Diff Question

[vecchioalfa]

I'm dazed and confused.......

[KevJTD]

yes, the tyre squash/different circumferance does depend on mnay factors lauren. car weight, tyre pressure, tyre brand even.
tyres are made within a certain spec. a 155 of one brand can be different size wise to another, they have a certain amount of tolerance to make it as they see fit. michelin usually make to the upper end of the tolerance. in the example of a 155 section width tyre it needs to be made between about 145 to 165 mm wide when on the nominal rim, measured across the widest part, not the tread width. michelin spec is 163mm for a 155 tyre....
not really relevant i know but shows the total inacuracy of calculations without exact data.

my suggestion would be to simply get the car on the tyres you intend to work out the data with, at the pressures you intend using, then chalk a line on the tyre corresponding to a mark on the ground. roll the car on 10 revolutions of the wheel and mark another line. measure that distance, divide by 10 and you have an accurate measurement of your rolling circumferance (i'd do 10 revolutions for accuracy to get rid of any doubt over the chalk lines being within a few mm of lining up)
way overkill i know, but if accuracy is needed it's the only way to be sure.

[AlfaCorseChris]

Thanks guys,

Kev, the box I bought now is off the 146, so I'm assuming its the same.

Current setup of my 33 seems a bit off, especially on 3rd gear.
Mine is 120 khm on 2nd and 170khm on 3rd. 4rth gear tested on dyno to rev limiter at 200khms.

So regardless of those results, the 146 is 90/140/180 approximately respectively from your results.

This means that its quite shorter by 25 to 30 khms per gear approximately.

This will definately be an improvement. But will it be the ideal improvement thats another story

Lauren, from what I see, early 1200/1300 ones have a long second (which will probably kill the engine till it builds up the revs) but also a short 4rth and 5fth gears, so that would mean higher revs on the motorways, driving like a maniac and going nowhere faster
Late ones, post 1982 seem more civilized and more torque efficient ? They do seem very close to the 146 box as well.
Or have you calculated those speeds to the stock rev limiter of 6300 rpms ?

[Alfasud1978]

Chris at 6000rpm I get around 75kph with 2nd and 105kph with 3rd if that helps. I can make a copy of my Alfasud's manual if you want. Has gear ratios, differential ratio and max speed per gear at 6000rpm.

[Veesix75]

Chris

2 things in using the 1200ti/early 1300ti gearbox :

1. Second gear doesnt kill the revs, all the 16v prod spec racers in the uk used a 1200 box, you have to remind yourself how often you use second gear in a track (not very often), and even on the road its not a long gear.
2. Third, fourth, fifth are close hence would be manic on a motorway, but you want them to be manic on a track - you spend most of the time in those gears

I know you use yours in the road hence the compromise you need.

For track, I've said before, best option I used was 1st and 2nd from a 16v, 3rd, 4th and 5th and Diff from a 1200ti with 14 inch rims.

[Spacenut]

Lauren, from what I see, early 1200/1300 ones have a long second (which will probably kill the engine till it builds up the revs) but also a short 4rth and 5fth gears, so that would mean higher revs on the motorways, driving like a maniac and going nowhere faster
Late ones, post 1982 seem more civilized and more torque efficient ? They do seem very close to the 146 box as well.
Or have you calculated those speeds to the stock rev limiter of 6300 rpms ?

No, I used the 7,200 rpm limit and the 205/45 x 16" tyres. Interesting point about the long second gear ratio, but as others have pointed out, I won't be spending much time in that gear, especially on the road.

My use of old-skool 205/60 x 15 tyres means that only the 1.2 Ti can give me decent acceleration through the gears. I'm really not bothered about the theoretical top speed - with my old post-1982 gearing giving 125 mph in 5th at 5800 rpm, the engine was never going to overcome the air resistance anyway. Even with the 1.2 Ti ratios, I can't see the Green Machine out-dragging a modern Corsa kiddie-kar (I just annoy them with stylish good looks )

Lauren

PS - thanks Kev, I will do some measurements and see what kind of correction factor I need to refine the analysis - I'm using some pretty low tyre pressures so sidewall distortion will come into it I'm sure...

[KevJTD]

Lauren, from what I see, early 1200/1300 ones have a long second (which will probably kill the engine till it builds up the revs) but also a short 4rth and 5fth gears, so that would mean higher revs on the motorways, driving like a maniac and going nowhere faster
Late ones, post 1982 seem more civilized and more torque efficient ? They do seem very close to the 146 box as well.
Or have you calculated those speeds to the stock rev limiter of 6300 rpms ?

No, I used the 7,200 rpm limit and the 205/45 x 16" tyres. Interesting point about the long second gear ratio, but as others have pointed out, I won't be spending much time in that gear, especially on the road.

My use of old-skool 205/60 x 15 tyres means that only the 1.2 Ti can give me decent acceleration through the gears. I'm really not bothered about the theoretical top speed - with my old post-1982 gearing giving 125 mph in 5th at 5800 rpm, the engine was never going to overcome the air resistance anyway. Even with the 1.2 Ti ratios, I can't see the Green Machine out-dragging a modern Corsa kiddie-kar (I just annoy them with stylish good looks )

Lauren

PS - thanks Kev, I will do some measurements and see what kind of correction factor I need to refine the analysis - I'm using some pretty low tyre pressures so sidewall distortion will come into it I'm sure...[/quote]
the michelin data gives a rolling circumferance of 1912mm for the 205/60-15 lauren. it'd be interesting to see what the real world figures give you with running low pressures

the lower rolling circumferance is incedentally what most of the manufacturers use nowadays as data for type pressure monitoring rather than using a delicate sensor in each wheel like the renault lagunas used to use. some makers are using wheel sensors again now as cars need to have them, we'll see how well they last...
the wheel speed data is collected by each individual ABS sensor which if the pressure drops thus causing the wheel speed to increase relative to it's opposing one then the car concludes that the reduction in rolling circumferance can only be due to a puncture. the process has been used on bmw's for many years. even with super stiff runflat tyres which hardly look flat even when they are, the sensors can detect a pressure loss even if it's less than 10psi loss which you'd think would be a tiny reduction is circumferance.
just shows that the difference between calculating the circumferance of a tyre using the diameter and Pi as opposed to what is the true circumferance in real road terms on a standard tyre must be quite a bit apart.....be interesting actually to calculate the difference in the 2 methods of calculation...

[Veesix75]

Good point Kev re the tyre pressure and radius. My BMW has the sensors, I was very dubious of them, but 134K miles later they work great and kick in at a 7psi loss of pressure and 40 series tyres.