When You Realize You Need Telemetry; Gearing Decisions

Our Datsun 1200 has had an ongoing conundrum for years.

The 56 series Datsun close ratio gearbox works really well but it's fragile.  The 60 series gearbox is bulletproof but there is a 2700 rpm drop between 2nd and 3rd gear.

The rpm drop may not sound like a lot but it represents a 19hp drop. 

Using 2nd gear coming off of the slow corners works for ever so briefly; I end up having to upshift just as the car drifts out to the exit curb. The huge rpm drop also means the shifts are slow; you can shift the car faster than the rpm drops.

My current solution has been to work hard to roll the car through the corners in 3rd gear.

So in an effort to get a better drive of corners I've decided to to from a 4.38 final drive to a.5.13. This makes it possible to use 3rd gear in the slowest corners.

On the surface this is an easy decision but looking at my spreadsheets it appears that I need more information. Using 2nd gear in places puts the motor more in it's powerband but it means 7 extra shifts per lap. There is also sections where going down to 2nd may not any lap time improvement.

My current telemetry consists of the extremely limited information I get from Track Addict which is not exactly state of the art.

All of this has made me realize I need to get some real telemetry.

FWIW, what you need is "data".  "telemetry" is data that is radioed back to the pits in real time so that your race engineer can analyze it and tell you that you need to box now and switch to a 3-stopper if you want to beat Verstappen. :)

That said, changing the rear end will not fix a large 2->3 RPM drop unless you change it enough that you never use 2nd any more (and the 3->4 and 4->5 drops are not just more of the same).

Are you going to run out of gear in 5th with that short of a rear end? Any durability concerns with the smaller pinion gear?

Cutting out 7 shifts per lap is a lot.

I know there are fancier solutions, but I've had decent luck using a gopro on a chest mount for "data logging" of sorts - can see all the gauges, my phone with the lap timer app, and what I'm doing and where I am on track even if cinematically it's a crappy angle.

In terms of speed/location data the big difference between what you get from TrackAddict vs. a dedicated data logger is GPS data rate and accuracy from the phone's built-in receiver, and you can close most of that gap with an external 10Hz GPS receiver. I think simulation would help more here.

There are two ways you could do that, either with a data analysis-type simulation tool that can calculate the car's gears and RPMs at different parts of the course, or with a driving sim that lets you test out the different gearboxes virtually. I found a Datsun 1200 for Assetto Corsa to start with here, it clearly has well beyond stock power already so it would need to be modified (probably as in brought closer to stock) to match your real one:

https://www.assettoworld.com/car/datsun-1200

Then you should find a track you can test on IRL.

Once that's done you can add an alternate gearing set and test the two back to back. I do this kind of testing with my 86:

https://viewsync.net/watch?v=QiU9Gh6NF-U&t=0&v=eYFJk0CS4jE&t=3.5&mode=solo

codrus (Forum Supporter) said:

FWIW, what you need is "data".  "telemetry" is data that is radioed back to the pits in real time so that your race engineer can analyze it and tell you that you need to box now and switch to a 3-stopper if you want to beat Verstappen. :)

That said, changing the rear end will not fix a large 2->3 RPM drop unless you change it enough that you never use 2nd any more (and the 3->4 and 4->5 drops are not just more of the same).

 

The 5.13 gear will rid the need for 2nd gear.

The 3-4 shift is a 1900 rpm drop and the 4-5 is 1300 rpm. Those shifts represent 10hp or less drop.

Top speed in 4th is 99 mph and 5th is 121 mph. 5th gear is an overdrive but it would only be used on one track layout and only for about 200yds.

So, I take it there are no other gear ratios available for the transmichigan?

I don't know what a 56 or 60 gearbox are, but could you use one from a Z car for instance, or out of a Datsun/Nissan pickup truck?