Author Topic: Transmission Ratios  (Read 10958 times)

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Offline rmcmullin

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Transmission Ratios
« on: June 26, 2016, 04:51:46 PM »
I'm in the process of ordering a Liberty five-speed transmission to be coupled to a Winters QC incorporating a 3.08 ring and pinion ratio.  The rear tires have a 30" diameter  The engine a normally aspirated, 365 cu. in.  SBC producing 711 HP and 533 ft. lbs. of torque at 7,000 rpm.  My plan is to run a turbo motor at some future date, and I would like to select the transmission ratios to accommodate both configurations and make the required changes in final driveline ratios via the quick change....to the extent possible. My problem is exactly what gear ratios to specify for the Liberty to achieve an optimum RPM split.

I have produced a spreadsheet that gives me the speeds in each gear and the RPM drop at each gear change, but am not sure how to interpret the data, based on my (admitted) inexperience. My general approach has been that at the release speed from the push vehicle (assumed at 25 MPH...too low?  too high?) the engine needs to have enough RPM to accelerate the car.  For the normally aspirated motor, I have assumed this to be about 2500 RPM, but this would likely be higher with a turbo motor.  Thereafter, it seems logical to me that the shift from first to second and second to third, etc. should have successively smaller RPM drops since the engine needs to remain in the higher end of the power band as the mechanical advantage decreases in each higher gear. Engine peak power and torque are made at 7000 RPM, and the negative slope of the graph after 7000 is much steeper then the positive slope before 7000, so I'm assuming it's better to execute the shift at 7000 rather than running the engine faster but at a lower HP and torque value.

I've tried some ratios in the spreadsheet with the following results:

Quick change ratio 3.23
30" tire
1st gear 2.42, speed at 7000 RPM=80, RPM drop in shift to 2nd=2144
2nd gear 1.68, speed at 7000 RPM=115, RPM drop in shift to 3rd=1707
3rd gear 1.27, speed at 7000 RPM=153, RPM drop in shift to 4th=930
4th gear 1.10, speed at 7000 RPM=176, RPM drop in shift to 5th=640
5th gear 1.00, target speed at 7000 RPM=192

Is there a guideline that defines what the respective RPM drop should be in each successive gear or does anybody out there have any experience that would be helpful?  Liberty offers a wide variety of ratios in small steps, and while I can always make a change if I make the wrong choice, I would prefer to base my gear selection on more than just luck.

Thanks for your help.

Roger McMullin
rogermcmullin@comcast.net  
« Last Edit: June 26, 2016, 04:58:14 PM by rmcmullin »

Offline Stainless1

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Re: Transmission Ratios
« Reply #1 on: June 26, 2016, 08:38:08 PM »
To me it looks like your top 3 are way to close together. I would think a 1 and a .9 would make a better 4th and 5th.  But then I'm a small motor guy....
:cheers:
Stainless
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Offline Interested Observer

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Re: Transmission Ratios
« Reply #2 on: June 26, 2016, 09:35:07 PM »

Offline Milwaukee Midget

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Re: Transmission Ratios
« Reply #3 on: June 27, 2016, 01:07:01 AM »
Looks like you're running the three mile.  You'll want to be pretty well all in by the time you hit the two.

Peak power is for bragging rights, but the curve of the graph is the tool you need to look at.  If the engine is solid and can be coaxed to pull past peak in the lower gears, you won't risk falling into the hole, and you will have accelerated closer to your target speed sooner in the run.

Engine peak power and torque are made at 7000 RPM, and the negative slope of the graph after 7000 is much steeper than the positive slope before 7000, . . . 

How steep is "much steeper?"

How tight would you feel safe spooling it up?  Taking it up to 8000 probably keeps you closer to the power band on the on the 2-3.

On the 3-4 shift, it's not what the engine is pulling at 7000, but what it's putting out at 6070.
On the 4-5 shift, it's not what the engine is pulling at 7000, it's what it's putting out at 6360.

You'll want to superimpose your dyno sheet over your proposed shift points and ask yourself, will those last 640 rpm get me those 16 mph before I run out of track?

As to whether or not the ratios are right for a turbo engine with longer gears, that's another graph.
"Problems are almost always a sign of progress."  Harold Bettes
Well, I guess we're making a LOT of progress . . .  :roll:

Offline wobblywalrus

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Re: Transmission Ratios
« Reply #4 on: June 27, 2016, 01:48:55 AM »
The publication, The Racing Motorcycle, Volume 1, by John Bradley ISBN 0 9512929 2 7 has the best discussion of this topic I have seen.  He uses the concept of driving and ultimate forces to determine gear ratios and shift points based on the engine's dyno curve.  It might be a good idea to read what Bradley says and then to fine a program or spreadsheet to do the number crunching.  The attached is a copy of a figure in the book to give a rough idea of the concept.

Offline mike f

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Re: Transmission Ratios
« Reply #5 on: June 27, 2016, 07:38:08 PM »
Hi to all,  After looking at all the post , most people forget many different factors.  The most important one is the track surface.  I believe the best way to go is with a basic set up for a new engine/car/driver combo.  After that fine tune it.  I have already changed 2nd/3rd/4th ratios at the track for different conditions, and I've done it for more then 1 guy.  The ratios you have are a good starting point and by all means DON'T over think it.  Remember a spread sheet and physics book never drove a race car.  Your feel for the car will be your most important tool.  Mike

Offline rouse

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Re: Transmission Ratios
« Reply #6 on: June 28, 2016, 08:50:40 AM »
I run a bike and have found that the faster you are going the closer the ratio needs to be. That way you have less hill to climb, so to speak, going up to the next gear. It's all about recovering from the gear change, if it recovers good OK, if not, tighten up the ratio.

What you have looks like a good setup to start with to me. Nothing like trying it out to see.

Rouse
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Offline rmcmullin

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Re: Transmission Ratios
« Reply #7 on: June 28, 2016, 07:40:02 PM »
I want to thank each of you for the valuable advice, which is much appreciated. I'll keep working the problem.

The last time I ran a car at El Mirage or Bonneville was 1958, and a lot has changed since then.  That car went 170 with a 299 inch, normally aspirated engine, and it only had one gear!  I was 17 years old and am returning (or hope to return) to the salt at age 74 to fulfill a lifelong dream. One thing hasn't changed over the past 60 years is the LSR fraternity. That's the part I remember most fondly.

Offline Stainless1

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Re: Transmission Ratios
« Reply #8 on: June 28, 2016, 09:14:27 PM »
Roger, you sound like a guy that wants to go 200... go with the 1 in 4th and a .9 in 5th.... that's a 212 if the motor has the grunt to go the speed... unless you are in a real dirty brick, give it a whirl... you can always swap gears in the QC. 
Stainless
Red Hat 228.039, 2001, 65ci, Bockscar Lakester #1000 with a little N2O

Offline SPARKY

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Re: Transmission Ratios
« Reply #9 on: June 29, 2016, 12:12:34 AM »
Go  giter done :cheers:
Miss LIBERTY,  changing T.K.I.  to noise, dust, rust, BLUE HATS & hopefully not scrap!!

"Security is mostly a superstition. It does not exist in nature, nor do the children of men as a whole experience it. Avoiding danger is no safer in the long run than outright exposure. Life is either a daring adventure or nothing."   Helen Keller

We are going to explore the racing N words NITROUS & NITRO!

Offline wobblywalrus

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Re: Transmission Ratios
« Reply #10 on: June 30, 2016, 08:17:23 PM »
Roger, that graph in Reply #4 was simply one I grabbed out of the book as an example of the procedure.  Looking at it close, this is the way I like to set up the transmissions.  A wide gap between first and second, a smaller but still wide gap between second and third, and the narrower and progressively smaller gaps between third and fourth, and forth and fifth.  Usually when setting up the gears I worry about getting the gaps between the ratios right.  The overall ratios are adjusted in the final drive, as said in a previous post.

The speed the shifts can be made is a factor.  It is hard to get a good performance from a wider ratio transmission if there is time for the car to slow down while shifting.  A slower shifting driver or linkage needs a closer ratio trans. 

Offline rmcmullin

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Re: Transmission Ratios
« Reply #11 on: July 02, 2016, 12:27:14 PM »
Thanks again for your help and insight. I've continued to work on this and it's become apparent that, while there are more than a hundred choices of ratios, many only differ by 0.01. Maybe this make a difference at the drag races, but I'm not so sure it does at Mirage/Bonneville. 

In answer to your question, I plan on using a Liberty transmission with an air shifter so the gear changes should be pretty quick.

I'll post a spreadsheet this weekend summarizing my preliminary ratio selections and would appreciate any feedback from forum members.

Offline SPARKY

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Re: Transmission Ratios
« Reply #12 on: July 02, 2016, 02:01:39 PM »
Ideally you would know what your torque dyno sheet looks like before you order transmission ratios
Miss LIBERTY,  changing T.K.I.  to noise, dust, rust, BLUE HATS & hopefully not scrap!!

"Security is mostly a superstition. It does not exist in nature, nor do the children of men as a whole experience it. Avoiding danger is no safer in the long run than outright exposure. Life is either a daring adventure or nothing."   Helen Keller

We are going to explore the racing N words NITROUS & NITRO!

Offline Rex Schimmer

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Re: Transmission Ratios
« Reply #13 on: July 02, 2016, 05:07:48 PM »
I worked on several 5 speed Hewlands when I was younger and if we did not have previous info on the ratios for the track we would guess what the top speed was going to be set 5th for that speed and then set low gear for our estimation of what the slowest corner would be then split the ratios such that the step between each ratio was approx  1/2 of the previous step. So looking if first to second was .8, second to third would be .4, third to fourth would be .2 and 4th to 5th would be .1. It was amazing how close this gess-timate worked out. If you check your original gear selection you are pretty close to this. Your gear difference is .74-.41-.17-.1 and per my guess method and using your first to second step of .74 the rest of the gears would be .37-.18 and .09 steps, pretty close.

As IO has previously stated it is horse power that makes you go fast so dyno plots of your horse power curve are what should be used not torque.

Rex
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Offline SPARKY

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Re: Transmission Ratios
« Reply #14 on: July 02, 2016, 11:54:51 PM »
REX  I have a slightly different take.

  I multiply my dyno torque sheet through my drive train ratios to get the the TE that is put to the ground---that is the bottom line what goes from the tire to the ground.  I almost don't care what the HP because it has to go through the drive train and tire size..  the torque cure and its RPM determines the HP curve
Miss LIBERTY,  changing T.K.I.  to noise, dust, rust, BLUE HATS & hopefully not scrap!!

"Security is mostly a superstition. It does not exist in nature, nor do the children of men as a whole experience it. Avoiding danger is no safer in the long run than outright exposure. Life is either a daring adventure or nothing."   Helen Keller

We are going to explore the racing N words NITROUS & NITRO!