How many teeth on your front and rear sprocket?

[nicks94tbirdsc]

I did a search to try and find this with very minimal success. As a new team we are just trying to figure if we have our sizes close to what others are using to make sure we are at least in the ballpark. of course the actual ratio you use will vary depending on engine setup and what your curves look like ect. If anyone wants to share how many teeth they use on the motor sprocket and differential sprocket it would be appreciated. Thanks in advance

[Storbeck]

Ours is 14-52. I know I've talked to at least a couple of teams with 14-48.

[Garlic]

I've seen a huge variation over the years. From 60-11 to 45-13. Seems like teams don't put as much thought into final drive as they should IMO, but perhaps they just have differing philospophies about it.

These cars have enough power and gears (generally) that it's not a huge disadvantage if you aren't perfect on this I guess.

[nicks94tbirdsc]

Thanks for all the input so far, although Id still like to hear more if people are willing to share. I think it just depends on your motors numbers and curves whether or not you need to make up for it with steeper gearing.

[jack]

13:39 = 3:1

[Denny Trimble]

Depends on your shift strategy. Many teams launch the car in 2nd gear and only use 2/3/4/5 on the course, avoiding neutral. These teams use a high numerical final drive ratio.

We've been a 1st-gear team for as long as I can remember, launching in 1st, and using 1/2/3 on course with ratios from 13:44 to 13:47 with F4i's and 20" wheels.

The 1-2 shift is interesting, because you go through neutral, and the RPM drop is large.

[nicks94tbirdsc]

Thanks for the input Denny. I believe we are planning on using first gear, but since this is our first car Im sure well play around and see what we like best. Currently on the car we have a 13:52. WE will jsut have to get everything going working properly and decide where we want to go with some testing.

[Frank]

"1st gear team" 13:46

[Daves]

2005 LeTourneau Univ. used 14/65. The drivers wanted to be able to burn out any time.

Speaking of sprockets, I've been looking into chains. Last year, I chose a 428 because it was supposedly lighter than a 520. This year, I realized I made a simple math error. The weight/100 links for a 428 chain and for a 520 chain cannot be directly compared since their pitches are different. Must multiply the 428 value by 5/4 to get the 520 equivalent. Here are the lightest chains from RK, DID, and EK I have found (all weight units are lbs, all strength units are lbs):

Weight/100 links 5-Series Equiv. Strength P/N
1.59 1.99 4300 EK Sport Series 420
1.68 2.10 3580 DID 420V
1.72 2.15 4200 RK "O"� Ring: 420 SO
1.79 2.24 4400 EK Sport Series 428
2.63 2.63 5870 DID 520ERS2
2.13 2.66 5800 RK 428 MXZ
2.2 2.75 5740 DID 428 NZ
2.27 2.84 5500 EK 428SHR
2.84 2.84 6600 EK 520 MRDL
2.84 2.84 7200 RK Heavy Duty: 520 GBTR Gold

You can see that the difference between the lightest 428 and lightest 520 is only 0.39 lbs, but that's for 100 links. Assuming about (60) links, that's only about 0.23 lbs lighter. However, the 428 sprockets are actually slightly wider (5/16" width vs 1/4"), so that 0.23 lbs may be regained in the sprocket weight. So essentially it appears that the 428 sprocket/chain setup maintains about the same weight with considerably less strength (4400 lb chain vs 5870 lb).

Has anybody found the weight/100 links of any titanium 420-series chains (like those made by Sidewinder)?

[Mechanicaldan]

If you haven't made one yet, I suggest you make a thrust chart. Take the torque curve of your engine and multiply it by the gear reduction and drivetrain until it gets to the ground at the contact patch.

It's amazing what an advantage you can gain with the right gearing. 40 ft/lbs can still make 1200 pounds of thrust to push a 450 lbs car into 13th place in acceleration. Only 60 HP. It was 13/50 gears.

This is basic physics. F=m*a F is thrust force from gearing up torque. M is your car mass. A is acceleration. There's 2 things you can do to make your car faster.

If the top speed of the course is 65 mph, then why would you not gear limit your car to around that speed and take advantage of the available thrust?

[Garlic]

Wow, I'm really suprised that you guys are running such tall final drives!

[Denny Trimble]

Originally posted by Mechanicaldan:
If the top speed of the course is 65 mph, then why would you not gear limit your car to around that speed and take advantage of the available thrust?

Because we have to shift (those of us without CVT's), and shifting takes time and effort (mental and physical).

One of our top-level goals is to make a car that is easy for a non-professional driver to drive well; our comp drivers usually have 6 months to 3 years driving experience in our cars. Sure, Michael Schumacher could drive an H-pattern 3-pedal car with 7 speeds that tops out at 70mph faster than I can drive our car, but he's not enrolled. On the autocross and endurance courses, the driver's realtime mental capacity is a limited resource. We choose to simplify things for the driver by running "wider" ratios to reduce the number of shifts. Our engine package is tuned around this, to provide a broad, driveable HP curve.

Your thrust chart is a very good idea, but don't forget that more HP means you can carry that thrust to a higher vehicle speed before you have to shift, and shifting to a higher gear means you get less thrust to the wheels. F=MA, sure, but the acceleration score is based on the integral of A over the distance of the accel event, including time spent coasting while shifting.

More thrust is always good, but running a 13/100 sprocket combination to give you 2400lbs thrust wouldn't help you much because you'd smoke the tires then run out of RPM at 10mph.

And another thing! Sprocket size is important in chassis packaging, smaller is usually better to get out of the way of everything else, but there's a limit on how small you can go on the front sprocket (search the forum for our discussions of that).

[Broderick]

We're mainly a 2/3/4 team and try to stay in the 42/11 or 44/11 range. But like Denny said, Packaging it all between the suspension and sub-frame ususally makes the final decision.

Ryan B.
Rutgers

[Daves]

there's a limit on how small you can go on the front sprocket (search the forum for our discussions of that)

For a CBR600, 11 tooth front with a 5xx series chain or 14 tooth front with a 4xx series chain. Custom made or special ordered, of course.

[Jon Weir]

Originally posted by Daves:
2005 LeTourneau Univ. used 14/65. The drivers wanted to be able to burn out any time.

Speaking of sprockets, I've been looking into chains. Last year, I chose a 428 because it was supposedly lighter than a 520. This year, I realized I made a simple math error. The weight/100 links for a 428 chain and for a 520 chain cannot be directly compared since their pitches are different. Must multiply the 428 value by 5/4 to get the 520 equivalent. Here are the lightest chains from RK, DID, and EK I have found (all weight units are lbs, all strength units are lbs):

Weight/100 links 5-Series Equiv. Strength P/N
1.59 1.99 4300 EK Sport Series 420
1.68 2.10 3580 DID 420V
1.72 2.15 4200 RK "O"� Ring: 420 SO
1.79 2.24 4400 EK Sport Series 428
2.63 2.63 5870 DID 520ERS2
2.13 2.66 5800 RK 428 MXZ
2.2 2.75 5740 DID 428 NZ
2.27 2.84 5500 EK 428SHR
2.84 2.84 6600 EK 520 MRDL
2.84 2.84 7200 RK Heavy Duty: 520 GBTR Gold

You can see that the difference between the lightest 428 and lightest 520 is only 0.39 lbs, but that's for 100 links. Assuming about (60) links, that's only about 0.23 lbs lighter. However, the 428 sprockets are actually slightly wider (5/16" width vs 1/4"), so that 0.23 lbs may be regained in the sprocket weight. So essentially it appears that the 428 sprocket/chain setup maintains about the same weight with considerably less strength (4400 lb chain vs 5870 lb).

Has anybody found the weight/100 links of any titanium 420-series chains (like those made by Sidewinder)?

I was on Carleton University's team last year and we used an RK 420 MXZ chain on 13/40 sprockets. If I remember correctly it was only about 60-70 links long. We saved quite a bit of space and weight, and we never snapped the chain. Oh, and this was using a single cylinder KTM making about 40ft-lbs.

[Daves]

Jon,

How much torque did Carleton have at the countershaft sprocket, though? Or how much force was transferred through the chain?

The internal transmission gearing of the F4i, for example, may vary greatly from the KTM. Although the 420 MXZ might still be adequate for the F4i.

[Underthefloor]

Daves,

We came to the exact same conclusion about 428 chain. It definatly wasn't worth the extra trouble.

[Andrew Del Donno]

Were any of you able to buy a front sprocket for a CBR 600 F4i with <13 teeth. We have found 13 tooth sprockets for a 520 chain to be fairly common, but i'm enticed by these 11 and 12 tooth sprockets I keep hearing mentioned. we really dont have the time to manufacture our own, and we're trying to be a 2/3/4 team, any helpful links?

[nicks94tbirdsc]

use the search and you can find the name of manufacturers who will make the 11 and 12 tooth sprockets for the 520 chain. Some teams sell them in the for sale section as well.

[Agent4573]

we use an 11/42 right now and are a 1/2/3 gear team on 20 inch wheels. That 1/2 shift was tricky in past years when we used an electro/mechanical shifter, but since we went back to a full mechanical we haven't had a single problem with nuetralling out on either the upshift or downshift.

[Underthefloor]

Why do some teams start out in second gear? Did you guys calculate the different acceleration speeds for both a first gear start and a second gear start with proper ratios for each setup? Was it just packaging?


It seems that if you use first gear you might not be able to take advantage of peak power because the rpm difference between first and second is so large but you could eleminate a shift point wich might even out on the accel run but would definatly help you on the auto cross and endurance.

Heres a link to two possible gear setups for our car. In the accel event at Detroit we only got up to the low 60s so if we had used the setup best suited for a first gear start we might have only had to shift once.

http://oregonstate.edu/~danj/Torque%20curves.doc

We are trying to figure out the best approach to choosing a final drive ratio. Any help or insights would be apreciated.

Thanks

[nicks94tbirdsc]

Some teams start in second gear to eliminate going through neutral, the large RPM drop, or both. Its just a preference.

[Cement Legs]

Originally posted by Andrew Del Donno:
Were any of you able to buy a front sprocket for a CBR 600 F4i with <13 teeth. We have found 13 tooth sprockets for a 520 chain to be fairly common, but i'm enticed by these 11 and 12 tooth sprockets I keep hearing mentioned. we really dont have the time to manufacture our own, and we're trying to be a 2/3/4 team, any helpful links?

We went in on a group purchase with 4 other schools to buy 12 tooth F4i sprockets from Sprocket Specialists. I think the minimum order for a custom setup was 8 or 10. Anyway we all bought at least 2 to get the purchase discount and they were roughly $30-40 a piece. There are usually other teams looking for similar components so a post to do a group buy might be a good idea http://fsae.com/groupee_common/emoticons/icon_biggrin.gif.

[Jon Weir]

Originally posted by Daves:
Jon,

How much torque did Carleton have at the countershaft sprocket, though? Or how much force was transferred through the chain?

The internal transmission gearing of the F4i, for example, may vary greatly from the KTM. Although the 420 MXZ might still be adequate for the F4i.

We had a 2:1 internal reduction ratio, and then our 1st gear was about 2.8:1 I think. Pretty similar to an F4i.

Depending on your weight and CG location, you might find that there is more force applied to the chain in an engine braking situation (assume hot tires and no slip).

This being said, we had replacements for both sprockets and the chain, just in case...
If you wanted to be extra careful, you could measure the elongation of the chain to determine wear and predict the lifespan.

[Cement Legs]

Does anyone have the formula to calculate the minimum distance between sprockets?

[Agent4573]

I'm not a drive train guy, but wouldn't minimum distance between sprockets be 1/2 the smaller diameter plus 1/2 the larger diameter? I don't see why you would need more space....

[scooter2131]

Chain wrap on the smaller sprocket becomes an issue when you move the sprockets too close together. You're front sprocket will start to look like a hockey player.

[Erik Whoa]

14-54.

How I look at it, why have such a low ratio if your top speed is limited? Isn't autocross about acceleration (amongst other things)?

I made a spreadsheet to see the expected speed at various rpm and gears. I shot for a ~70mph top speed.