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Bugeye Diffy Marking

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Hap Waldrop said:
Hopefully Ray MCcaleb is reading this, if I'm not mistaken he plans on running a 3.7 with the Rivergate 5spd, that sounded like too tall of gearing with the 5 speed, but I have no experience with the 5s pd., maybe you guys can shed some light on this for Ray.
Click to expand...

I've done a little research on this before making the decision and put together a spreadsheet that helped in the decision making process. I offered it up once here on the list...unfortunately the discussion evolved into hypotheticals (environmental variables, etc.). Never the less, leaving wind resistance, humidity, and planet alignment OUT of the analysis and using the strict givens, engine rpm, transmission ratio, diff ratio, and tire size, the chart tells me that with a Rivergate 5-speed (Datsun trans.) and a 3.7 rear end, the final drive ratio (which can be inferred from the relative MPH column) will be slightly less in first, second, and third gears, equal in fourth (1/1), and greater in fifth than with the stock transmission (4 speed) and the 3.7 or 3.9 diff. The stock 4-speed and the 4.22 diff would yield a lower speed (higher ratio) in all four gears. Keep in mind, however, that production street cars with the 4.22 were only produced with the smaller displacement engines installed, not the 1275.

Fact is, I have one each of the common differentials (3.7, 3.9, and 4.22) and will experiment with each until I find the one that best suits my driving needs.

Here's the chart...pretty small to fit here, but you can save it off and open it with a viewer that can magnify.

Ray
 

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Two things Ray... You will probably find that the rolling circumference of that tire is small than what you list, and that tire might be a little tall for the car. I made a similar spreadsheet using 165/70R13, but when I put those tires on my car I found the rolling circumference significantly less than what I had calculated.
 
Great detailed info on the gearing topic here.

Gearing and driving style, that is the question!

To me, I'm a pure pleasure driver in my LBCs. No racing or hole shots, occaisional spirited run through the gears. However, the biggie to me is how the car runs at highway speeds and in traffic. NOT on Interstate highways, which I avoid like the plague.

My benchmark test for gearing is what RPMs am I pulling at about 60 MPH (normally my crusing speed on highway trips). I like no more than about 3,500 RPM at 60, and prefer about 3,000 RPM at 60. Anything above that for longer trips is too buzzy for my tastes.

Actually, the stock (4.22) gearing with the Datsun 5 speed box looks pretty good (e.g., 3000 at 60). But I would not like the 4.22 without the overdrive (5th) gear. Mighty buzzy.

Very interesting . . .
 
That spreadsheet is great, really gives a good idea what the relative differences are. The comment about rolling circumference is likely correct, with tire compression due to weight it is likely a few inches less. Nevertheless, the relative differences will hold true.

Seems that since I have a 4.22, that looks to be in good shape, I will start there.

Thanks all for the input. :thumbsup:
 
Trevor Jessie said:
Two things Ray... You will probably find that the rolling circumference of that tire is small than what you list, and that tire might be a little tall for the car. I made a similar spreadsheet using 165/70R13, but when I put those tires on my car I found the rolling circumference significantly less than what I had calculated.
Click to expand...

Thanks for the info, Trevor. It's been a while since I put the spreadsheet together, but as I recall, the tire data came from an online source...I believe the Kumho tech site.

A few minutes ago I went upstairs to where I have the new wheel/tires stored and measured the circumference of one. The sheet lists 72.89" and the actual measurement, as close as I could determine was about 72 3/4". Close enough for me. Now will that vary a bit due to tire pressure, temperature, rotational speed...I suppose, but you've got to start somewhere.

The tire is tall but fits the way I wanted. It is also wide and will require that I roll the flat on the (square) wheel arch slightly. The entire fender will not have to be bowed out, just the flange on the arch bent from the stock 90 degrees up to 180. The gain of 3/8" should be all I need.

Here's a photo of the flange and clearance required (about 1/8"); there are more on my site of the wheel in the wheelwell here:

https://www.raysmg.com/rays_mg_midget_62_sprite_bodywork3.htm
https://www.raysmg.com/rays_mg_midget_62_sprite_bodywork6.htm

Ray

P1010016.jpg


Ray
 
I'd like to add a couple thoughts. Seems that your chart is running a little fast for the rpms on the 3.9 chart. Mine is not quite 80 at 4,000. What I found with a strong 1275 with this setup was that the motor needed 4,000 rpm to pull hills on the highway. With the SC it isn't a problem. I've thought of a 3.7, but have been happy enough with the 3.9 and just haven't done it.
 
I think you will find that wind resistance (and all the other parasitic drag from the drive train) are major contributors to reduction of maximum speed. Result is that more engine torque is necessary to obtain the theoretical top speed than the engine can give. Bottom line, lower top speed and necessity for a lower diff ratio.
 
I know at Dayotna in a Spridget 1275 race car, a 3.7 would is nice, atleast you won't be sitting there at 8000+ rpms for what seems like forever with the 3.9 :smile: Caculations are nice but Ray has all the diff ratios if he needs to change, sometime charts lie. I know someoen who did a 5 speed tranny in a MGB that had a overdrive 5th .069, he ended up upping the gear to get it in the right RPM range it was way too low with .069 and then3.9 rear, of course tire diameter comes into play, and Ray will be running like 3+" than we run on the race cars which will make the car taller cruising> One things for sure though we'll all know when he gets the car going :smile: I looking at a rather unique 5spd application for my B, and I want to be able to cruise at about 3200 rpm at 80 mph, anyhting lower would chug the engine.
 
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