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Login to post Forums » Grassroots Motorsports » compact IRS rear differentials w/ low numerical ratios

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Dec. 31, 2011 11:00 p.m. JamesMcD Reader

I am trying to design a small car to utilize a Honda Goldwing engine and transmission. The primary drive (in the transmission case) reduction on these is 1:1.59 according to my research. So I need to use a rear differential with a low numerical ratio to make the whole combination work. I am trying to avoid going over 1:5 for the combined final drive ratio. The hang up is that this is a small car, so I want to use a smallish and light-weight diff., but smallish cars tend to have high-ish numerical diff ratios. (What is the tallest gear set you can put into a Miata 3rd member?)

Does anyone have any suggestions?

-James
Dec. 31, 2011 11:44 p.m. Chas_H New Reader

Miatas come with a 3.63 final drive in most places other than N.America. Does the Goldwing trans have reverse?
Jan. 1, 2012 12:06 a.m. Streetwiseguy Dork

Find yourself a 7.5 out of an early 80's Cutlass with a 2.29, and I think thats about as tall as you will ever find. Low 3's should be fairly simple in lots of American stuff.
Jan. 1, 2012 1:14 a.m. Will Dork

8.8 Ford can take a 2.73.
Jan. 1, 2012 1:50 a.m. rotard HalfDork

Chas_H wrote:

Miatas come with a 3.63 final drive in most places other than N.America. Does the Goldwing trans have reverse?

Yes.
Jan. 1, 2012 7:43 a.m. bigdaddylee82 Reader
Copy and pasted from my "Delusional GRM 201X Drivetrain" notes:
- Ford 7.5" 2.73:1 3.08:1 (Track Lock LSD avalible)
- BMW E30 325e/es 2.79:1 2.93:1 (LSD option on e, LSD on es)
- Hummer H1 2.56:1 2.73:1 3.08:1 (Torsen LSD or Locker)
I don't think the Mazda and Subaru offerings are "compact" enough for my idea, they both have a pretty long pinion.
- Lee
Jan. 1, 2012 7:49 a.m. DeadSkunk Dork

Chas_H wrote:

Miatas come with a 3.63 final drive in most places other than N.America. Does the Goldwing trans have reverse?

I'd love to have a 3.63 ratio in my Miata for highway cruising.
Jan. 1, 2012 8:25 a.m. Knurled Dork

bigdaddylee82 wrote: I don't think the Mazda and Subaru offerings are "compact" enough for my idea, they both have a pretty long pinion.

The Mazda diff as used in the Miata is actually pretty short as far as flange to axle centerline is concerned.

I have "similar" ideas that require a differential that has as short a pinion as possible. Everything I've measured so far is quite a bit longer. As you shorten the pinion, you make the pinion bearings closer together, which isn't good for gear mesh reasons.

Bear in mind that Mazda has made several different lengths of pinion. IIRC the Miata uses the shortest one. Fortunately it's also the most common. (The FC RX-7 probably has the longest)

If you're real, real lucky, you can find a 3.31 from an early 80's Mazda Diesel pickup. http://www.solomiata.com/RingandPinion.html
Jan. 1, 2012 8:30 a.m. Curmudgeon SuperDork

Don't forget you can also tune the overall final drive ratio with tire height. Shorter tire = more RPM, taller tire = less RPM.

Let me suggest this: find what engine RPM you want to run at cruise at, say, 70 MPH, plug a commonly available rear axle ratio into that, then see how many tire RPM/mile are needed for the ratio you want to achieve and see if there's a reasonable tire height that will help you accomplish that.

Also, what is the final drive ratio on the 'Wing's rear R&P? A quick Google says somewhere around 3:1-1.

EDIT: Come to think of it, gas engine forklifts with real short tires will often have oddball gear ratios. Might be worth going to a forklift graveyard and counting revolutions on older Datsun/Toyota stuff. A lot of that type thing will be derivatives of passenger car stuff.
Jan. 1, 2012 8:34 a.m. RossD SuperDork

There was talk that an awd escape has a 1:1 ratio rear axle, but I neither was able to confirm or deny this talk.
Jan. 1, 2012 8:42 a.m. Curmudgeon SuperDork

This page says 2:93-1. Not mine, can't confirm/deny, may warp your spine. http://www.vibratesoftware.com/html_help/2011/Ford/Ford_Escape_2001-2012.htm
Jan. 1, 2012 10:39 a.m. erohslc HalfDork

I wonder what kind of ratio those agricultural/industrial vehicles use (Cushman, Deere, etc.)? How about Quads and such?
Jan. 1, 2012 10:45 a.m. dean1484 SuperDork

Can you change the gearing in the Honda motor?
Jan. 1, 2012 10:56 a.m. FlightService Dork

In reply to JamesMcD:

Why a goldwing engine and transmission in a car? I am making the assumption you want it for the reverse.

The reverse is electric and is run off the starter motor. so....

Also I do not know of a re-ratio kit that is available for the Goldwing, it was made for the American touring market, it is built in the US. Some of Honda's ATVs have different gear kits based on the market they are destined fors regulation for off highway vehicles.

If this hasn't moved you in a different direction. The rear diffs I can think of is

Subaru's but I believe they are around 4 +/- .25 region

Miata

Plethora of ATVs and Side by Sides.

all of these are high numerically.

How tall of a tire are you looking at? You can compensate alot there.
Jan. 1, 2012 1:29 p.m. Keith SuperDork

Knurled wrote: .

If you're real, real lucky, you can find a 3.31 from an early 80's Mazda Diesel pickup. http://www.solomiata.com/RingandPinion.html

You won't, that r&p is fiction. Every time someone finds one, they discover the diameter of the ring gear is wrong. Bad info, but persistent info.
Jan. 1, 2012 1:36 p.m. cwh SuperDork

How about the R160 Nissan unit? I believe that is in the Subaru as well, but you would have to go to aftermarket where I'm sure you will find proper ratios.
Jan. 1, 2012 2:24 p.m. erohslc HalfDork

The Subie's are all long snout. AFAIK, lowest ratio you can find is around 3.55
Jan. 1, 2012 3:47 p.m. Curmudgeon SuperDork

Another angle: Gear Vendors has a line of overdrive units, maybe one is suitable. They are not exactly grassroots priced, though.

http://www.gearvendors.com/index.html

Maybe you could check with a local power transmission distributor (for conveyor belts etc) and see if they have something which could be adapted. Or even build your own using that Morse chain and sprockets like the ones used in 4x4 transfer cases.

I have never seen the innards of a Gold Wing so sorry if this is a dumb question: is it maybe possible to change the primary drive ratio by replacing the gears with something custom, or even with gears from another year model?
Jan. 1, 2012 3:52 p.m. bearmtnmartin New Reader

Triumph Herald or Spitfire. Very compact IRS and came in ratios at least down to 3:27. Won't handle huge torque but would be really good behind a bike engine.
Jan. 1, 2012 3:59 p.m. Curmudgeon SuperDork

The 3:27-1 Spit type diff was the one used in the 1969/1970 GT6+, other GT6's used a 3:89-1. Spitfires from 62-74 had 4:10-1, all the 1500 cars used a 3:89-1.
Jan. 1, 2012 4:13 p.m. fasted58 SuperDork

Paging Kevlar... I bet he'd have some ideas w/ his MC transplant madness/ knowledge
Jan. 1, 2012 5:18 p.m. erohslc HalfDork

To change ratio, could use a set of quick change gears, relatively cheap (~ $60) Would need to fab a housing, as well as shafts with splines matching the gears. Would also reverse the rotation (unless you used 3 gears), but maybe that would work out better, depending on relative location of GW engine/trans and diff.

HD Morse chain looks like the winner here; used in lot's of vehicles, proven, cheap (used), compact, robust, etc.
Jan. 1, 2012 5:33 p.m. Curmudgeon SuperDork

The Morse chain or quick change gear setup could also be mounted on the back of the engine asy., saving some packaging aggravation. Hell, grab an old Jeep transfer case (or even one from, say, a Suzuki Sidekick or Samurai) and you now have a lot of the goodies you need, you'd just need to run the stuff reversed so the input would turn slower than the output (make it a step-up box). You would need to fab a housing but that's not insurmountable.

Another possibility is a Gilmer belt like the Harley guys use for primary drives. Not 100% sure that would withstand the pounding it would likely get in your app, but it would certainly save the aggravation of building an oil tight housing for gears, chain, etc.
Jan. 1, 2012 10:55 p.m. JamesMcD Reader

Thanks for all the replies.

I'm not married to the idea of using the Goldwing motor. The Goldwing motor was mostly driven by packaging constraints and the fact that they have an electric reverse which I hope would be good enough to get me out of a parking spot. The trans on them is also nicely tucked up against the back side of the motor.

Here are the Goldwing transmission ratios:

(1) 2.375 (38/16)

(2) 1.454 (32/22)

(3) 1.068 (31/29)

(4) 0.843 (27/32)

(5) 0.686 (24/35)

Primary Reduction is 1.591 (78/49)

As you can see the gears in the trans are rather tall. I figure I can live with a combined final drive ratio of 1:5.25 - 1:5.5 (gives 70mph @ 3750-4000 rpms), so:

5.5 / 1.591 = approx. what my rear end ratio needs to be.

I have designed the car around NA Miata wheels and tires. I can't really go taller on the tires to help my situation due to packaging issues. The car is berkeleying small, think 110% of a Berkeley, with a roof. (75in. wheelbase.) It's like a GT micro-car; an answer to a question nobody has asked.

I am open to hearing of other small (short from front to back) drive trains that don't involve chains. It's going in the front, and RWD car transmissions kinda end up occupying space I need for my feet, so that's another constraint that led to the Goldwing.

It may be that there is no good solution and that my car has to get bigger.
Jan. 1, 2012 11:01 p.m. carguy123 SuperDork

Is the tranny separate from the Goldwing motor ala Harleys?

As I understand it the Goldwing output shaft turns the right way to use a car diff so that's a plus.

Another plus is that if you look at dynos you see the Goldwing engine is almost identical to a Miata engine.

It's light. It's small.

As I understand it most if not all larger motorcycles have a first gear that will take you to at least 60 mph. That's the problem with using MC engines in a car. But yours seems small and light enough so that shouldn't be a clutch issue.

Do they make a heavier clutch for the Goldwing?

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