Diff Ratio Calculations

[gearb0x]

Hey Guys,

My 20V Ae71 Had its first day out on a gravel hill climb yesterday, managed to fry a set of rally tires so looking for some of those in 14" if anyone has any

More to the point, I was having lots of trouble with my diff/gearbox ratios. Does anyone have any good gearbox/diff ratio calculators or can offer any tips with my problems?

I currently have a T-18 T50 with a 4.1 final drive, when Im coming into corners its relatively low speed but It was generally right in between 1st and second, when in 1st i was bouncing off the limiter and in second I was doing 4000rpm, Not ideal when trying to slide on gravel. I want to work out what diff ratio would be the best to try and get it so I'm using maybe second and third if its possible as I cant quite affort a close ratio T50

I had the same problem when I was running the 2TG, i was just hoping the 20v might be able to power out of it, which it can, but it kept coming on song at the wrong time and spinning me. If you watch this video you can see its obvious I was valve bouncing the 2tg then grabbing second and dying in the ass http://www.youtube.com/watch?v=hWwk9634_dc

Any Help would be appreciated

Regards,
Phill

[andrew_mx83]

there are plenty of ratio calculators around just have a goodle or spend 5 mins setting up an excel spreadsheet


i have a pair of 15" falken slicks for sale.... nothing in 14" tho sorry

[oldeskewltoy]

Hmmmmm, instead of screwing with gears.... you might want to screw with tire/wheels.

Maybe look @ 13"

[Xauterus]

Hi
I think what you are starting to discover is that you will require a close ratio gearbox to suit what you are doing. With your stock box the ratios are just way too far apart. The 20v really likes the revs and is not a particularly torquey motor. if you try dropping your diff ratio a fair bit further you will bring the ratios slightly more together. Are you using full revs in top gear now? you need to know your highest speed for track and work out what diff will give you max revs in fourth gear.
Corey

[gearb0x]

heh its autocross, i only use 1st and second as it is

max speed is 100+ if that.

I know a close ratio box is my best bet but its a little out of my price range hence the investigating diff ratios, someone else at the track mentioned dropping to 13" on the rear as well, I need 14" on the front to clear my brakes

I am currently playing with a gear ratio calculator but its hard to estimate the revs I will do into different speed corners with different ratios because the wheels are spinning (gravel) as opposed to not spinning on tarmac

[wombat]

What RPm is too low? cos obviously your RPM is low when changing gears on the way out.

My first suggestions would be get a high ratio diff. like dunno a 4.3 or a lil more. I think there is a 4.5 available. you'll be out of 1st quicker, but it means you be able to get into second earlier and hopefully that will fix your having to change as you get out. plus if you need it on fast course you can use 3rd down a straight.

[Norbie]

http://akb.norbie.net/Public/Gearing.aspx

Play around with that to get an idea of revs/speed for different diff and transmission ratios. I've even thrown together a nifty graph (link at the bottom) so you can see the effect of changes in real time.

[gearb0x]

well I'm still getting the tune right & haven't had it a dyno so I'm not 100% sure where peak torque is, but atm it is dropping to 4000 which is way out, id rather it being over 5 when i shift.

Playing with a gear calculator it looks like If I go to an albins gear set (which is the closest thing that i can afford if i rebuilt the box my self) I will still need a short diff to make it useful, and effectively I will only use 1st gear with the albins ratios.And drop into second on the longer straights. With the Albins set 1st -> second change will drop to around 5300-5400rpm which looks ideal.

At 600$ x 3 + misc rebuilt parts I think it will be the last thing I try

I am looking at trying a 4.778, knocks off around 35kmh off the top speed but I cant see my self doing 200kmh+ in this car too much.

One problem with dropping the rolling diameter, I think it will make the tires wear a little quicker, I fried my tires on the weekend as it is The more "available" secondhand 13" sized rally tires are only 10mm less rolling diameter than what I have, so i will have to drop the profile a bit to see any noticeable difference)

[Billzilla]

For rallying you want a 4.7:1 diff.

[wombat]

I think Sheldrake has a 4.77 for sale at the moment buddy.

if you change those gear ratios won't that mean taking off will be really bad if you can use 1st gear for so long?

[gearb0x]

yeah i know thats why im trying to work it out to make sure 4.7 is good for me

[Billzilla]

A 5.125:1 is also good if you can find one.

[gearb0x]

OK, just doing some research to try and find the easiest way of getting a nice short ratio, these are the 2 options Im looking at:

1. Buy a 4.778 R&P or complete center and put it in with welded spiders
2. Buy a YN22 tarago diff, which is 30mm wider according to the internet and comes with a 4.3 stock (also according to the internet)

Swap the mounts, new tailshaft, etc (not sure if these have good handbrakes & if the cables will work on the ae71 though)

Next step would be to remove the 4.3 and put it in a safe place for any tarmac driving I might want to do. The F series opens up lots of wierd and wonderful CHEAP ratios like this puppy

http://cgi.ebay.com/ebaymotors/TOYOT...Q5fAccessories

5.71 would get me out of first VERY quickly and ill be in the middle of second for the sweet spot in tight corners, with a small drop in revs when I shift to 3rd.

It will also cruise at 5000rpm on the highway So slightly insane but for racing no faster than 120kmh it could be a goer

[gearb0x]

Also from what ive seen 4.778 are around 600 new just for the R&P, and running around 350-400 second hand + fitting costs. So as long as i do most of the YR22 conversion my self It wont be tooooooooo much more expencive (I will have to farm out the tailshaft)

[AL510]

with out taking a change in speed or wheel spin into account because of a different gear ratio the revs you will achieve will be a simple %. if you decrease your gearing by 20% you will increase your revs by 20%.If a particular corner is 4000 rpm with a reduction of 20% you will be at 4800 rpm.4.1 is 87% of 4.7 and so on. wouldn't it be that simple?

Cheers, Al.