Landracing Forum
Tech Information => Technical Discussion => Topic started by: McRat on February 26, 2009, 12:35:41 PM
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Are larger diameters are more likely to sustain higher speeds?
What about aspect ratio? Is a short sidewall better or worse than a taller profile? I know taller profiles will support more weight in general, but I don't know about speeds.
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Centrifugal force calculator:
http://calctool.org/CALC/phys/newtonian/centrifugal (http://calctool.org/CALC/phys/newtonian/centrifugal)
If you could find two tires that were exactly the same weight but different diameters the larger tire would have a higher force. Smaller and lighter is the key.
The size and aspect ratio are only part of the equation that involves the tire construction, inflation pressure, and the quality of the build. It also involves the frame, suspension geometry, and load that the tire maker doesn't have control over.
What you want is someone else to do the math and testing.
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Centrifugal force calculator:
http://calctool.org/CALC/phys/newtonian/centrifugal (http://calctool.org/CALC/phys/newtonian/centrifugal)
If you could find two tires that were exactly the same weight but different diameters the larger tire would have a higher force. Smaller and lighter is the key.
The size and aspect ratio are only part of the equation that involves the tire construction, inflation pressure, and the quality of the build. It also involves the frame, suspension geometry, and load that the tire maker doesn't have control over.
What you want is someone else to do the math and testing.
Thanks for link!
Yes, I'm working with a professional testing lab. But experience trumps everything IMO, what sometimes makes sense on paper doesn't always translate the same on the racetrack.
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Centrifugal force calculator:
http://calctool.org/CALC/phys/newtonian/centrifugal (http://calctool.org/CALC/phys/newtonian/centrifugal)
If you could find two tires that were exactly the same weight but different diameters the larger tire would have a higher force. Smaller and lighter is the key.
The size and aspect ratio are only part of the equation that involves the tire construction, inflation pressure, and the quality of the build. It also involves the frame, suspension geometry, and load that the tire maker doesn't have control over.
What you want is someone else to do the math and testing.
But Dean, didn't they make tires huge (36" +) in the olden days to reduce serface speed and keep them together?
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What about the larger contact patch with the taller tire? Depending on HP and speeds you add another factor into the equation.
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What about the same height and larger contact patch? The tire I ran last year had about 60% wear, so it had a much larger contact patch. Does more traction override the additional rolling resistance?
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I would think for El Mirage or the short course at Bonneville, traction would more important than rolling resistance, at least in my case.
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Centrifugal force calculator:
http://calctool.org/CALC/phys/newtonian/centrifugal (http://calctool.org/CALC/phys/newtonian/centrifugal)
If you could find two tires that were exactly the same weight but different diameters the larger tire would have a higher force. Smaller and lighter is the key.
The size and aspect ratio are only part of the equation that involves the tire construction, inflation pressure, and the quality of the build. It also involves the frame, suspension geometry, and load that the tire maker doesn't have control over.
What you want is someone else to do the math and testing.
You have it backwards. The larger tire will have less force on it then a smaller one. Centripital acceleration (in g) is given by the v^2/(32.174*r), where v is velocity in feet/sec, and r is the radius in feet. Changing r will cause an inverse change in g, where changing v will have an exponential change in g.
To get the force needed to hold agianst that centripital acceleration, just multiply g-force by the mass of the object that is being spun.
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Man I wished I had payed more attention in math classes. :-D Learning this stuff over 40 sucks!
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You are correct!!! Number and the ABC is a bit*h when your over 50. But thanks for homework now I got work to try and figure out
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Here is my question since I can't find my gearsplit software backup. I have a final gear ratio of 3.67 and a 4th gear of 1.13 and a 5th gear of .91
In choosing a speed that I think I can hit should I match a tire to fit the speed with peak horsepower in 4th or 5th gear?
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Here is my question since I can't find my gearsplit software backup. I have a final gear ratio of 3.67 and a 4th gear of 1.13 and a 5th gear of .91
In choosing a speed that I think I can hit should I match a tire to fit the speed with peak horsepower in 4th or 5th gear?
What is your 3rd ratio. Personally if it was mine I would pick the gear where there was less rpm drop between it and the one before it.
The spread sheet on my site at......
http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html
..... will give you the rpm drop between any two gears at any rpm shift point.
c ya,
Sum
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3rd is a 1.44 - doing some math it looks like shifting at redline the RPM drops a couple hundred RPM (give or take) above peak torque. With speed loss during the shift, RPM probably drops to either peak torque in the 3/4 shift to a touch below in the 4/5 shift.
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3rd is a 1.44 - doing some math it looks like shifting at redline the RPM drops a couple hundred RPM (give or take) above peak torque. With speed loss during the shift, RPM probably drops to either peak torque in the 3/4 shift to a touch below in the 4/5 shift.
If you can't change the rear gear ratio and/or find the right tire dia. then you are going to have to go with which ever one is going to get you the right overall ratio for the speed you think you can run. If you can make either work with tires and/or rear get I would go with the 4/5 final shift since it has the least amount of rpm drop. With Hooley's Stude we have a 7% drop going from our 1:1 third to our .93 overdrive and that is great. I like the "touch below in the 4/5 shift", but will be interested to see what others think.
c ya,
Sum
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What I do not have enough knowledge about is the torque properties with gear ratios versus tire diameter. Dad always mentioned a 1:1 final gear ratio out the trans as being ideal. Don't know if that is a myth or not.
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What I do now have enough knowledge about is the torque properties with gear ratios versus tire diameter. Dad always mentioned a 1:1 final gear ratio out the trans as being ideal. Don't know if that is a myth or not.
Our transmission is 1:1 in what is the normal 4th gear, but they can and did put a overdrive cluster where 3rd usually is, so the direct through gear (1:1) is now our 3rd gear. True we are giving up some HP running the overdrive since it is now not direct (straight through). The problem we have is that with the Ford rearend we are running we are geared as high as we can be in the rear with 2.47 gears so have to resort to the transmission to run faster at less RPM's where our motor makes it's power.
A cam change or a switch to one of Sparky's GM rears would solve that problem, but as always time and money can rear it's ugly head. If Hooley sells this car and builds a new one like he has talked about it would probably have a land speed racing type quick change with the low number ring/pinion in it. Then we could run the 1:1 gear like you mentioned and that would be the most ideal.
c ya,
Sum
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It really (always!) gets down to horsepower and how much you have at the engine speed that happens when you make the shift from 4th to 5th. Using your numbers: 3.67 rear end ratio, 1.13 4th gear and .91 5th gear and assuming three things: 1. You have enough horse power to reach your maximum velocity at your maximum horse power rpm.2. you are running 30 inch dia tires 3. Your maximum hp is at 8000 rpm. ( I don't know what you have for an engine), but using these numbers your maximum speed would be 213 mph and you would shift from 4th to 5th at 172 mph. When you shift from 4the @ 8000 rpm to 5th your engine rpm will drop to 6430 rpm. Now if you look at the horse power required at (I am looking at aero power only) 172 mph then to go 213 you will require 1.9 times as much horse power to go 213 as you need to go 172 ( Hp required is a function of the speed cubed.) so that says you need at least 52% of your maximum horse power at 6430 rpms to not have the engine stop pulling. So you need to know your horse power curve, not your torque curve. Again this is hp required to over come aero loads only and does not include rolling resistance and any other frictional drags that may be taking horse power.
This also shows you why big blocks with power glides work at the salt. The gearing my not be optimum for maximum acceleration but neither are the traction conditions so maybe dropping the big block into high gear and letting it lug a little actually could help acceleration by not having to much power available to spin the tires.
Lots of combinations that work and some that don't, the trick is to be able to look at the "cause and effect" and be able to tell why the do or do not work.
Rex
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I am beginning to understand. Now if you drop my max speed by 70mph we are getting there. My torque curve is pretty flat, it varies about 10% from 2100RPM to redline at 7200. So I am what I think is 60% of HP or better after the shift. I do not have a numbers printout but a curve plot for one of my dyno runs from last year. We are headed back to the dyno in a couple of weeks with the throttle bodies on my little 16V 4 cyl. I just want some ideas for tires as we get closer to the first Maxton meet. I have 2 options for rim sizes that allow me to tuck the wheels further inside. Right now they stick out a touch (1/2-3/4") where the air-dam can't cover. I think I have an idea of what to use. THANKS!
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wolcottjl,
I guess I didn't understand that you are running at Maxton, in a case where you have a pretty limited distance to accelerate and also much better traction than at Bonneville, going with "optimum" gearing is pretty important and certainly with a small engine to get absolute maximum acceleration you need to have gearing that maximizes the " area under the curve" and that is done by using ratios that maximize the use of your engine horsepower thru the gears. Using something like Sum has described is probably what you need you be considering. If you have a dyno chart of your engines output from that you can optimize the gearing by plotting the speed thru each gear and making sure that the rpm drop does not drop below pike torque. By doing this you will maximumize your acceleration.
I see Maxton as a long drag strip as opposed to Bonneville where you have lots of room but pretty limited traction. One other note I would highly suggest that you run the litest and smallest diameter flywheel and clutch combination you can find, it will have a pretty big effect in the lower gears.
Rex
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Thanks, that opens up another question. Flywheel weight, I read somewhere that you did not want to take too much weight off of a flywheel for running at Bonneville.
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wolcotjl,
Clutch/flywheel weight is again different from Maxton to Bonneville. At Bonneville you don't need the quick engine acceleration response that a light clutch/flywheel provides you as it can cause traction loss I would not expect that to be the case at Maxton.
Rex
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Rex,
Thanks a bunch.
Joel