Transmissons

red eye · 20 Dec 2010, 13:58 (Ref:2806528)

Hello everyone!

I need to work out ratios for a 6sp box with final drive for a uni assignment, Im having trouble with the formulas etc.
Any body know of any websites that have the formulas so I can work out how to use them?

I got this so far, im trying to decipher my crappy handwriting and remember what the lecturer was saying while he wrote this down lol

power ratio x gear ratio x final drive =?
1.6 x 29/27 x 44/14 = 5.4

I need to work out were the power ratio came from

tristancliffe · 20 Dec 2010, 17:59 (Ref:2806595)

Vehicle Speed in km/h = ( 2 x pi x RPM x Rolling Radius of tyres ) / ( 1000 x gear ratio x final ratio )

Speed in m/s = (2 x pi x Rolling Radius) / (60 x RPM x gear ratio x final drive)

The Bosch Automotive Handbook (mine is the 3rd edition, but I bet there are newer ones, ISBN 0-8376-0330-7) is a great source of information for things like this.

Red Dog · 21 Dec 2010, 21:01 (Ref:2807121)

Quote:

Originally Posted by red eye

Hello everyone!

I need to work out ratios for a 6sp box with final drive for a uni assignment, Im having trouble with the formulas etc.
Any body know of any websites that have the formulas so I can work out how to use them?

I got this so far, im trying to decipher my crappy handwriting and remember what the lecturer was saying while he wrote this down lol

power ratio x gear ratio x final drive =?
1.6 x 29/27 x 44/14 = 5.4

I need to work out were the power ratio came from

Not sure what you mean by 'power ratio'

What is the brief? I have a spreadsheet that will do any combination of ratios that you want - but are you being asked to suit the ratios to the power band of the engine perhaps? After all, that is the whole point of the gears.

red eye · 22 Dec 2010, 13:16 (Ref:2807347)

Red dog, I do not know were power ratio came from ether!

the brief is to specify ratios for a 6sp box with integral final drive and clutch with a predicted top speed of 220mph

engine= 500kw @12,000 RPM, usable power from 8,000 RPM.
Flywheel= 150mm
tires= 16/26/15

I thought I had all the formulas needed in my notes, well I have to a degree its just that there a bits missing

tristancliffe, regarding The Bosch Automotive Handbook does it have all the formulas used in the motor and motorsport industry? I might be lucky and get this book before the dead line

red eye · 22 Dec 2010, 13:52 (Ref:2807380)

Quote:

Originally Posted by tristancliffe

Vehicle Speed in km/h = ( 2 x pi x RPM x Rolling Radius of tyres ) / ( 1000 x gear ratio x final ratio )

Speed in m/s = (2 x pi x Rolling Radius) / (60 x RPM x gear ratio x final drive)

The Bosch Automotive Handbook (mine is the 3rd edition, but I bet there are newer ones, ISBN 0-8376-0330-7) is a great source of information for things like this.

is this the same book? http://www.waterstones.com/waterston...dbook/5934857/im seeing prices from £26 to £160!

tristancliffe · 22 Dec 2010, 14:26 (Ref:2807396)

I wouldn't say it has all the formulas used, no, but it does have quite a lot, and is a useful read for anyone working with cars.

I guess the tyre sizes are in inches - 15" wheels, 16" wide and with a diameter of 26"? In which case you'll need total top gear ratio (final x gear) of about 4.225. Maybe have first as 9.170. And then space them equally between, with smaller gaps near the top. What is the final drive ratio? Do you have a power or torque curve?

Hope it helps.

red eye · 24 Dec 2010, 14:37 (Ref:2808176)

Are road tires marked differently to race? If so there is no mention of it in the brief, so i am assuming its 15" wheels, 16" wide and with an aspect ratio of 26%

back on the Bosch book, silly question OI know but are they the same book?
just don't want paying £160 for a book when I can can spend it on my son

tristancliffe · 24 Dec 2010, 17:44 (Ref:2808241)

Yes, some race tyres don't quote aspect ratio, but overall diameter. The tyres on my car are 250/570R13 - 250mm wide, 570mm in diameter and for 13" wheel rims. All are nominal, so the actually rolling circumference is slightly less than 570mm.

Yes, I think that's the book. Sorry, I missed your last post somehow, which is why I didn't reply.

phoenix · 26 Dec 2010, 14:49 (Ref:2808577)

As set, the problem only reveals that there is no useable power beyond 12,000 rpm and useable torque from 8,000 rpm. There is no indication of where peak torque occurs so not even a guess can be made about the shape of the torque curve against engine rpm.

Without knowing some more detail about the torque curve of the engine I can see no way to select gear ratios for this problem using any kind of science, engineering or maths. Apart from top gear and final drive which can be selected to match the 220 mph anticipated top speed, there can be no right or wrong answers for the remaining gear ratios. Although by making some assumptions about the torque curve some workable and half-sensible ratios could be selected, in practice there could be a solution which makes optimum use of the available torque.

jonathanj · 26 Dec 2010, 16:52 (Ref:2808594)

I have looked at this , and there is several ways of coming to a conclusion , simple maths will give you the required ratios , but you already have the ratios noted as I assume the 29:27 is the top gear (1.074) , 44:14 is final drive (3.142) and you say the tyre size is 16/26/15 , the power ratio could be the inverse of the power to weight ratio ie weight to POWER RATIO that is sometimes used .

This could just be your lecturer giving you too much information to see how you come to a conclusion , I do it sometimes , it means that you can lead yourself up dead ends , can you confirm your tyre size please then we might be able to help , go here http://www.michelin.com/corporate/EN...petition-tires to look at tyre sizes and learn a bit about them , remember these are part metric measurements with imperial rims but you might get the idea .

phoenix · 26 Dec 2010, 17:36 (Ref:2808604)

Quote:

Originally Posted by jonathanj

This could just be your lecturer giving you too much information to see how you come to a conclusion , I do it sometimes , it means that you can lead yourself up dead ends

I agree - why else would the diameter of the flywheel be given?

jonathanj · 26 Dec 2010, 19:27 (Ref:2808633)

Only the diameter is given , not its mass , therefore there would be no point in giving this as far as I am concerned as you could not calculate its inertia or anything useful .

Start with 6th and work backwards using a rev drop chart that you produce using the information you have theoretical top speed would always be @ max revs in top , so 12,000 rpm equals 220 mph at 8000 rpm that so 18.33 mph / 1000 rpm , get yourself some graph paper an work it out .

bdwoody · 26 Dec 2010, 21:33 (Ref:2808649)

this may be of some use .http://www.ukmotorsport.com/misc/gea....92&fifth=0.88

JohnD · 27 Dec 2010, 14:12 (Ref:2808777)

Why not use basic principles?
You have the tyre specs, you can work out crcumference. You could allow for tyre compression and a derived rolling circumference, but I'll bet you could use the theoretical value without invalidating the result. Then you know what wheel revs for any road speed.
Differential constant, anything in front of the gear box irrelevant, you now compare engine speed range with wheel revs, to decide the gear ratios you want.

Reliance on formulae and recipes for calculation can be useful and speedy, but you really need to understand what you are doing, and to be able to check those results against reality.
Or am I being simplistic?

John

snailpace85 · 27 Dec 2010, 21:46 (Ref:2808866)

Just a thought - might the power ratio be the primary ratio i.e. ratio between engine and gearbox input shaft. Applies to motorcycle units (unlikely at 500 kW!) and also some very high end motorsport applications.

As above though - the limited information implies that you need to think things through in your own way and the assignment is designed to check this. Bear in mind that peak power revs is lower than the revs at which usable power runs out. Gear ratios will often be adjusted up or down from the initially calculated values to suit particular critical corners on the circuit.

Scott

red eye · 29 Dec 2010, 15:40 (Ref:2809222)

Thank you for replying guys!

The brief was to specify 6 gear ratios, final drive and clutch. For a 500kw engine with max rpm @ 12000rpm and usable power @ 8000rpm.

The power ratio equation (power ratio x gear ratio x final drive =?
1.6 x 29/27 x 44/14 = 5.4) in my first post is what I wrote down in my lecture it is a formula I was hoping to use in this assignment. My lecturer has a habit of writing on the board while explaining I kinda got lost trying to catch up with everything.

As this a motorsport assignment I think I will use 16mm wide/ 26mm dia /15in rim, I don't see why I can not.

I think this assignment is about using formulas. I am not sure how I am meant to do this, I know I cant specify 1-5th but 6th and the final drive to give me 220mph can be worked out using power ratio x gear ratio x final drive. If I work out 6th and the final drive ratios and provide an explanation why 1st to 5th can not be work out due to the lack of a power and torque graph, should get a D- at least?

red eye · 29 Dec 2010, 15:43 (Ref:2809223)

Quote:

Originally Posted by jonathanj

Only the diameter is given , not its mass , therefore there would be no point in giving this as far as I am concerned as you could not calculate its inertia or anything useful .

Start with 6th and work backwards using a rev drop chart that you produce using the information you have theoretical top speed would always be @ max revs in top , so 12,000 rpm equals 220 mph at 8000 rpm that so 18.33 mph / 1000 rpm , get yourself some graph paper an work it out .

Can you explain how to make a rev drop chart please.

tristancliffe thank you for confirming that, I will put an order in for that book.

red eye · 29 Dec 2010, 15:49 (Ref:2809226)

can someone point to a website were I can read up on and reserch gearboxes and gear ratios please

phoenix · 29 Dec 2010, 17:42 (Ref:2809244)

Quote:

Originally Posted by red eye

can someone point to a website were I can read up on and reserch gearboxes and gear ratios please

Try here: http://www.thecartech.com/subjects/a...Auto_eng_3.htm

As for your tyre size, I would say it is definitely 16 inches wide, 26 inches in diameter and fitting a 15 inch rim. A 26mm tyre would be just over an inch in diameter - on a 15 inch rim? What cloud do you live on? Looking at Avon Motorsport tyre website this would need 782 revolutions to travel one mile. 220 miles would require 220 x 782 revolutions in one hour. Per minute that is 2867.333 revolutions required to obtain 220mph. If the engine is revving to 12000 rpm then the overall ratio of top (6th) gear would need to be 4.1851:1.

If you have a free choice of final drive ratio, you can choose how to obtain that overall ratio by combining the final drive ratio and 6th gear ratio.

If you must use the 14/44 final drive then 6th gear must be 4.1851/44*14 which is 1.3316. If you must use the 29/27 top gear then the final drive would need to be 4.1851/29*27 which is 3.8965.

I have no idea what the "power ratio" is - I suggest you speak to your tutor, a fellow student who attended the lecture or the lecturer to check what you should have written down.

jonathanj · 29 Dec 2010, 19:02 (Ref:2809263)

Right , the power ratio is the reduction ratio into the gearbox , it is making a litle bit of sense now , so the usable rev range is reduced , ie the input speed is between 5000 rpm ( 8000 engine speed divided by the 1.6 ) and 7500 rpm ( 12000 divided 1.6 ) .

I went to Hewland and did this quickly . have a look at this link .

http://www.ratiochart.com/images/53100.jpg

hope this helps , pm me here if you like and will explain more , btw I am a motorsport lecturer

JJ68 · 11 Jan 2011, 14:31 (Ref:2814237)

Hello Red eye.. assuming we go to the same Uni -your assingment is due 24th Jan?

From what i can see in my notes the power ratio should read primary ratio, which is:

overall gear ratio = primary ratio x gear ratio x final drive ratio

output torque = input torque x gear ratio

output speed = input speed / gear ratio

power = 2xPixNxT
N = torque speed (RPS)
T = torque (Nm)
power in Watts

Speed = No. of revs at rear wheels / sec x circumferance
(circumferance = Pi x D)
(1" = 25.4mm)

JJ68 · 11 Jan 2011, 16:26 (Ref:2814282)

Would anybody know what ratio 6th gear would have to be to achieve 220mph with a primary ratio of 1.6 and a final drive ratio of 44/14?
Iv had a look around the Hewland website but theres so many variations of ratios, Im just looking for a point in the right direction if possible please?

John

jimbob · 11 Jan 2011, 17:20 (Ref:2814306)

25/30?????

that give a top speed of 225.75 MPH

but is tht gear ratio possibe?

phoenix · 11 Jan 2011, 17:22 (Ref:2814309)

With 782 tyre revolutions per mile I calculate a 29/24 gear pair giving 221.2 mph @ 12,000 engine rpm.

But why am I doing your coursework? It's very simple maths!

JJ68 · 11 Jan 2011, 17:58 (Ref:2814323)

Cheers Guys. But im still completely baffled by this subject to be honest...