tyre Profile (%):
Tyre Size (inches):
CD (Co-efficient of drag):
Frontal Area (feet):
RPM and Power (BHP @ wheels)
2000:
3000:
4000:
5000:
6000:
7000:
8000:
9000:


I've been sitting on it for years now with the hopes of one day selling it but I can't see that ever happening now. If anyone wants a look, send me your email and I might send it over... It really is pretty handy and might prove that I'm not talking shite!

I assure data quality for a living, so please trust me when I say I wouldn't put my name to it if I thought for a second it was wrong! For what it's worth, Engineers (especially in the rail industry) rarely know their arse from their elbow in my experience, so a few words at the bottom of a paper don't hold much weight for me...

Clearly it doesn't!

Add just 64 cells and you can arrive at a set of accurate figures for F. Then a=F/m will work fine and all will be well.

To be honest some of the stuff on here is far beyond my understanding. THis helped a lot http://craig.backfire.ca/pages/autos/horsepower

I am pretty certain that changing the final drive ratio DOES affect acceleration times and maximum speed.

But to be fair, I am not clever enough to prove it, so I am enjoying and learning from this thread. 8)

I must say this is quite facinating but I dont think you have understood SediciRich's point which is the key to this discussion. You have not provided a decent arguement against it either. You cannot refer to your spreadsheet as proof in doing so. We dont know how it works or if it is even correct.

looking at the whole speed range of the coupe and comparing accelerative ability of 2 identical coupe's, one with shorter gearing (15% shorter)and one standard

standard vs shorter gearing (per 1000rpm)
1st 6 vs 5.1
2nd 10 vs 8.5
3rd 15 vs 12.75
4th 20 vs 17
5th 25 vs 21.25

looking at the speeds from 1000rpm to 7000rpm in each gear
with the shortened ratios the coupe can accelerate to 35.7mph in 1st before it needs a change to second so from 0mph to 35.7mph it will accelerate faster than the standard coupe

from 35.8mph to 42mph the standard coupe will accelerate quicker as it will stay in 1st

so in first the shorter geared coupe can accelerate quicker for the first 36mph then the standard coupe is quicker for 6mph

from 42mph to 59.5 the shorter geared coupe will accelerate faster, an increment of 17.5mph

after which point it will require a change to 3rd so the standard coupe will be quicker from 59.6mph to 70mph as it can stay in 2nd, an increment of 10mph

from 70mph to 89.25mph the shorter geared coupe will be faster again over a 19mph increment

i don't know about anyone else but i spend 99% of my driving below 90mph and from 0mph to 90mph the shorter geared coupe will be faster over 72mph of that speed range hence making it usefully faster in everyday situations

if you want to continue to expand the comparison to almost max speed then from 89.26mph to 105mph the standard geared coupe will be faster by virtue of the shorter geared coupe having to change in to 4th, an increment of 16mph

the short geared coupe is then faster over the increment of 105mph to 119mph, an increment of 14mph before it has to change in to 5th allowing the standard geared coupe to be faster from 119mph to 140mph an increment of 21mph

therefore from a total of 0 to 140mph the shorted geared coupe can accelerate faster between speed ranges 0 to 36mph (36), 42 to 59mph (19), 71 to 89mph (18) and 106mph to 119mph (13) giving a total of 86mph over which it will accelerate quicker

with the standard gear coupe able to accelerate quicker from 36 to 42mph (7), 59 to 70mph (11), 89 to 105mph (16) and 120 to 140mph(20) giving a total of 54mph over which it can accelerate quicker,

20mph of that increment being over 120mph which really is of no consequence in the real world

so basically the higher the speed the less impact the shorter gearing has, in fact over 120mph due to the standard coupe's gearing it will be quicker after this speed, so if you live in Germany and regularly use the autoban then stick with the standard gearing as it will likely be quicker, if however you live in the uk and drive normally the shorter geared coupe will be quicker in everyday driving

Trappy it doesn't appear you know what this sheet is doing I do not understand why you give it so much credence in that case. The sheet has far more vaibles then your A vs B comparison which displays a fundemental error in you understanding irrespective of other variables.

Now if we put the variable of a torque curve behind the wheel torque forumla Road speed= (rpm * tyre circ)/(overall gear ratio *1050) with a set of gears (overall ratio * engine torque = axle torque) we can end up with a table which can show the drop in axle torque, if this drop is overly large then the car's acceleration will be diminished, at we go up the gears the torque drop will be less but wind resistance will increase meaning less of a torque drop is acceptable. If the gear change puts the engine at and RPM where the torque is greatly reduced then the situation is really compounded. Race gear sets reduce this torque drop compared to road boxes, and high level MS will use different ratios for different tracks.

Now this XL sheet has variables for traction and that is important too, if the tyres cannot provide the traction to tolerate the wheel torque then we cannot utilise that wheel torque and lower gearing here which increases that wheel toque will be pointless - that is clear but your fundemental dismissal of lowering the FD to improve acceleration is incorrect when we assume (quite reasonalbly from a rolling start) that the wheel torque will not overcome the tyre's traction.