Diff advice and moral support.

[JohnK]

One more thing that really needs to be fixed :-(

I tried to ignore this as I was wrestling with other things that seemed a lot more important but it's beginning to be as conspicuous as a sore thumb. My "kit" came with a really tall rear end (11:39, or 1:3.5455). Because my primary goal in putting the "kit" together myself was to see what I could come up with in the handling department, I ignored this, but . . .

Someone observed that Sevens spend most their lives between 60 and 80 mph; I've got just lots of HP without having to do anything other than wire up the stock ECU correctly; while I expect to spend time on the street, I really hope to do track days and possibly some SCCA Solo competition; my Racing by the Numbers software makes it really apparent that running tall gearing is awful if you want the car to accelerate 'smartly'; the Honda S2000 drive train comes with a 4.10 dif; . . . Have I missed anything? Is there any conclusion to come to than "It's really stupid to run such a tall rear end! (never mind the question, "How come you were delivered a "kit" with such wrong gearing?") ?

 

As a feeble defense I'll offer that, had I not been doing things like redesigning and figuring out how to fabricate the front suspension so the links wouldn't tear out the R&P when I both turned the steering wheel and an inside wheel happened to hit a bump at the same time, I certainly would have addressed the problem earlier.

 

So, how to proceed? I'll offer that the best approach seems to be to buy a Subaru 4.11 gearset, do the gruntwork of getting the shimming right, and bear the hazard of having to do the shimming over if I don't get it right the first time.

 

I'll support this approach with the following.

 

I know of no way of telling exactly which diff I have, and don't what to find out that there's something subtle that interferes with something else by having the rear end self destruct once assembled, trashing all the special fitting of the one-off pieces. Thus, only replacing the gears, leaving everything else the same, seems like the safest course. (Anyone know how to decode the numbers stamped on the diff - and do they signify anything relevant to this issue?)

 

Another complicating factor is the complexity of the rear suspension. By some great stroke of luck the "kit" came with inboard brakes, which is great in terms of handling and very cool in visual appeal, and it SEEMS in all appearances to actually be correct (he says crossing his fingers - no noise when run up on blocks and especially when the brakes are applied). But this means that the axles, with their welded-on brake rotor flanges on the inner CV joint, have to match the diff - perfectly (one-off machining job ensures that the calipers and rotors are in alignment). I expect that the parts have worn in together, but at this point I wouldn't bet on anything. If I get a used diff and there's any difference in width, some very expensive machining may have to get tossed - not to mention gremlins lurking elsewhere.

 

So new gears seem the safest.

Pictures of the assembly provided for the terminally curious.

 

Suggestion, comments, watchouts, . . . polite laughter?

[rzempel]

Isn't it the final ratio of diff plus transmission that's important...not just the diff gearing? What's your gearbox--are its gears low enough to account for the high gearing of the diff?

[Dave W]

Also the rolling radius of the tires should be taken into account. I would put the trans in high gear, and turn the engine over by hand [5 or 10 rev] and count the tire revs. I would bet that you do not have a the typical over-drive top gear but a 1 to 1 ratio.

Dave W

[Kitcat]

Ditto what above posters say.

 

And, isn't it only relevant in 1st and top gear? If you mostly auto-X, a tall 1st gear can be a real asset as you may never have to shift out of first. When I was running auto-x's here in Cincy, the courses were very tight and the dedicated auto-x'ers wld set their gearing up to stay in 1st (top speed 55-60 mph).

 

If you drive a lot on the highway, (to and from track days?) then a tall top gear makes everything better (less noise, better mileage). At the track you wont need 1st except to launch wh/is irrelevant at a track day given the rolling starts, and you will never be in top gear except maybe a place like VIR with a mammoth straight, but probably not even there.

 

Mike M.

[rnr]

One suggestion not related to you original question but to Ultralite Diffs in general is that you should definitely consider adding additional bracing to protect the diff from torque. It looks like you are running the stock 3 point configuration that at least 4 people have snapped (including me). I'd also recommend strengthening the front attachement point inside the transmission tunnel.

[m wirth]

RNR, would you have some pics to post of the design you arrived at for extra bracing? thanks

mike

[m wirth]

JohnK, here is a nifty calculator I have had since the '60s, I'm sure there are others available out there still. Gives you a way quickly to compare tire diameters vs gear ratio, there may be new ones on line what with all these fancy computers and all.

mike

[Mondo]

Here's the brace I have on my diff. I like your brake rotors... mine are more like motorcycle rotors. You'll definately need more than the brake bias bars to adjust the brakes.

Edited February 5, 2011 by Mondo

[WestTexasS2K]

You can swap the diff out for any R160 Subaru diff. 3.54 3.90 4.11 4.44 are the most common. Not all diffs have the LSD units. Usually if the diff has a gold cadium plating on the bolts it is a lsd unit. For most tracks I prefer the 4.44 diff. it gives you great acceleration out of the corners and is great for 99% of the tracks. The car will cruise 75 mph at about 4400 rpm on the street with this arrangement. It is fine for shorter drives a few hundered miles no big deal. Do 5k miles in a week and the engine noise at that rpm can become painful. With the naturally aspirated motor I highly recommend the 4.44. The Honda engine doesnt have alot of low end torque so the 4.44 really helps that out. You just start off in second gear most of the time and can do 60 before you have to shift. You can still spin the tires in 3rd gear when Vtec kicks in.

 

I recommend the diff brace as well. The diff wants to twist under hard acceleration. It can and will tear the front diff support out eventually. When you take the diff out where the front bolt mount is I take and weld and 3" by 6"x 1/8 steel plate on top to reinforce that area and then use large washers on both side to spread the load out. Be sure to check and make sure you have clearance for the rear u joint.

 

It is important and Mondo needs to do this as well. Where that rear brace is attached in the picture supplied by Mondo those tabs on the chassis are extended off the chassis and can flex and break over time. You need to put a long bolt in the top mount where upper control arm is attached and then sleeve spacer that fills the gap in between so that the brace is tied into upper brace. This locks everything together and makes it very stable. I have a few diffs at the house I will see what gear ratios I have if your interested in another diff. It is usually cheaper to by another rear end than to buy a gear set and rebuilding it.

 

I also recommend to replace the rear bushings with a ABS plastic bushings or Urethane material like used in aftermarket suspension bushings. I bought some ABS rod material from McMaster car and machined it down to replace the rubber bushings that come with the kit.

[JohnK]

Bingo! This is the kind of stuff I've needed to see and hear. Thanks all, especially WestTexasS2K's descriptions and Mondo for the picture of the brace, ... and m wirth - your calculator looks like it'd be much easier than having to boot Windoze! See end for notes on gear ratios.

 

WestTexasS2K, when you say "You can swap the diff out for any R160 Subaru diff." is "the diff" MY diff? - as per my concern about my brake setup with all its special bits. I'm a bit weighed down at this point by all the info you've provided, but will consider swapping out the entire diff (changing just the gears SOUNDS a lot safer but I have not done such before). I would like to know if you do have a 4.11 unit available and what you'd sell it for. However, with shipping from TX it may make more sense to source it locally ?

 

Diff mounting. I saw earlier where people had torn out the forward boss - and welded a thick bead around both sides of mine in order to increase the support. As per the attached pic, I do not have a chassis that I can attach a brace to - so I guess it's back to the dwg board. Ideas/suggestions appreciated here. I can see where twist would be a problem, and will change the rear mount material to Urethane.

 

Mondo, et al. - Brakes were the first thing I tackled. Wilwood told me that the steel rotors were for drag race apps only - single hard usage application followed by long cooling off, otherwise they distort over time into a bell shape. Coleman machine has a pretty wide selection of cast iron rotors that they're set up to machine to spec. I spec'd out a set of Wilwood calipers that'd allow me to run Diamond 13" wheels, figured out the caliper piston sizes I needed along with master cyl piston sizes front and rear to give appropriate balance along with force, and figured out the widths and diameters to fit caliper to rotor. We'll see how well balanced it is when it finally gets put into service.

 

 

What follows is a set of calculations showing the difference in possible acceleration with my current setup and then switching to a 4.11 diff and Hoosier 13" tires. Assuming that more acceleration is better, ... :-)

Also, for doing Autocross, WestTexasS2K's observation that a diff ratio as low as 4.44 still allows 2nd gear to go from start to 60 mph makes this a killer combination for this type of sport, and emphasizes how much I need to get rid of my tall ratio diff.

 

 

Tran. Gear Ratio 1.21

Ring & Pinion Ratio 3.5455

Final Gear Ratio 4.28

 

Chassis Total Weight (lb) 1725

Drive Line Efficiency 85.0%

Engine Torque (lb) 168.0

Engine Torque Per Wheel (lb) 84

LR

LR Tire Loaded Radius (in) 12.844

LR Acceleration Force (lb) 286

Possible LR Acceleration (g) 0.166

RR

RR Tire Loaded Radius (in) 12.84

RR Acceleration Force (lb) 286

Possible RR Acceleration (g) 0.166

 

Possible Total Acceleration (g) 0.331 ***

 

-----------------------------------

Tran. Gear Ratio 1.21

Ring & Pinion Ratio 4.11

Final Gear Ratio 4.96

 

Chassis Total Weight (lb) 1725

Drive Line Efficiency 85.0%

Engine Torque (lb) 168.0

Engine Torque Per Wheel (lb) 84

LR

LR Tire Loaded Radius (in) 10.85

LR Acceleration Force (lb) 392

Possible LR Acceleration (g) 0.227

RR

RR Tire Loaded Radius (in) 10.85

RR Acceleration Force (lb) 392

Possible RR Acceleration (g) 0.227

 

Possible Total Acceleration (g) 0.455 ***

[RDROCKT]

John,

 

I do not want to confuse or dispute anyone by relating my experience.

 

If you are going to get serious about autocrossing, the conventional wisdom today is to run 13" bias ply slicks on aluminum wheels. This will be consistent with the inboard brakes devised to reduce the unsprung weight in the rear of the car. When considering the needs for this purpose, I have found that the 4.11 LSD rear axle ratio is pretty much optimal. Launches in 2nd gear are all the tires will stand for grip and a short shift to 3rd gives you a top speed somewhere near 70mph. On all but the tightest courses, you will be able to avoid time consuming gear changes.

 

This same set up has given me 141 mph on the front straight at Gateway Internationals' road course still pulling in 6th gear.

I have no idea what the street use rpm will be but I am sure you can calculate it for any speed range.

 

My car weighs 1432# driver, fuel, ballast and all.

[WestTexasS2K]

I agree the 4.11 is best choice if you are running 13 wheels I didn't see that in the initial discussion. The final drive ratio would be about the same as with 4.44 with 25 in tall tire. This would also give you decent rpm in street mode with 17" tires.

 

I know Wilwood doesn't recommend the steel rotors for road racing, but I can honestly say I have run thousands of miles on the track without a hint of brake fade or warped rotor one. They are lighter than a large cast iron rotor. I don't hesitate to use the steel rotors.

 

Is your 1700 car weight with you in it? That seems to be on the heavy side.

[JohnK]

Chuck and WestTexasS2k,

Thanks very much for this info. It means a great deal to me because you both have a lot of practical experience and have been able to assess the results of changes made to the car.

I don't anticipate doing autocrossing because I don't think my reflexes are up to a competition that is that intense, but we'll see. I brought that subject up to reinforce the point of what I felt was appropriate gearing, since there were views that running a tall ratio was not a big deal. Chuck, your experience for a setup with 4.11 and 13" wheels is what I was hoping for and then some. I didn't expect that it would still have reach at 141 MPH.

 

The weight I got for my car is something I'm not comfortable with, even though I spent quite a bit of time trying different setups and sequences. And, yes it's with my weight in the driver's seat, a half tank of fuel, and all fluids. I used a lever-divider setup with bathroom scales and could not come up with very consistent readings - I was looking for +/- maybe 10 lbs and got more like 25. The problem I think is due to suspension loading of the corner placement of the scales - even though I was using solid struts in place of shocks, (home-made) slip plates, varying the scales looking for bias, trying different ways of placing the car on the scales, ... I'm tempted to spend the money with Longacre on proper scales but really think the problem is with my procedure. Zakiras Garage is down the street - maybe I'll see if they're up to doing that for a customer.

 

WestTexasS2k, It's good to hear that your experience with steel rotors indicates no problem. Wilwood was really adamant in their advice - maybe they just doin't know about Sevens :-) Perhaps the cast iron will result in a more solid feel - but at least they'll allow me to fit 13" wheels (after I get the short sump and the Accusump fitted).

 

I've done a bunch of things to the car that SHOULD make differences in specific areas. When I finally get everything dialed in (like shocks, ARB rates, brake bias, ...) I'll be looking for people experienced with the base car or at least a Seven to spend some time at the wheel to get some feedback - AFTER I spend some time enjoying the thing after all this work :-)

 

 

Thanks again very much for giving me (and other Listers who may be just as interested) the benefit of your experience.

[S1Steve]

Hey John,

 

Put me on the sign up list for the field testing and feedback. I don't know how much info

 

I can give , but it will be fun. 6 years was a long time to wait..

[Kitcat]

John:

 

Performance Alignment in Blue Ash has scales and can also align the car with you in it, corner weight it, etc. All the Cincy racers get their cars done there. They are 20' from you.

 

Mike M.

[JohnK]

Hey John,

 

Put me on the sign up list for the field testing and feedback. I don't know how much info

 

I can give , but it will be fun. 6 years was a long time to wait..

 

You're on the list! It'd be great to have you try out my work on a real live car rather than just a bunch of numbers loaded on a chip. Spring is coming, stay tuned and wish me luck!

[powderbrake]

John:

 

A couple notes regarding the Ultralite.

 

#1) West Texas s2k and rdrockt are the Ultralite experts, and you can take what they say to the bank.

 

2) My Ultralite comes in at about 1420 pounds, less driver, so we are very close on weights. I don't worry about the weight, it is what it is, and the S2000 engine will rapidly get me to my level of incompetance.

I corner weighted it at Run & Gun a couple years ago, and was able to adjust the spring seats to get it very evenly balanced.

 

3) I don't have the inboard brakes, I have the 12" Wilwoods all around, and I had to shift the bias bar on the master cylinders all the way over to lighten the rear braking. On the track that still wasn't enough, so I added a Wilwood Brake Bias Valve right under the dash where I can reach it while driving. That did the trick to keep the rears from locking up. I think you probably have smaller rotors in the rear.

 

4) A few track days, and mostly street driving have not created any problems with the differential mounting sysytem on my car. Of course, I haven't looked in the last couple years, because it hasn't needed any maintenance.

 

5) I have the 4:44 diff, and it's a hoot when you get on it. Driving at 70mph,

it runs somewhere near 4000 rpm. Like Loren said, its fine for those typical 150 to 250 mile blats that I take. The wind noise is still louder than the car noise, so the RPM really isn't a factor.

 

6) I didn't check any of you numbers, just a note that the transmission output is at a higher RPM than the engine input in 5th and 6th gears, they are overdrive gears.

 

I have lots of pics from my build so if I can be any help , just ask.

[WestTexasS2K]

I second Jerrys advice on the bias valve. You can get it close enough with the balance bar for the street, but on the track where are braking harder and tires are hotter you will lock up the back prematurely. I also installed bias adjuster in the cockpit to make it much easier to adjust on the fly. I also went from the 1.75 to the 1.38 caliper to get the balance bar more centered.

[Mondo]

Sometimes I wish I could just load my car up and drop it off with Loren for a week or two and let him make all the little modifications & adjustments he's learned over the years.

[WestTexasS2K]

Hey John I found this chart on the net aprox 8 years ago. I dont know how accurate it is. It helped find a few diffs and it was correct on the cars I looked at. It might help you find something local if you want to go that route.