Diablo gear ratio

I suppose these ratios came out from SE30 Jota development program, so most likely they were available in 1995 already, but as an option for those who know.

 

An update.
Here's a nice video to look at.

[ame]http://www.youtube.com/watch?v=fPk5HTljbEc[/ame]

So the power flow is:

[main shaft]
↓
[secondary shaft]
↓
[rear transfer ratio]
↓
[rear output shaft or viscous coupling]→[front transfer ratio]→[front output shaft]→[front diff]
↓
[rear diff]

Rear transfer ratio
is a ratio of a gear machined into secondary shaft vs. output shaft gear (machined within on 2wd) or viscous coupling gear (separate piece)

Front transfer ratio
is a ratio of 2 separate gears, one driven off the front of the viscous coupling and another off the end of the front output shaft (sits below the main shaft)



That being said... the ratio of front and rear VT diffs are different:
2.81:1 front
2.41:1 rear

2.31×1.59=3.67 - 1st gear ratio on the viscous coupling
3.67×0.81×2.81=8.35:1 - final ratio of the 1st gear on the front wheels
3.67×2.41=8.84:1 - final ratio of the 1st gear on the rear wheels


Of course, the diameter and circumference of the front 235/40 ZR17 and rear 335/35 ZR17 tires are different.
Front/rear diameter 620 vs 666 mm (hehe)
Front/rear circumference 1947 vs 2093 mm
All make up for 7.5% difference.

*drums*

8.35×107.5%=8.97 (front final 1st gear ratio corrected to tire difference)
8.84≠8.97

So that makes the front underdriven, i.e. it runs slower than the rear.


Is that all??? Not yet!

The standard VT was equipped with 235/40 ZR17 tires instead of 245/40 ZR17 tires as fitted on standard 2WD Diablo.
245/40 ZR17 to 335/35 ZR17 difference is 6%
8.35×106%=8.85
8.84≈8.85
So, I guess, originally it was designed to have 245/40 ZR17 tires. Maybe, keeping the viscous coupling loaded made better handling in some turns, but it's worth to check on your own.

Anyone tried 245s?

 

Okay, so I can confirm in order to have a motion correct wheel speeds, the rotational difference of 6% must be compensated by 6% difference in tires.

OE VT specified
235/40 ZR17
335/35 ZR17
offer 7.5% difference

2WD specified tires
245/40 ZR17
335/35 ZR17
offer the correct 6% difference

If still in doubt, check out the 18 inch wheels tire specs
235/35 ZR18 - 622 mm
335/30 ZR18 - 658 mm
offer a 6% difference once again.

So I've forwarded some questions to Italy, Valentino Balboni replied:

However, he haven't seen the calculations I've done by the time he answered...
The VC failures make no sense, but arm travel - maybe, because 1st setting on Koni selector is quite soft. But I don't know if 2WD has any rubbing with 245/40ZR17...
I wanted to ask Luigi Marmirolli, the question is still in the way.

 

After some studies I came up with a better gear ratio solutions for the Diablo transmission.
So far I think further parts will need to be made:
- primary shaft
- secondary shaft
- 1st gear on the secondary shaft with synchronizer rings
- output shaft gear (either 2wd output shaft or ring gear on the Viscous Coupling)

This is not a big number of parts that I thought it would be.
My calculations proved me wrong, that altering 3rd or 4th gear will make the car worse. A 2nd gear ratio can be slightly altered to achieve a perfect feel and at the same time can be left alone as the difference is minor (5%).
The key benefit is that the car will have much faster acceleration in part throttle and a less loaded engine. The top speed should drop, but still over 300 km/h, if you have a standard 5th gear.
The kit is calculated for the 2.41 rear end.

I wonder if there are brave ones who'd be also interested in a quicker transmission for their Diablo, in case, whether I should make few

The key benefit is that the kit is calculated for the 2.41 rear end. If you have a 2.53 rear end, you will have even lower top speed, likely 280 km/h or so. We assume that the rev limiter is on 7300 RPM (later cars had it higher).

My point is to make an early VT agile without altering the looks anyhow. I think there's fair enough power and making bigger power will not solve the driving issues of the overloaded engine and clutch due to the very long 1st gear ratio.

The proposed kit will contain only transmission parts, one won't need to change ring & pinion, adjust bearing loads and other stuff, which makes installation harder. Actual manufacturing of ring&pinion gears is quite easy. The speedometer also works from the output shaft, so the speedo reading would not be altered as the ratio is changed before the output shaft (rear transfer ratio).


p.s. Don't be sorry about the low top speed, it's better to achieve 300 km/h fast than wait few minutes to get to 320 km/h.