differential calculations

[himanshudave]

Hello, i am a beginner. I need certain guidelines to start my work over the drivetrain and some detailed information on differential i.e differential calculations etc. whatever u think can be helpful.

[himanshudave]

Hello, i am a beginner. I need certain guidelines to start my work over the drivetrain and some detailed information on differential i.e differential calculations etc. whatever u think can be helpful.

[J.R.]

First make sure you need a differential for your car! Dropping it is a good way to lose weight/ cost, and we placed 14th in the enduro running a solid rear at MIS this year. If you're a beginning team, keep the car as simple as possible, building these things takes a lot more time than you think.

If you're still convinced you need one read the Carol Smith Books, and Race Car Vehicle Dynamics by Milliken. That should help you out to start.

[Natho]

There are many different paths you can take in regards to differential/drivetrain in general. In formula sae you see a wide range of setups depending on the characteristics of the vehicle. Most teams seem to run some sort of limited slip diff to enable enough tractive effort through cornering.

Im no expert but for the driveline in general it is important to understand the sort of input loads to the system. The amount of torque the driveline does see primarily depends on the performance of the tires. But if you are starting from scratch u will need to analyse what kind of torque values the driveline is being exposed to as a product of the torque multiplication factors thru the gearbox and the final reduction ratio. From here u can get a baseline of what ur design constraints are. Also note that the driveline experiences shock loading under acceleration/braking and if a proper fatigue analysis cannot be done then appropriate shock loading safety factors should be applied.

Hope this helps!

[himanshudave]

@ J.R
can u please explain the statement "first make sure you need a differential for you car".
I mean can we really proceed further without having a differential in our car?

@Natho
thanks for the help. Actually i have read certain reports on drivetrain made by some teams. The reports are helping a lot but still i would appreciate if you can suggest some more links or if u can mail me any of related information. my email id is himanshudave.bits@gmail.com.

[Grant Mahler]

Yes. You can proceed without a differential.

There are reasons to do so, you will have to do research to figure out why. Diffs are not necessary.

[J.R.]

Some of the reasons are $2000 USD off the cost of your vehicle is your running a LSD, and if you set up your suspension to unload the inside rear tyre, then you can go along the corner without scrub. This is one of the things that we are working on for our car for next year. We run a solid rear axle out of a sprint car, 7075 Aluminum, which only cost us $250 USD. If you still want to run an independent rear suspension (we use satchell link), then you can run a spool, which weighs less.

http://www.formulastudent.de/academy/pats-corner/advice...ts-column-january-2/ (http://www.formulastudent.de/academy/pats-corner/advice-details/article/pats-column-january-2/)

Check out this link, (posted a couple of weeks ago) It explains some more reasons for using a spool. The "virtual differential" part is a little unclear, as a differential lets wheels travel at different speeds, and doesn't have much to do with slip ratio of the tyres, although, you can make one tyre slip more by unloading it, so it kind of makes sense.

If you don't already have it, have your team purchase tyre information from Milliken (link below), and also a copy of Race Car Vehicle Dynamics, read about tyres, and then use this to determine if you need a diff. or not. Hope all this helps!

http://www.millikenresearch.com/fsaettc.html

[exFSAE]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Some of the reasons are $2000 USD off the cost of your vehicle is your running a LSD, and if you set up your suspension to unload the inside rear tyre, then you can go along the corner without scrub. </div></BLOCKQUOTE>

So you're doing one thing.. "reducing scrub" to make the car go around faster.. by doing something else which will decrease your lateral grip (having massive load transfer)? I'd think.. to corner better you NEED the most lateral force.

Yea, you can have a car with a solid, locked rear axle (though not even Nascar is this extreme..), maybe its just my preference but I'd always run a differential. Particularly if it's tunable.

You can have a LSD and set it up so in something like a skidpad there's no "scrub" (0 or near-0 preload and the diff is "open") but on acceleration there's a heap of locking force applied and it hooks up and puts the power down. Its expensive, sure. But its a race car http://fsae.com/groupee_common/emoticons/icon_smile.gif And we all know how much of a joke the cost report is.

With a permanently locked rear end I'd imagine it being way overstable up until the point you're lifting wheels. And you're not ALWAYS lifting wheels. I'd think itd plow through a slalom for example. Would be interesting to see a US/OS plot of the UB car going around the comp circuit.

In any event, whatever you choose.. differential setup has a HUGE effect on vehicle handling. Wish I'd realized this more earlier!

[Mike Macie]

exFSAE, our team knows a diff will outperform a solid axle in most situations. Our team had a small budget and we wanted to design a simple car that would score well in every event and i think we did that.

The most asked question our team gets is how does it handle with the solid rear. We tell them it's like driving a go-kart. It's not as bad as you think. Sure it does understeer a little bit but it's not what most of you skeptics make it out to be. As for lifting the inside rear, it's something the team wants to look into. The current car does not but the tire data shows that your not giving up that much lateral force. We will see what the actual trade offs are from testing.

Overall i think the solid rear was the least of our worries for our dynamic event results. At the time our engine was making 30hp, pictures of our front suspension during cornering were quite disturbing, and our car is overweight at 430lbs for what it is. But we still managed a 18th place finish for car in its first design cycle. Fix these issues and i think the solid axle briggsmobile can crack the top ten.

As for the OP, pretty much every diff has been discussed here in detail before. If you can provide some more info on what your trying to do, you will get a better response.

[oz_olly]

Hey guys,

UNSW@ADFA ran a solid spool last year after our custom Torsen housing failed (due to lack of damage tolerance from a previous gear failure). We were looking at not being able to compete the Monday afternoon before the comp. But after an almost teary phone call to Pat Clarke he convinced us not to give up too soon. So we quickly designed up a spool and got it into the car once we arrived at the competition. As far as testing and comparison go it is hard to say because we didn't get enough driving time to really evaluate it. We were however able to post times in autocross and skid pan and the only change we made to set up was to go from 3 deg caster to 5 deg. The final drive gears failed completely in the last autocross run but the reports from the drivers was the car was very responsive and felt good. Rather subjective comments I know but it has prompted our team to look into this issue furhter.

Pat talks about the 'virtual differential' concept. He likened it to how a train can corner with solid axles in that one wheel rides up on the rail while the other rides down giving a change in effective wheel radius. That's how I interpret it any way. (Pat if you read this and my understanding is wrong please way in).

The thing that I am curious about is if you are going around a corner even without accelerating booth wheels have to spin at the same RPM, therefore one or both wheels have some slip ratio. If the inside rear wheel is at a negative slip ratio i.e. the axle is spinning at the outside wheel speed. Then the inside wheel will have a longitudinal force (braking) if you take this force about the c of g then it is opposing the yaw moment of the car contributing to understeer.

I apologise if what I have said is completely obvious but I believe it may generate some good discussion and I hopefully it will check my logic.

Cheers

Olly

UNSW@ADFA

[carbon_black]

Just to chime in Olly, but I interpreted Pats "virtual-diff" the same way you did. It makes sense in my head.

Pat also has some very inspiring thoughts written up on it over here : http://www.formulastudent.de/academy/pats-corner/advice...ts-column-january-2/ (http://www.formulastudent.de/academy/pats-corner/advice-details/article/pats-column-january-2/)

[ben]

As with most things in FSAE, provided you justify what you did you can go either route.

Yes you can make a spool work on these cars. However the vast majority of "real" race cars have limited slip diffs of some variety - so for your future career it might be more valuable to learn how to setup and tune the diff and the car around it.

Ben

[JD232]

When we formed our team at our uni, the first thing we did was to make a car completely out of scrap and compliant to FSAE rules (to attract sponsors who will give us money to build a shiny expensive one).

Now the differential was costing a lil too much so we decided to go for a solid rear. I dont know about other countries, but here in India the Horse carts have a special rear axle, they just have the one wheel powered. And well, we thought what the hell... so we had a car with only one powered wheel--the left rear one!!

We used Royal Enfield 350cc engine on that car, not too much HP on that but it worked well. As you can predict while we were turning right it was downright awesome, but the amazing thing was the performance wasnt much affected while turning left either.

Well its difficult to say whats the best strategy. So you have to choose ur pic urself...

[J.R.]

Silverback,

Which Uni are you studying at? Did you go to any competitions, if so, how did the one wheel thing work there? It's an interesting idea, running like a go-cart. Did you run toe/camber adjustment or anything to try and negate the pushing effect?

[rjwoods77]

J.R.,

Easy buddy. Think about what he is saying. Remember when we were driving Lazer Zone or what ever that was. They had one wheel drive as well and didn;t do shit when you turned them the one direction except for spinning the tire. Also to do that with our setup would be crazy.

[JD232]

We are based in Alwar, India...
We are putting final touches to go for FSAE Japan in Sep'08...

No, we didnt make any particular adjustments... frankly back then we werent all that comfortable with these adjustments.. (camber, toe... the basic parameters you know)... and we are still learning http://fsae.com/groupee_common/emoticons/icon_wink.gif

The car I talked abt earlier, cost us just over $1000... pics are on the website (under TI-07)

Arpit Gupta
Team Leader
Torque India Racing Club
http://www.torque-india.com

[J.R.]

Rob,

Wasn't actually thinking about doing it on OUR car, just something that I was wondering how it would work/ IF it would work well for cars that turned both ways, and if there were adjustments that COULD be made to minimize the performance loss.

Arpit,

Good luck in Japan, I'll be looking to see how you guys do, especially in cost http://fsae.com/groupee_common/emoticons/icon_smile.gif ! If there is any adjustability in your suspension, you may want to test with camber toe caster, etc. to see if you can make the car handle better.

Good Luck!

[JD232]

Thanks John...

Yes we do have adjustable suspension, we have been doing some testing to check what fits best, but really concentrating on passing all the tests.

I can give you the number at the end of my cost report if you want...

Regards

[psid]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by himanshudave:
@ J.R
can u please explain the statement "first make sure you need a differential for you car".
I mean can we really proceed further without having a differential in our car?

@Natho
thanks for the help. Actually i have read certain reports on drivetrain made by some teams. The reports are helping a lot but still i would appreciate if you can suggest some more links or if u can mail me any of related information. my email id is himanshudave.bits@gmail.com. </div></BLOCKQUOTE>


Himanshu, I am basically starting exactly from where you did. Where did you find the reports for the drive train calcs of other teams?..Thats gonna a help me lot,pls let me know about the reports..Thanks!