Differential ratios

kjm · Postby **kjm** » Wed Jun 27, 2012 11:16 pm

Greetings learned folk. Is it possible to get a 5:1 ratio in an MX5 differential easily in Australia? I'm interested in converting a Mazda to electric drive by coupling the 6000RPM max motor direct to the diff. A 5:1 reduction should give me ~136km/hr top speed with 14" wheels & maximise the torque. The standard 4:1 ratio could leave me a bit short of torque on hill starts with no gearbox.

timk · Postby **timk** » Thu Jun 28, 2012 12:37 am

You can get a direct swap 4.7 ish out of a Kia Sportage apparently. I don't know the details but I do know a few people are running them on here with 5 speed boxes.

Guran · Postby **Guran** » Thu Jun 28, 2012 9:12 am

Mazda Motorsports apparently do ring & pinion sets in 4.44, 4.875 and 5.125. Not sure where you'd get them though. This info comes from Norm Garrett's book.

toppertee · Postby **toppertee** » Thu Jun 28, 2012 9:19 am

Yes you can get a 5.1 but it's aftermarket from Mazda or Kia
Made from the same 7' ring and piñon
Just remember with your electric motor it has instant toque, so hill starts will not be a issue.
You could always use a reduction gearbox inline, check SEW
But top end will be more of a problem with your car I would think
Are you running a dc or ac
Also depends on your. Controller as well

hks_kansei · Postby **hks_kansei** » Thu Jun 28, 2012 9:48 am

Worth also noting that the replacement crown and pinion sets mentioned are for the 1.8 diff (7in crown)

NOT the 1.6 diff (6in crown)

toppertee · Postby **toppertee** » Thu Jun 28, 2012 10:15 am

Guran wrote:Mazda Motorsports apparently do ring & pinion sets in 4.44, 4.875 and 5.125. Not sure where you'd get them though. This info comes from Norm Garrett's book.

4.875 is out of a front diff out of a Kia sportage about $200 for the whole diff if you can get one.

MiGoreng · Postby **MiGoreng** » Thu Jun 28, 2012 10:21 am

Toppertree 4.77 is from the Kia front diff. 4.875 is an aftermarket diff. Mazdaspeed makes 4.875 & 5.125 diff for the 1.8, you can get them through our sponsor MiataRoadster or Yahoo Japan. I'm currently running 4.875 on my NA8, bang for buck for torque increase.. Not so good on highway though lol

toppertee · Postby **toppertee** » Thu Jun 28, 2012 1:43 pm

MiGoreng wrote:Toppertree 4.77 is from the Kia front diff. 4.875 is an aftermarket diff. Mazdaspeed makes 4.875 & 5.125 diff for the 1.8, you can get them through our sponsor MiataRoadster or Yahoo Japan. I'm currently running 4.875 on my NA8, bang for buck for torque increase.. Not so good on highway though lol

Right you are.
Yep I've pulled one out of the kia, works well. Nothing like pulling 4 grand in 5th at 100km/ph. :mrgreen:

When I was looking around, I'm pretty sure you can get the 5.1 ratio from KIA may be cheaper than the MS unit. A few of the 4x4 guys are using it.

http://www.solomiata.com/Drivetrain.html

flycasta · Postby **flycasta** » Thu Jun 28, 2012 4:20 pm

I will have a couple of 5.125:1's in about 4 weeks.

Have another on order coming from Japan in the next few days will be here in approx. 1 week

kjm · Postby **kjm** » Fri Jun 29, 2012 11:47 pm

tnks for the heads up the 5.125:1 sounds promising, do you have a website flycasta? Don't believe everything you read about electric motor torque coppertee. My first attempt at this was the all electric rear transaxle out of a Lexus RX400H, 130Nm of motor torque with an inbuilt 6.9:1 diff for 900Nm out & it's a dog, a guy in the US has one in a 1000kg conversion & says it wouldn't pull the hat off his head. The new AC motor I'm looking at is 240nm x 5.125 should give me 1230Nm, I'm hoping the extra 37% will be enough.

Postby **NitroDann** » Sat Jun 30, 2012 12:03 am

Im no electric expert but if the same principles work then the only important thing is HP output.

Unfortunately with no clutch you need to make thousands of NM of torque at 1rpm to make any useable amount of horsepower to take off from a stationary start.

Dann

Postby **NitroDann** » Sat Jun 30, 2012 12:11 am

Also are you saying that 136k/h is the speed the car will reach at 6000rpm? You should find out how many HP is required to get an NA to do 110kph, and find out if your electric motor makes that HP at the rpm it will be spinning to do 110.

The only practical way I can think of on top of my head is to get a car with a throttle position sensor, do 110 on the flat, no wind, roof up. Check throttle percentage, and then get on the dyno, same gear, same RPM same throttle percentage and the dyno will give a HP readout. Within 10%.

Dann

Postby **NitroDann** » Sat Jun 30, 2012 12:52 am

So for example even if you can get 10rpm out of the motor in driveline twist upon a stationary takeoff it still takes 21,338NM of torque to make 30hp and give an even moderately zippy takeoff.

Perhaps a torque convertor with lockup would fix these issues?

Dann

flycasta · Postby **flycasta** » Sat Jun 30, 2012 10:12 am

No sorry I dont have a website I just play with Mazda diffs on the weekends and have a full time day job. I keep some ratios in stock because I get asked for them a lot.

Dan have you ever seen this?

http://www.youtube.com/watch?v=UAYrsEOxqYc

Duane

Beelzeboss · Postby **Beelzeboss** » Sat Jun 30, 2012 2:21 pm

NitroDann wrote:Im no electric expert but if the same principles work then the only important thing is HP output.

Unfortunately with no clutch you need to make thousands of NM of torque at 1rpm to make any useable amount of horsepower to take off from a stationary start.

NitroDann wrote:So for example even if you can get 10rpm out of the motor in driveline twist upon a stationary takeoff it still takes 21,338NM of torque to make 30hp and give an even moderately zippy takeoff.

That's not how it works, acceleration is based on torque at the wheels, not HP.

Think about it, according to your theory if you loaded up the drivetrain with an electric motor and then let off the brake, since it's doing 0rpm then it would need infinite torque to move which is clearly not the case. If it's producing a lot of torque at 0rpm in a light chassis then it's going to take off like a stabbed rat, even though it's not making any HP when stationary.

It's just physics. Apply torque, you get angular acceleration of the wheels.

HP is important if you take into account different gear ratio's which change the torque at the wheels. Higher rpm in a lower gear provides more torque at the wheels and therefore faster acceleration. However for an electric motor which only has one gear it's largely irrelevant.