Landracing Forum
Tech Information => Technical Discussion => Topic started by: rmcmullin on June 26, 2016, 04:51:46 PM
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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
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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:
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See: http://www.landracing.com/forum/index.php/topic,14044.15.html
reply #15 ff.
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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.
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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.
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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
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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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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.
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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.
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Go giter done :cheers:
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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.
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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.
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Ideally you would know what your torque dyno sheet looks like before you order transmission ratios
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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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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
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I guess I really don't understand this last post. Maybe someone could help me out.
I do have a Dyno sheet on the engine, and the HP and torque curves leak at 7000 RPM. The engine ran on the dyno at 8000, but HP and torque are diminishing. From playing the numbers it seems to make sense to peg the shift point at about 7500 which will keep the engine higher on the power curve at the gear change.
I'm beginning to think that first gear isn't too critical since it will be difficult to prevent wheel spin. The shif to second will likely come early thereby giving a long pull in second gear up to 7500. 3,4, and five result in progressively smaller RPM drops and, therefore, keep the RPM well up in the power band, 6000 to 7000.
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Sparky is a big fan of tractive effort (TE) and prefers to deal with that and torque, probably because it is a more intuitive quantity to him. Since the torque and power are directly related, either method can be made to work. Most deal with power, since that is what makes things go and is generally what is quoted off the dyno.
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The torque and power peaks cannot both occur at 7000 rpm unless the torque just STOPS there. Otherwise, your general approach makes sense, it is just a matter of how accurate you want to be as regards optimizing gearing and shift points.
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"I'm beginning to think that first gear isn't too critical since it will be difficult to prevent wheel spin. "
I do not run a TQ converter because I have a fairly light car 3500#
I run a PG/GV which gives me 4 trans ratios 1.76 1.37 1.00 .78
I run this at B'ville with a 2.14 RA with a 16" Mickey which is about 24.75 diameter
MY FDR in 1st is 3.77 1/od 2.93 D 2.14 D/od 1.67
I do this because I have two other tire sets 23.5" on 15" and 26" on 18" wheels
I assign the diam. of the 16" a value of 1 and put all this on a "Dream Chart" spread sheet by multiplying the TQ by the FDR by the tire factor. I plot TE and Speed for each gear by using the dyno TQ numbers
With your Liberty you can do with gear ratios what I am doing with tires.
The bottom line is you need the biggest number of TE at the Speed you are needing to run in my case my target speed is 315 so I need the biggest number at about 320 I changed my car to be cleaner and I changed my eng to move the TQ curve up a little higher in the RPM band.
RPM 1st/TQ 1st/speed 2nd/TQ 2nd/Sp 3/TQ 3/sp 4/TQ 4/sp 5/TQ 5/sp
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putting this on a spread sheet will give you a greater understanding of what your car is doing on a run especially if you have a peaky eng with some big jumps or drop off in power
I/O said it best this is what helps me "see" the direct connection from the info on my dyno sheets to the race track speeds I am shooting for.
others see better with graphs---others with HP OUR "Mileage Varies"
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Here's my spin on it - pun intended! Torque & traction equal acceleration and HP equals top speed. This is true in drag racing, land speed and everything in between. Traction varies between the venues but torque for a given power-plant would stay the same. The torque applied at the contact patch produces the force of thrust that gets us down the track. Torque is the ability to do that work and HP is the amount of that work that gets done in the time or space allowed for doing it! If we plot the torque curve vs the gear ratios [torque multiplication] we can determine the maximum acceleration possible in each gear if the traction is 100% available. For our purposes three examples are given. All three have the same transmission gears and torque but the overall gear ratio and vehicle weight are changed to see the effects. First chart just runs out of gear at 175 mph but we want to go 200 mph and our wind tunnel tests and dyno results say we can do it so we change the overall gear ratio. Notice the curves are the same they just get a little flatter [less acceleration] and the end points go faster. We shift at the same rpm but at different mph. When we add the weight then the end points stay at the same mph but we get less acceleration. Maybe have to move from the short course to the long course? If we plot force (thrust) instead of acceleration the curves and shift points look the same. If we have wider transmission ratios then there are no intersection points and we just shift at red line. If we assume that 0.6 g is good B'ville traction and 0.4g is not so good then you can see that each combination will have different launch and run results. For some the first three gears are totally useless. [Think push start!] You can see where the throttle has to be managed in the other cases! These charts also assume the course conditions are the same all the way down - good luck with that assumption! These trends agree pretty well with the seat of my pants experiments - how about yours? I am always amazed at how well empirical rules of thumbs work (most times/some times?)! While this exercise is useful for planning or refining a particular setup sometimes you just have to run what you brung to have fun! Theory and reality do agree that once the limits are reached it's still add gear & weight then mo' torque & HP!
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Yepppers Mr. Woody !!!!!!!!!
I have head B'ville called a 4 mile drag race with a 1 mile trap there is not much difference between my last mile speed and my exit speed and if ther were an exit speed at the 4 I bet there would not be much difference there either for my car.
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Though not exactly on topic, you suggest that your engine makes PEAK HP and TQ at the SAME rpm. I am not an engineer and only a very amateur engine builder/assembler but this is very unusual and my sense from reading is not likely and/or not desirable-even possible?. Do others have this experience? Dynoroom? As Woody has said, TQ accelerates you but HP is where your speed is overcoming aero and mechanical drag. Sparky mentioned 4 mile drag race with a long trap. In that setting, your gears and shift points want to rock over the HP curve so the HP before and after the shift are more or less the same. If you shift at peak HP RPM, you lose-looking for max HP under the curve. Drag racers for sure shift in this fashion and though there are arguments about it, they always shift this way, more or less.
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OK, I guess I "misspoke." Peak power is 711 at 7000 RPM Peak torque is 540 and occurs at 6500 RPM but is 533 ft. lbs. at 7000 so the curve is pretty flat.
I didn't anticipate my question would generate such a quality and quantity of comment. You guys are a lot smarter about this sort of thing than I am so I'll have to spend some time digesting the info you have provided.
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And like some ( not all) of the responders) I like your gear split, especially the 4-5 jump as you want to keep the engine as close as you can to peak HP and as Midget said, enter your last timing trap pretty much all in. My engine is small and peaky. 4th is 1.23 and drops me farther off my curve than I like. Depending on engine set up, power peak for me is 9000 or 9500 and I shift at 9800-10K (digital tach has some trouble keeping up with the engine sometimes so not always sure) but it drops close to 8000 on the last shift. I'd much prefer 8800 or so which your 1.10 gear would do. In another car we used a Jerico out of a Nascar Pocono car where they run in 4th part time and 3rd part time. 3rd gear was 1.09. Worked well but 1st was about 1.50 so we had trouble launching that particular set up. Adjust your final with the QC and tire size.
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RMAC with an eng like yours--you may well benefit by the torque multiplier tables you can see and compare your TE over a wider range than one might think in my old car I was able to run almost exactly the same speed at 7800 in direct on 28" tires as I was on 24.5" tires in od at 6850 .. the chart said I had 15'# more the smaller tire in od at the same speed and lo and behold I went back with the smaller tires and ran a little faster and got the hat.
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Woody:
Are you computing the acceleration shown on your graph by using A=F/M where F is the product of engine torque multiplied by the final ratio in each of the gears, and M=weight if the car?
This has been a really interesting discussion. It's my first entry in the forum. Are they all this enlightening?
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Calculating the “acceleration” possible from merely the tractive effort and car mass would seem to be a pointless and potentially misleading exercise. Such a value would only be reasonably representative at push-off, before significant drag forces enter the picture. And even then, it would be erroneous since the low end acceleration would likely be limited by the friction coefficient. So then, what is the point? Just stop at tractive effort or do it including all the pertinent forces.
Torque is the ability to do that work and HP is the amount of that work that gets done in the time or space allowed for doing it!
While I am reluctant to criticize Woody’s notion, and his concept stated just above may “work” for him, it is in conflict with the actual definitions of torque, power, and work. Torque, per se, isn’t the ability to do anything. Torque acting at a rotational velocity is (horse)power, and that is the ability to do work. Further, horsepower is not an amount of work. An amount of work is just that--work or, energy. Horsepower acting over a period of time will expend an amount of energy and do an amount of work.
When people adopt different and erroneous notions as to what these quantities really represent, it is no surprise that they become confused and distressed when discussing them with others.
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" Are they all this enlightening?"
Well no--- some can be a lot more confusing :-D But that is the good part---it is trying to lift the veil of confusion that lead to a deeper understanding of how ones best way to get there..
ultimately it just all boils down to---Total Drag vs TE and or thrust---