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Jaggs
06-10-2010, 12:38 AM
Hey last time we used a manual tranny gearbox with ( 4 F + N) for our Baja vehicle. It is a gearbox used in 3 wheelers(meant for public transport) here in India(Gear ratios- 31.48:1,18.70:1,11.40:1 and 7.35:1). This time we plan to use a custom made gearbox to get better output and acceleration. How to go about choosing the appropriate gear ratios and the process?? Plz suggest..[smilie=bs_help.gif]

nutz4sand
06-10-2010, 03:05 AM
Consider a CVT? (Variable ratio tranny)

Any other major changes to the buggy? (like added weight etc?)

How did the gear ratios you listed above work for you last time????

minibajaman
06-10-2010, 12:36 PM
Well you can easily calculate the theoretical top speed in each gear based on engine RPM, gear ratio, and tire size. Then assume some losses (maybe 10-15%) due to engine not having enough power, aerodynamics, drivetrain efficiency, etc. Shoot for a top speed in your highest gear of around 35 mph, as it's tough to get a Baja car to go much faster than that. Then pick your other gears to give progressively lower speeds. I can't say in what increments they should be spaced out, as I have only built CVT buggies and never a manual gearbox.

Jaggs
06-11-2010, 11:32 PM
@nutz4sand
The reason we are not considering CVT is that it is expensive here. All the CVT's here are imported . So it adds to the cost. Moreover, CVT tuning is hell of a job. From, whatever i hv read, I see that manual performs better in events like endurance and hill climb....Last time the vehicle performed fine with those gear ratios,but I think we can improve the pick up soomewhat...

Jaggs
06-11-2010, 11:33 PM
@minibajaman
Thanks...

Jaggs
06-11-2010, 11:37 PM
I wanted to ask one more thing... The speed of the vehicle was calculated using gear ratio instead of the final drive ratio...I saw these calculations on net.. How can this give a correct value? They didnt consider the transmission ratio(and hence the final drive ratio)..

nutz4sand
06-11-2010, 11:53 PM
@nutz4sand
The reason we are not considering CVT is that it is expensive here. All the CVT's here are imported . So it adds to the cost. Moreover, CVT tuning is hell of a job. From, whatever i hv read, I see that manual performs better in events like endurance and hill climb....Last time the vehicle performed fine with those gear ratios,but I think we can improve the pick up soomewhat...


There are a lot of people who would disagree with you on that.

I can understand they might be $$$$ or very hard to get though.

As for the gear ratios you need to drive it around (if you still cn) with the tranny you have/had in it and see if say first gear is a little to deep then making it a bit taller would help.

When you shift from first to second does second pull good or wind out to quickly or does it bog any?

Do that for each gear to see if you think the next gear needs to be deeper or taller.

Building your own gearbox may end up costing you a ton more than a good CVT. A cvt is not that hard to tune if you get a decent one. Once its dialed in it will make a gearbox machine of near equal weight and HP look bad usually every time.

Still its your baby and you can do it the way you wanna. The old gearbox may be simply able to have better quality bearings and a lot slippery-er lube to free up some power? If you do not manage to do a complete fab up on a custom gearbox in time.

Good luck with it either way.

Jaggs
06-15-2010, 02:47 PM
@nutz4sand
I think we can optimise the gear ratios on what u r suggesting... One more thing, i wanted to make calculations to calculate the speeds achievable with a given gear ratio. But I saw these calculations on net and saw that the calculations have been performed using the gear ratio rather than final drive ratio.. How wil this return a correct value??
The gearbox that we used last year had gear ratios as- 31.48:1,18.70:1,11.40:1 and 7.35:1.

I checked a motorbike gearbox with the gear ratios-
14.8:1,19.8:1,7.26:1,5.32:1.

But even if the gear ratios are different, cant I use a different differential for both to get the same final drive ratio. As far as I understand, we can match the final drive ratio for a particular gear but the final drive ratios for rest of the gears will vary acc. to the differential that has already been chosen..Plz correct if I am wrong..:D

nutz4sand
06-15-2010, 05:25 PM
Calculating speeds on a given ratio can be misleading if you do not have the power to push the machine in the higher gears.

You seen to have it right on the "different differential" as after the gearbox you can adjust with a chain and sprockets or other means but as you also mentioned this will affect all the gears equally in front of it.

Might put you closer in the ball park with some. But make others near unuseable? Gotta do a lil homework to figure out if its worth it over what you had!

Jaggs
06-20-2010, 08:24 AM
And what about the tire diameter? Does it contribute to the max. speed that can be achieved?? I read somewhere the diameter doesnt matter for off road applications..

minibajaman
06-20-2010, 08:48 AM
Yes tire diameter absolutely matters for your gearing calculations. It won't affect the max speed of a Baja SAE buggy too much, because you need to make your gear ratios suit the tire size you are using. For the same gear ratio, larger tires will in theory make the vehicle go faster, but the engine won't have the power to pull a higher speed.

Convert RPM of the wheel to forward speed by:
(RPM of wheel x tire diameter x PI) / 12 / 5280 x 60
(using standard units inches, miles per hour)

nutz4sand
06-20-2010, 08:59 AM
And what about the tire diameter? Does it contribute to the max. speed that can be achieved?? I read somewhere the diameter doesnt matter for off road applications..


If you really read that somewhere I would NOT take anything I read there serious again!

First off for the Reasons stated above by Minibajaman.
Tire size and gearing go hand in hand and its important that the motor CAN pull the tire in the higher gears. Or else they are wasted.

Tire size also affect ground clearance. VERY important offroad.

Tire size also affects the tires footprint it has on the ground. A larger tire will tend to have more tire on the ground at any time and give better traction.

Jaggs
06-25-2010, 12:02 AM
Hey guys
We are considering two options now..
1. Using our old manual gearbox with the addition of one gear and making it a 5 speed gearbox.
2. Using a CVT from CVTech IBC along with a 2 speed gearbox.
I want to get a rough idea which one will perform better. May be we will implement both and see which one is giving better results.
Btw consider the last 2-3 yr baja events out there. The teams getting the best acceleration time..Do they use CVT ot manual??

minibajaman
06-25-2010, 09:29 AM
2. Using a CVT from CVTech IBC along with a 2 speed gearbox.
I want to get a rough idea which one will perform better. May be we will implement both and see which one is giving better results.
Btw consider the last 2-3 yr baja events out there. The teams getting the best acceleration time..Do they use CVT ot manual??
Your option 2 is exactly what the team I used to be on did last year. In the end, they found that it was difficult to pick two gear ratios that were actually useful with a CVT. It ended up that they just used the low gear all the time, as the high gear just gave up a little too much torque. The only way a two speed is useful I think is to make the lower gear super low for crawling/towing and then make the high gear the same as if it was one speed.

For performance, CVT is almost always better. There has only been a handful of teams to use a manual gearbox and have it work well. It requires an expert driver to get the best performance from it. The main problem is the Briggs engine does not have enough RPM range or power to really use a multi speed gearbox the way it is meant to be used, so you have to shift very frequently when accelerating and going up hills. Also, since there is no electric start, you have to be careful not to stall it during the events.

Most top 10 overall cars are CVT. The last competition I went to (2007), my team was using a Comet CVT with single speed gearbox and placed 6th in acceleration and 2nd in the endurance race. This year the team was also top 10 in both accel and endurance with a CVTech drive system.

Buggy Builder
06-25-2010, 10:28 AM
Amen to Bajaman's comments. Find the widest possible range CVT. Make and educated guess on the final gear reduction. Build it and test it. Make a change to the final gear reduction and test again.

With ten or 12 hp available, your top speed will be around 30 maybe 35 mph on level ground. When I was in Minibaja, we used an 8 hp engine and we could get around 25 mph.

the most important thing you can do is get the car built early and begin a testing program.

One other comment, Minibaja is a program for college engineering students. Hit the books and figure this stuff out.