Choosing gear ratios

[Jono]

Leading on from my last topic on SC/helical (and to avoid going too off topic in that one), thought I'd start a new thread on gear ratios.

I've got the gearcalc Pc program which lets me plug in any ratios I like, gives speed at given rpm, rpm drops between gears and a nice little graph.

Ignoring desired speed and rear end ratio for now, what sort of difference between each gear is reasonable? I'll be using a 4-speed box, with 1:1 top ratio.

Looking at bar chart of gear ratios, is it better to have a straight diagonal line through them or more of a decreasing gradient curve? I suppose the other way to look at it is should gear ratios get closer from 1st to 4th or try and keep gaps more or less the same (which would seem to make gearchanges easier if rpm drop is the same each time?)

I might be answering my own question here ,but I think in an ideal world you would have an even drop, but in reality to keep a reasonable first gear ratio and still a good top end speed (without going to wide on ratios) you would need uneven drops and a taller (by this I mean lower speed but may have got wrong term) first gear for good "off the lines"

Is there a rough and ready approximate value that is considered a reasonable rpm drop between gears?

Just re-read this and it doesn't seem very clear, hopefully some of you clever bods on here can make sense of it...

Cheers
Jono

 

[Russ Noble]

Jono,

Depends what you're doing with it!

But anyway closer gear ratios as you go up the box.

For circuit work I would go with a high first gear that will get you out of a hairpin without having to find the next gear while still exiting the corner. I would think about 70 mph first for a GT40, my 5 litre TR7V8 was 58mph and that was fractionally low. I'm sure you'd be able to light a 70mph first up off the line. Then I would gear 4th for whatever top speed you need, say 170 mph, then split the other two about equally say 105mph 2nd, and 140 mph 3rd. However if there is one particular circuit that you race at most often then I would adjust those speeds if possible to suit particular critical corners on that circuit.

For the street you might want wider spaced ratios, a lower first for easier take off and parking and a higher 4th for more relaxed highway cruising. Say maybe 50, 100,150,200 mph respectively 1st to 4th. Maybe leave 3rd at 140 mph. Really it's a matter of personal preference.......

Given the same scenario for a five speed I would go 70,95,120,145,170 for the track and 50,90,125,160,200 for the road.

The above would be my personal preference...... Others may disagree.......

Cheers

 

[wanni]

Leading on from my last topic on SC/helical (and to avoid going too off topic in that one), thought I'd start a new thread on gear ratios.

I've got the gearcalc Pc program which lets me plug in any ratios I like, gives speed at given rpm, rpm drops between gears and a nice little graph.

Ignoring desired speed and rear end ratio for now, what sort of difference between each gear is reasonable? I'll be using a 4-speed box, with 1:1 top ratio.

LOOK TO THE POWER AND TORQUE DIAGRAM PARALLELY.
TO EACH DROP, REFER IT TO THIS DIAGRAM AND WATCH IF YOU HAVE POWER AVAILABILITY.
MY OPINION IS THAT YOU SHALL USE A 5 GEARS AT LEAST.

Looking at bar chart of gear ratios, is it better to have a straight diagonal line through them or more of a decreasing gradient curve? I suppose the other way to look at it is should gear ratios get closer from 1st to 4th or try and keep gaps more or less the same (which would seem to make gearchanges easier if rpm drop is the same each time?)

IN ORDER THAT BY INCREASING THE SPEED, THE RESISTANCE IS INCREASING AS WELL, BETTER TO HAVE DECREASING CURVE.

I might be answering my own question here ,but I think in an ideal world you would have an even drop, but in reality to keep a reasonable first gear ratio and still a good top end speed (without going to wide on ratios) you would need uneven drops and a taller (by this I mean lower speed but may have got wrong term) first gear for good "off the lines"

NEVER MIND ABOUT THE RATIO, WHICH WILL BE ALWAYS TOO SHORT. iF YOU NEED ACCELERATION FROM STANDING, USE A VERY HEAVY FLYWHEEL. IT WILL STORE THE ENERGY YOU NEED TO PROPPEL THE CAR.
SET UP THE 1ST GEAR RATIO AT 75 KMH WITH THE 5TH AT 220 KMH

Is there a rough and ready approximate value that is considered a reasonable rpm drop between gears?

Just re-read this and it doesn't seem very clear, hopefully some of you clever bods on here can make sense of it...

Cheers
Jono

cHECK MY RATIO CHART BELOW

 

[Jono]

Thanks,but 2 issues with 5-speeds, the main one is that my budget is rapidally spiralling out of control, and i just don't think I can stretch to a five speed jerico or tex racing, the four speeds are a bit cheaper. 2nd, most performance/road race 5spds still have 1:1 top, so while I can see the advantages of closer ratios due to the extra gear, I'm not gaining anything in terms of top speed or cruising economy (not a main priority but...), and still have to put up with running out of revs at top speed or fitting a taller diff ratio.

 

[Russ Noble]

Jono, the answer is still the same, closer ratios as you go up the box so that your bar graph of speeds in each gear is a straight line.

If you're stuck with a 1:1 fourth then you choose diff ratio to give the desired top speed (or cruising ability, whichever is your main criteria) and then find or configure a box with the right spread of ratios to give you the 1st gear you need.

You may have to compromise the 'ideal' for what is available.

Cheers

 

[Jono]

Yes, think it will be a case of choose 1st and 4th, then fill in the gaps, bound to be a compromise, but unless I win the lottery (unlikley as I don't play) I will have to make do.

I am really confused now, I downloaded the spec sheet from Jerico, and although it says all 4th ratios are 1:1, within the table of ratios it gives a number of 3rd gears which are less than 1.0. presumably they would just be for 3spd boxes?

 

[DBLDREW]

Thanks,but 2 issues with 5-speeds, the main one is that my budget is rapidally spiralling out of control, and i just don't think I can stretch to a five speed jerico or tex racing, the four speeds are a bit cheaper. 2nd, most performance/road race 5spds still have 1:1 top, so while I can see the advantages of closer ratios due to the extra gear, I'm not gaining anything in terms of top speed or cruising economy (not a main priority but...), and still have to put up with running out of revs at top speed or fitting a taller diff ratio.

I’m not sure what your budget is but you can pick up a 5 speed Tremec TKO 600 road race for under $2000. It is good for over 600lb/ft of torque. The gear ratios for the road race box are…

1st- 2.87
2nd- 1.89
3rd- 1.28
4th- 1.00
5th- .82

If you are shifting at 6000 rpm the rpm drop per gear is..

1st-2nd 2049 rpm drop
2nd-3rd 1937 rpm drop
3rd-4th 1313 rpm drop
4th-5th 1080 rpm drop

 

[Jac Mac]

Yes, think it will be a case of choose 1st and 4th, then fill in the gaps, bound to be a compromise, but unless I win the lottery (unlikley as I don't play) I will have to make do.

I am really confused now, I downloaded the spec sheet from Jerico, and although it says all 4th ratios are 1:1, within the table of ratios it gives a number of 3rd gears which are less than 1.0. presumably they would just be for 3spd boxes?

Jono,
No you are correct, you can use what was 3rd as an OD top by using the correct combo of ratios if you so desire- eg; 0.89(3rd,now OD) 1/1( top ) 1.3 (2nd) 2.3 (1st). Just means flipping the selector arm over on that pair of ratios.
Dont know why You would want to though unless you were on an Oval type circuit where fuel conservation was an issue. Remember the major reason behind all of this stuff is spelt 'NASCAR'. For example its not uncommon for them to run say a 1 / 1 top and 1.05/1 or similar 3rd on some tracks to get more acceleration off the corners .

One other thing,I see one hopeful Harry above using 200mph as a target top speed, forget that unless your going to be all day on the Autobahn or at Bonneville. Around 165/175 is a much more realistic figure for most circuits. Plenty of ratios for 9" diffs if you need more.

Jac Mac

 

[Russ Noble]

Jac Mac,

200mph just chosen coz some guys have said elsewhere on here 2500 rpm cruising at 70 mph. Assuming approx 7000 rpm max that works out.

As I said depends on personal choice.

Cheers

Harry

 

[Jono]

dbldrew
Thanks but I think I'm set on the jerico /tex route now, as reportedly muhc stronger and not that much more expensive...

Reason I was thinking overdrive was partly top speed. I have been working on a rpm limit of 6k which I realise may be a little low, but just estimate based at looking at crate engines. World posted their dyno chart for their new alloy block in my alloy vs. steel thread, which revved to 6400, so I've been comparing 6k 6.5k and 7k to see what effect that has.

I'm worried on two counts, (perhaps unduly) about a 7k engine, firstly about the extra cost for reliable internals, and also low end power / torque, which I guess would suffer in a higher revving engien built for peak power? Not sure what effect this has on power band either but I'll try and keep on topic and save that for the engine forum.

I'm working on a 245/45/R16 tyre with rolling diameter 24.68" / 627mm (standard tyres are 23.2" / 589mm dia. so not sure how much bigger I can go height-wise. Width will be taken care of with arches.

For 1:1 top gear, 3.09 diff this gives top speed of
143mph @ 6k
154mph @ 6.5k
166mph @7k

with 2.89 diff
152 @ 6k
165 @ 6.5k
178 @ 7k

wasn't sure if 2.89 was getting a bit tall and if I select 1st gear to suit this if the other ratios may end up a bit wide.

To be honest as Jac Mac says, if I don't like it I can always change diff ratio to make up for it if I don't like it, as this would be much cheaper than a new gearset. I'm not really sure how often it will see those kind of speeds anyway.

Assuming top speed about 150 - 160, 50/90/125 for 1st/2nd/3rd would seem ok based on Russ's post.

Thanks for everyone's help, really appreciate it.

 

[Russ Noble]

Jono,

Just to give you an example, my TR7V8 ran a Ford 8" 2.8 diff, 23.7" tyres and a WC T5. This gave speeds in the gears at 6500 rpm


2.95 1st 55mph
1.94 2nd 84 mph
1.34 3rd 122 mph
1.00 4th 164 mph
0.74 5th 222 mph

I never really used 5th on circuits but it was good on Targa as it reduced engine revs on the really long straights although it never went a lot faster than it would in 4th.

Prior to that I had a TR7 3.9 ratio diff which kept lunching itself. That gave speeds in the gears at 6500

1st 40
2nd 61
3rd 88
4th 118
5th 164

Needless to say 1st never got used. Second was used off the line but the end result was that lap times were virtually identical and speeds in the relevant gears were not dissimilar. The disadvantage with that setup was the dogleg 5th which you were in and out of all the time. A real PITA.

I much preferred the later setup using the Ford diff and one gear lower. That was also a good cruiser on the road too, it would lope along at 100mph at under 3000 rpm. It was very effortless and deceptively fast A to B. The disadvantage was that first was slightly too low on the track. Excessive revs in first out of the hairpin eventually destroyed the motor and directly led to my GT40 project. Not all bad!!

Cheers