Hi

Assuming a rolling diameter of 25" (...is this about right?) for a 17" Ruby wheel and using the 5.125:1 axle ratio, a Speed to Revs graph works out like this:

Sandy of this Forum let it be known a little while ago that he had done something similar as a MS Excel spreadsheet,

see
http://img300.imageshack.us/img300/3329/speedcalcszr6.jpg

for a copy........ because I don't know how you get images onto here

Algie

I should have added that on the original spreadsheet, you can enter the apprpriate axle ratio in the RH columns (under Sandy's 'CV 9998' or 'Normal RP Saloon' headings) and the engine revs cascades out accordingly, Sandy has another spreadsheet too which works from the base of the engine speed through the ratios to give you a road speed.

Obviously the data at top LH corner can be altered for different tyre sizes/diameters etc and I suppose on that basis is not exclusive to A7's.

I understand this all started from wondering at what revs the engine was at when he heard the engine go through it's harmonic stage - e.g. the engine went quiter. (37.5 mph, which explains that line in his table).

Algie

See the back of Bill William's book Austin Seven Specials he includes speeds against revs for various axle ratios and tyre sizes.

Colin,
My own sums, using 5.125, 4.50x17 with 681mm diameter, 785 revs per mile in top gear, came out at 14.9mph per 1000rpm. Looks like the same as your (cleverer) graph.

I tried to lay this out with the comman axle ratios and tyre sizes in a Gearing page on the Speedex site. http://myweb.tiscali.co.uk/speedex/index_files/Page1138.htm See if the two things line up? Note that the Speedex site has moved ISP as per the addresses here.

5.125 and 4.50x17 is probably a bit overgeared for a Ruby or saloon. 5.625 or 5.375 is probably a better combination with this size tyre or the axle would be better if you could get some 4.00x17's. Its not a bad gearing for a lighter tourer or special.

Dave

Hi Dave

If I put your rolling diameter (681mm) into my calculations I get the same figures.

The only figure that is uncertain is this rolling diameter - everything else follows from that. Direct measurement from floor to wheel centre of the 17" rear wheels with 4.5 Armstrong tyres on my Ruby gives a radius of 320mm and therefore a diameter of 640mm (25.2"). The graph shown above was therefore based on 25".

I imagine that this figure depends on a number of factors such as tyre type, pressure, operating temperature (?), weight of vehicle etc. that would all make a small difference.

Yes, the Ruby is over-geared - generally climb even slight hills revving in third rather than straining in top.

Regards

Colin

.

On the topic of rolling radius I pose the following...

If we measure the radius from wheel centre to ground and multiply by 2pi we get a figure for the distance travelled for one wheel revolution.

If we take a tape and measure the actual circumference of the inflated tyre we get a larger number, and thus further travelled per wheel revolution.

It would be my interpretation that the second figure would be the more correct, and that the difference is taken up by the 'footprint' or flattening of the tyre against the road surface. Otherwise there would be enormous tyre wear as the tread scuffed to try and keep up, and loose the difference???

The flattening and flexing of the tyre against the road is probably the major cause of tyre temperature. Unless your Seven has sufficient power to spin the wheels that is.

Thoughts anyone?

Mike

It struck me that the rolling diameter might be measured more directly.

With the help of a large spirit level, I marked a point directly below the centre of the wheel on the tyre and on the floor, before rolling the car forward exactly one turn of the wheel, as checked by the spirit level, and marking the floor again. The distance rolled was then measured.

This distance was 1990mm, giving a rolling diameter of 633mm, which is 24.9" and 809 revs per mile.

Regards

Colin

Has anyone taken into acount the fact that as a wheel spins faster the diameter of the flexible tyre gets greater due to centrifugal force? I presume at low speed the measurement of the distance travelled by one revolution of the wheel is fairly acurate but at high ( relatively ! ) speeds it may be closer to the circumfrence measurement of the tyre. Just a thought !

Study some text books on suspension design and you will see that some of the above is correct.

The flattening of the tread does occur , and creates a " standing wave" around the tyre , this wave can resonate at certain speeds leading to rapid overheating and failure of the tyre. I believe this is related to the carcass design amongst other things and thus accounts for different speed ratings on tyres.
I think you will also find that drag racing tyres grow as Malcolm suggests, which is probably heat and centrifugal force.

steve

.

Steve,

Thanks for pointing me in a sensible direction, in fact there is some quite good stuff to be Googled I find - I should have looked there prior to making silly statements. Never-the-less it has raised an interesting topic which I am gently persueing. Just waiting for the weather to dry up to try running a wheel for a revolution on some old wallpaper to get a print. Certainly a quick check with some chalk marks yesterday confirmed your own findings.

My next question - to myself - is this .....

If there are 100 blocks of tread pattern around the circumference of the tyre, but the chalk marks on the ground are only 97% of a circumference apart where did the other three go???!!!

That will probably do for a wet Sunday



Mike

If you think of a tyre as a tank track, rather than a round disc, you'll get closer to the truth. The distance a car goes forwards per wheel rev isn't pi times the wheel diameter, or even 2pi times the static radius. Its the length of the (relatively inextensible) tread belt. To know with any accuracy, you'd need to ask the manufacturer to meadure it for you at various speeds, but you could push the car forwards a decent distance, say 10 wheel revs and measure how far you'd gone.