Gear ratio vs. rpm

General Technical Assistance

Moderators: WILD BILL, livewire, 2kidsracin, Pool Man, chili man

Gear ratio vs. rpm

Postby Kern Family Racing » Mon Aug 13, 2018 12:55 am

Anyone know or have a formula to figure out gear ratio to drop engine 200 rpm on top end? Right now I am at 11/70 on sprockets for a 6.36 ratio. I am hoping to pick up some mph as well.
Scott and Jack
Kern Family Racing
 
Posts: 126
Joined: Tue May 31, 2016 7:12 am
Location: Greenfield, IN

Re: Gear ratio vs. rpm

Postby RNorton » Mon Aug 13, 2018 1:43 am

It really depends on whether you are overdriving the clutch or not....

If you are over driving the clutch and jackshaft speed is following engine RPM a good rule of thumb is as follows.

Current Engine RPM * (New Ratio/Old Ratio) = New Engine RPM

If you are not maxing out the overdrive decreasing jackshaft ratio can actually increase RPM depending on the setup. A lot of setups really wont see a large movement in this case however, only when you carry it to extremes. From a 4.58 gear to a 2.72 gear our finishline RPM jumped up 1000 RPM for instance. In a case were the clutch is not over-driving typically you'll add primary weight (or loosen the secondary spring if your peaking to high at the shift).

If we had some more info on your setup it might be possible to give you a better answer.
RNorton
 
Posts: 93
Joined: Wed Jan 11, 2017 7:10 am

Re: Gear ratio vs. rpm

Postby zrt-racing » Tue Aug 14, 2018 7:48 pm

Running at a 6.36 ratio I would assume the car is getting well into overdrive.
Formulas and such are great...but track testing is my suggestion for tuning.

An easy switch would be change the drive gear to 12 and see your results. My thought is this will get you close to the 200 maybe a little more. If its off figure out what change it made on your combo and go from there to divide to the exact ratio you need.

A little more info would be great, as well as what your trying to achieve.... many ways to reach a goal.
zrt-racing
 
Posts: 179
Joined: Mon Mar 02, 2015 8:41 pm

Re: Gear ratio vs. rpm

Postby Kern Family Racing » Wed Aug 15, 2018 9:46 am

This car started as an outlaw car and has gear and chain size bigger than #415, but the jackshaft gear and chain are worn out. I have a new #415 11 tooth for the front, new chain, and I just need a new rear #415 rear gear so I was going to replace it anyway. I think I will drop the rear from 70 tooth to 67 tooth to get it just under 8000 which is where I would like to stay for a BZ1.
Scott and Jack
Kern Family Racing
 
Posts: 126
Joined: Tue May 31, 2016 7:12 am
Location: Greenfield, IN

Re: Gear ratio vs. rpm

Postby Kern Family Racing » Sat Aug 25, 2018 8:18 am

Man I hate messing with this car, but the drive gear is shot and we need to change it. I decided to only go a few teeth less on the axel gear because the car (and driver) is winning with this combo. We are currently at 11/70 and I changed to 11/67. I am hoping for a little more MPH on the big end, but it seems like I end up doing the opposite of what I want to accomplish every time I “think” I am using logic. The chain and sprockets are bigger than 415 so I have had a hard time finding anyone that sells bigger than 415 or I would just replace the chain and front sprocket. They are pretty worn out so I need to swap before I have a bigger issue.
Scott and Jack
Kern Family Racing
 
Posts: 126
Joined: Tue May 31, 2016 7:12 am
Location: Greenfield, IN

Re: Gear ratio vs. rpm

Postby WOLFRACING » Sat Aug 25, 2018 8:56 am

Can you post a graph of a run?
WOLFRACING
 
Posts: 1498
Joined: Mon Jan 16, 2012 7:20 am
Location: Illinois

Re: Gear ratio vs. rpm

Postby Kern Family Racing » Sun Aug 26, 2018 9:17 am

Here is a common run. It runs within a few thousandths of 9.55 with a 410 pound package. I’m sorry, I can’t figure out how to get the picture to post correctly, but you can see the initial start and it stays right at 8000 all the way to the finish. I am launching him at 4400 and the clutch engagement is around 5500. His worst light has been a .030. Usually he is right around .010 or less.

Image
Scott and Jack
Kern Family Racing
 
Posts: 126
Joined: Tue May 31, 2016 7:12 am
Location: Greenfield, IN

Re: Gear ratio vs. rpm

Postby RNorton » Sun Aug 26, 2018 9:58 am

A taller gear is not going to reduce your RPM, you need more arm weight. You might pick up MPH for a given ET but you will lose MPH overall. Your car does not have enough power to pick up MPH for a given ET.

Gear will probably not accomplish what you want and will slow your trees down however.
RNorton
 
Posts: 93
Joined: Wed Jan 11, 2017 7:10 am

Re: Gear ratio vs. rpm

Postby Kern Family Racing » Thu Aug 30, 2018 12:41 am

Just thought I would report back. Changing the gear did increase mph by 1 and lowered rpm by 100, but sacrificed 60ft. The E.T. was the same with this gearing.
Scott and Jack
Kern Family Racing
 
Posts: 126
Joined: Tue May 31, 2016 7:12 am
Location: Greenfield, IN

Re: Gear ratio vs. rpm

Postby WOLFRACING » Thu Aug 30, 2018 9:00 am

Where is your max rpm happening? You didn't show the end of the run. Do the rpm flair up at the end? If so, you have then run out of clutch, and you are just gaining rpm with mph.
I don't recall if this is an overdrive clutch. If you want to pick up et, you could run higher numerical ratio and not run out of clutch at the end. Your flatline during the run is affected by clutch setup.
BTW, the hit looks pretty harsh on the rpm graph and there is a little tire spin at the hit as well. Is the belt by chance slipping, causing the spike in the engine rpm?
An easy way to lower your flat line would be to add just a bit of weight to the arms. That should drive it down a bit. That may or may not be productive with the ET. You will just have to see what the combo likes.
WOLFRACING
 
Posts: 1498
Joined: Mon Jan 16, 2012 7:20 am
Location: Illinois

Re: Gear ratio vs. rpm

Postby Kern Family Racing » Thu Aug 30, 2018 11:05 am

I will try to post a better graph. You can kind of see it in the small box underneath the large graph. The rpm is pretty rock steady after the hit all the way through the run. The gentleman I purchased the clutch from had it set up this way and said it was normal to do this. It seems to work ok for us and the worst finish we have had is runner up, but I agree that it is strange and would like to have a smoother hit and better 60 foot. We are currently at 11 grams per arm with the purple spring and launching at 4400 rpm. The clutch does not engage until 5400 rpm and this is not something I am used to. With a 410 pound package, what kind of gear ratio should I be looking for. We have 11/67 for a 6.09 ratio. I want to keep the car no more than 8000 rpm to make this BZ1 live as long as possible. My son has been having a little issue with reaction time and I have played with the rpm during test and tune nights to see if that will help, but it only seems to make it worse for him to try and get used to. As always, thank you for your advice!
Scott and Jack
Kern Family Racing
 
Posts: 126
Joined: Tue May 31, 2016 7:12 am
Location: Greenfield, IN

Re: Gear ratio vs. rpm

Postby Flathead1 » Mon Sep 24, 2018 9:58 pm

Your chain and sprockets are a 420. I have used them for 5 years now. There is nobody I found who has those gears anymore. Bill at gaged has a few left and the guy at nitram who made 420 has a few also depending on what teeth you need. We still run a 420 chain on a 415 rear with a 420 front because of the gearing I have.
Flathead1
 
Posts: 9
Joined: Tue Feb 03, 2015 7:55 am

Re: Gear ratio vs. rpm

Postby Kern Family Racing » Tue Sep 25, 2018 4:33 pm

I couldn’t even get the chain to seat in the gears because it was so big. As an update to this thread, I put our backup Craws on and we have had much better luck. I am launching him at 4300 and clutch engages about 4600. We are going to play a little bit with that, but the RPM stays where I want it on the big end and we have been more consistent with this setup. The 60ft went from 1.95 to 1.99, but we went from 5.99 330 to 5.95 and still hitting the same mph. Now I just need to make the Huddleston work like this one and I will be happy.
Scott and Jack
Kern Family Racing
 
Posts: 126
Joined: Tue May 31, 2016 7:12 am
Location: Greenfield, IN


Return to Tech Talk

Who is online

Users browsing this forum: No registered users and 4 guests