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I now have a rebuilt Overdrive with 28 %ratio. A 3.4 axle and tyres with rolling circumference of 79.78 inches. So what will my rpm be in top at 70 mph and in Overdrive top at 70 mph. Also what will my speed be at 5000 rpm in top gear and in Overdrive top?

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30 minutes ago, Drewmotty said:

If you could reach 5000rpm in top gear o/d you’d be doing 142 mph!

111 mph in direct top. 

....best of luck with that :-)

 

(  Tom you must have had a comprehensive rebuild !! I hope you are happy with with it all)

 

Drewmotty

I used to use minty lamb. 

 

Would you you be able to share your calculator ?

H

Edited by Hamish
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15 minutes ago, RogerH said:

Hi Saggy,

        an apprentice friend of mine back in 1966 had a TriBsa motorbike that could easily do 70 in first.  quite frightening.

 

Roger

All his mates would have beat him off the line tho !!!

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BSA used to make a close ratio gearbox called a RRT2 which was supplied with a model called a Gold Star, this is the gearbox that go the bike to 70 in 1st, the big draw back was you had to slip the clutch until 30.

Not a lot of use in London but every one wanted one

George

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17 minutes ago, tomfpurves said:

Thanks everyone.Yes its a comprehensive rebuild.However silly me I said the axle was 3.4 to 1 it is now 4.1/1 so what does that mean at 5000 rpm in direct top and od top.

Tom

Very racy Tom 

its what pete cox suggested to me. 

But would have been too much with my smaller tyres. 

H

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11 hours ago, Hamish said:

 

(  Tom you must have had a comprehensive rebuild !! I hope you are happy with with it all)

 

Drewmotty

I used to use minty lamb. 

 

Would you you be able to share your calculator ?

H

Hamish,. Here is one that I have used. Download the excel file.

http://buckeyetriumphs.org/technical/Calculator/road_speed_vs_engine_speed_calcu.htm

Cheers  Peter W

http://buckeyetriumphs.org/technical/Calculator/road_speed_vs_engine_speed_calcu.htm

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Thanks Peter

Just need to suss out my gearbox ratios now. 

A 6 box in a 3a !!

H

Edited by Hamish
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It's a nice exercise, these gear calculators in Excel, but the arithmetic is so NOT quantum chromodynamics.  It's shoppng list arithmetic: So many apples at  2/6 a pound etc

Without overdrive, the 4.1: 1 diff converts 5000 rpm to 5000/4.1 = 1219.5 rpm.   

that will cause a rolling corcumference of 79.78 inches to travel 79.78 x 1219.5 inches/min = 97291.71" in one minute, and 97291.71 x 60 = 5837502.6 inches in one hour 

as there are 63360 inches per mile (1760 yards x 3[ft/yd] x 12 [ins per ft])  this is the same as 93 miles in that hour (nearly the same as Andrew Drewmotty, probably do to rounding in the Excel)

The same easy calculation has to be preceded by increasing the revs if the O/d is engaged.    A 28% O/d will raise the revs to 5000 x 128 or 6400rpm, where upon the same arithmetic will give you a speed  under O/d of 119mph.

All done on a sheet of paper and, I admit, a calculator, although sums like 5000/4.1 are easy, if lengthy, by long division.   And the computing aphorism applies, "Garbage in, garbage out".   Unless you know how to do the sums, how will you check the result?

John

 

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15 hours ago, tomfpurves said:

Thanks everyone.Yes its a comprehensive rebuild.However silly me I said the axle was 3.4 to 1 it is now 4.1/1 so what does that mean at 5000 rpm in direct top and od top.

Tom

Tom

did you have a LSD fitted with this set up. If so which did you go for ?

H

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I suppose the answer will be “It won’t make that much difference”, but does the circumference of the tyre not increase the faster you go, due to centrifugal force?

The tread part gets bigger and the side walls pull in.

This would result in going further for each revolution of the wheel, so RPM per mile would be different for different speeds.

(Or won’t it make that much difference…)

Charlie.

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2 hours ago, Charlie D said:

 

I suppose the answer will be “It won’t make that much difference”, but does the circumference of the tyre not increase the faster you go, due to centrifugal force?

The tread part gets bigger and the side walls pull in.

This would result in going further for each revolution of the wheel, so RPM per mile would be different for different speeds.

(Or won’t it make that much difference…)

Charlie.

Yes that is relevant.  Something tells me the carriage office require(s) London Taxi cabs with a mechanical taximeter to have 8 ply tyres minimum, to restrict/control the throw out of tyre diameter at speed, which would affect the measured distance travelled(charged)

Peter W

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Hi Charlie,

               there is apprx 1/4 ton weight on each tyre (TR4A apprx 1 ton in weight)

If we say a tyre weighs 20lbs and the foot print takes up 10% of the circumference.

 

Then we are looking at a 2 lb weight being thrown out at speed  - would the 1/4 ton (560lb) counteract this????

 

I would suggest it would by a long way.

So the rolling circumference may grow but the rolling radius would remain the same.

 

Or..........

 

Roger

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Roger,

I think I understand what you mean, but then, as Peter says, the carriage office is concerned enough to specify stiffer tyres on Hackney Carriages to keep things more constant.

I also started thinking that it will make a difference between new tyres (say 8mm tread depth.) and worn out tyres (say 2mm tread depth).

I worked out (possibly wrongly) that at 5000 revs in top, it might make 2MPH difference, on a 165/15 80 tyre.

All academic really, I know. Maybe I should get out more.

Charlie.

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