View Full Version : Interchangable Sprocket for different events

10-15-2012, 08:42 AM
I am considering designing a range of sprockets to suit each event, ie. acceleration, sprint etc. If i were to do this, would the car require rescrutineering for each sprocket change?

10-15-2012, 08:50 AM
I believe there is a distinct list in the rules, of what you can and cannot change after going through tech inspection.

10-16-2012, 01:29 AM
This is the rule Ace was referring to:

Once the vehicle is approved to compete in the dynamic events, the ONLY modifications permitted to the vehicle are those listed below. They are also referenced in Part S of the Formula SAE Rules – Static Event Regulations.
a. Adjustment of belts, chains and clutches
b. Adjustment of brake bias
c. Adjustment of the driver restraint system, head restraint, seat and pedal assembly
d. Substitution of the head restraint or seat insert for different drivers
e. Adjustment to engine operating parameters, e.g. fuel mixture and ignition timing, and any
software calibration changes
f. Adjustment of mirrors
g. Adjustment of the suspension where no part substitution is required, (except that springs, sway
bars and shims may be changed)
h. Adjustment of tire pressure
i. Adjustment of wing angle, but not the location
j. Replenishment of fluids
k. Replacement of worn tires or brake pads
l. The changing of wheels and tires for “wet” or “damp” conditions as allowed in Part D of the
FSAE Rules – Dynamic Event Regulations.
m. Recharging low voltage batteries
n. Recharging high voltage accumulators

You can adjust/change you belt or chain, but it looks as if you can't change sprockets. Maybe if you designed your system to have two (or more) sprockets always on the car (like a bicycle sprocket) and then just changed the chain, that might pass.
IMO though, don't worry about it. You'd only realistically care about a different final drive ratio for acceleration. But honestly, optimizing that ratio might gain you a few hundredths, maybe a tenth, which equates to about a 5 point gain overall. But a tenth of second of wheel spin off the long completes negates that gain. So unless you have a legit launch control system that can launch the car perfectly every time, don't worry about it.

Michael Royce
10-19-2012, 08:16 PM
Adam is correct. The answer is definitely, NO!!

Michael Royce,
Technical Scrutineer,
Formula Student.

10-20-2012, 02:18 PM
What if your chain tension is adjusted via differently sized sprockets? http://fsae.com/groupee_common/emoticons/icon_wink.gif

10-21-2012, 05:20 AM
You could easily "break" a sprocket, exchange it and ask for a re-scrutineering. This usually takes less than 2 minutes and should not be illegal. You are of course taking the risk that a scrutineer might not be available and thus lose runs in dynamic events.
I would, as RobbyObby did before, question, whether it is worth the hassle.
This rule, by the way, has an enforcement problem, in my opinion, as it should be impossible to seal or mark all the affected parts which are not allowed to be changed.
Therefore I think, that many teams got away with changing parts in the past without re-scrutineering, because they broke a part and exchanged it and the team did not know that re-scrutineering was necessary.
Unless another team protests this is usually not discovered, if it does not make the car illegal in general. A changed sprocket for example will not be discovered in Parc Fermé, which by the way not all competitions do implement anyway.


Fred G
10-24-2012, 07:20 PM
If a vehicle breaks down, or suffers a crash of some sort. I have seen Mr Royce walk up and tear scrutineering sticker off the vehicle...

Michael Royce
10-25-2012, 11:05 AM
"Tear", no. "Remove", yes, if I felt that the damage was such that we needed to check out the repair. I think that the scrutineers at all the events do the same thing.

10-26-2012, 04:47 AM
I think that the scrutineers at all the events do the same thing.

We do.

10-26-2012, 02:37 PM
What if some cone sprockets with two or three alternative ratios were permanently fitted and scrutineered ?

12-26-2012, 11:55 AM
This is of less interest to the IC cars but of high interest to those building Electric cars. With the electric cars one can simply turn the motor down to say 40% power for the Endurance event and then changing over the final drive ratio becomes very important to gain back some of the lost accel.

12-26-2012, 10:49 PM
Uhm....I do not think so! Efficiency is a matter of power, and power remains constant no matter what the troque/speed ratio is. Varying the gearing can only alter the latter, while not affecting the first one.

12-27-2012, 12:25 AM
What you say is true only if you are talking about mechanical power.

When converting electrical power through a motor into mechanical power the conversion and the efficiency is far from linear.

Basically the largest electrical loss will be due to resistive losses around the system, but mainly within the motor.
The dreaded I^R copper loss turns watts into useless heat.

The way to get highest efficiency from a motor is to run it at the highest voltage and the lowest current, which means high rpm, low torque, and a high numerical gear reduction.

You want to see some fairly high numbers on the tachometer, and hopefully very low numbers on the amp meter. That will give both highest recoverable amp hours from the battery, and lowest conversion losses in the motor.

A bit of data logging and experimentation with gear ratio will be required, but if you can maintain a higher constant road speed with less amps showing, you are definitely headed in the right direction for power efficiency.

12-27-2012, 11:46 PM
But why would you want that to be different in other events and change it to endurance?

Going with the highest possible rpm (limited by BEMF etc.) seems like a quite good route anyway.

12-28-2012, 02:05 AM
Cause in endurance is the only event you care for efficiency. Tony this is true; however I cannot think any time you spend cruising, most of the time you are either at full accel. or braking. When tbe latter happens, you want to have the highest possible current to charge your battery, i.e. the exact opposite ratio you wanted when using it as a motor. An electronically controlled CVT (like the Suzuki SE CVT) would be neat, if its efficiency was higher.Anyway, your ratio is pretty much restricted from your motor max. rpm and the designed max. speed.

12-28-2012, 02:31 AM
By oversimplifying this a bit:
High rpm, low torque and high numerical reduction in endurance would translate into high rpm, high torque and high numerical reduction in autocross.

So the change in gear ratio you'd do for endurance (better efficiency) would also be beneficial for autocross (more torque)?

The motor efficiency map Harry posted in the thread about gear reductions justifies using one ratio, as the torque drop starts in such high revs.

12-28-2012, 06:49 AM
Yes, but (for that particular application)IMO 750NM already more than enough. In general, a high-revving (lets say 15000 rpm as many of the motors out there) would benefit from such a system.In such a case though, reduction would also be necessary from the dictated top speed. So if you have a motor that needs a reduction of 14:1 to give you the desired torque but a ratio of 10:1 to get the top speed you want, what do you design for? As I see it, either you go for a higher torque motor and run a single reduction, or for a multi-gear (2gear? CVT? You name it...)
Again IMO the increased complexity (i.e. reliability, weight, resources and cost) of such a design makes the choice of a more torquey motor inevitable.

01-07-2013, 03:20 AM
Ummm, just build a "manual two speed Gearbox" for your electric vehicle then? Shouldn't be that hard...