tomfpurves Posted November 10, 2018 Report Share Posted November 10, 2018 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? Quote Link to post Share on other sites
Drewmotty Posted November 11, 2018 Report Share Posted November 11, 2018 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 :-) Quote Link to post Share on other sites
Hamish Posted November 11, 2018 Report Share Posted November 11, 2018 (edited) 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 November 11, 2018 by Hamish Added Quote Link to post Share on other sites
iain Posted November 11, 2018 Report Share Posted November 11, 2018 Wow that has some very long legs..... Iain Quote Link to post Share on other sites
saggy Posted November 11, 2018 Report Share Posted November 11, 2018 Hmmm, 70 MPH in first I reckon! Quote Link to post Share on other sites
RogerH Posted November 11, 2018 Report Share Posted November 11, 2018 Hi Saggy, an apprentice friend of mine back in 1966 had a TriBsa motorbike that could easily do 70 in first. quite frightening. Roger Quote Link to post Share on other sites
Hamish Posted November 11, 2018 Report Share Posted November 11, 2018 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 !!! Quote Link to post Share on other sites
harlequin Posted November 11, 2018 Report Share Posted November 11, 2018 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 Quote Link to post Share on other sites
tomfpurves Posted November 11, 2018 Author Report Share Posted November 11, 2018 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 Quote Link to post Share on other sites
Drewmotty Posted November 11, 2018 Report Share Posted November 11, 2018 92mph and 118mph Quote Link to post Share on other sites
Hamish Posted November 11, 2018 Report Share Posted November 11, 2018 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 Quote Link to post Share on other sites
BlueTR3A-5EKT Posted November 11, 2018 Report Share Posted November 11, 2018 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 Quote Link to post Share on other sites
tomfpurves Posted November 11, 2018 Author Report Share Posted November 11, 2018 Thank you very much.Most helpful.Minty does not seem to be available these days. Tom Quote Link to post Share on other sites
Hamish Posted November 11, 2018 Report Share Posted November 11, 2018 (edited) Thanks Peter Just need to suss out my gearbox ratios now. A 6 box in a 3a !! H Edited November 11, 2018 by Hamish Detail Quote Link to post Share on other sites
john.r.davies Posted November 11, 2018 Report Share Posted November 11, 2018 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 Quote Link to post Share on other sites
Drewmotty Posted November 12, 2018 Report Share Posted November 12, 2018 No Excel here.....the rounding was all my own :-) Quote Link to post Share on other sites
Hamish Posted November 12, 2018 Report Share Posted November 12, 2018 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 Quote Link to post Share on other sites
Charlie D Posted November 12, 2018 Report Share Posted November 12, 2018 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. Quote Link to post Share on other sites
BlueTR3A-5EKT Posted November 12, 2018 Report Share Posted November 12, 2018 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 Quote Link to post Share on other sites
RogerH Posted November 12, 2018 Report Share Posted November 12, 2018 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 Quote Link to post Share on other sites
Charlie D Posted November 12, 2018 Report Share Posted November 12, 2018 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. Quote Link to post Share on other sites
tomfpurves Posted November 13, 2018 Author Report Share Posted November 13, 2018 Hi Hamish its fitted with a Qaiffe diff. Interestingly I have just noted that it will be 60 tomorrow having been built on 14th November 1958! Tom Quote Link to post Share on other sites
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.