That'll be handy, he RX8 hub would make it a lot easier.

I'm not really interested in all new arms/linkage to pull this off. I don't think it's necessary, and especially if cost is the barrier, it's a lot easier to just make uprights that are compatible with factory arms.

 

I agree, just throwing it out there because options are so limited.

I’m honestly really surprised they aren’t available. They exist for most of the other popular time attack cars like EVOs etc.

Anyone actually analyzed the camber curves? With all the FDs that have competed in time attack in Japan, it seems like this would be available if it was beneficial. I feel like I’m missing a key bit of info here somehow. Were they readily available in the past?

 

RE Amemiya used to offer a reinforced front knuckle, but I have no idea if geometry differed from stock.

 

I've never heard of any of he Japanese shops doing them. Moonface used to sell lower arms with elongated pivot studs, they were very expensive. I've heard that some racers in California made them years ago, but when I called the machine shop that supposedly made them, the guy said he no longer had the fixture.

 

As far as I can tell, Japanese tuning shops mostly don't mess with major suspension components like arms because they have to be certified be in order to be street legal.
A lot of the adjustable aftermarket arms that are available here actually aren't road legal.
You also have to get your registration papers changed if you change arms, etc. So it's a hassle.

 

I saw several Gs of force especially at the top and they havent cracked yet thankfully. Although hacked is a strong word billy haha i know what you mean.

I built the drop spindles for the front but didn't get a chance to install them. The rears were a test and they made the car handle sooooo much better. i also welded the diff an inch higher so that the axles wouldnt bind with the lower ride height. The car felt incredible with 1-2 inch lower ride height. noticed the difference immediately. not so rolly even with the same roll bars and basic geometry. and much softer dampers that should have allowed more roll. Cant wait to get the fronts on and see how it affects the car.

 

This is exactly (theoretically) the benefit, and what I was looking for in getting them made. You can run a more compliant suspension without roll.

Rob, I'm not sure I've seen your solution, is it replicable?

 

"Anyone actually analyzed the camber curves? "

a great question.

i did a camber curve evaluation in 1999 when i bought my FD. the FD has a true race car camber curve.

the dynamics of a camber curve separate the men from the boys ontrack.

the dynamic is generally referred to as "camber gain on bump." the beauty of the double A arm suspension, versus any strut suspension, is that by varying the effective lengths of the upper and lower control arms (as well as the static angles) you can pull the top of tire in on bump/compression to offset body roll.

having a double A arm suspension, of course, does not mean you have a roadcourse winner. most double A arms have little or no camber gain...

proper camber in a turn is everything.

in a right turn the LF rises into the wheel well. if the upper A arm scribes a tighter arc than the lower A arm the tire will gain negative camber to offset the body roll. most cars are engineered to understeer/push into a corner. the thinking is John Q is better able to deal with the car going straight rather than snapping around. front grip is deliberately reduced. the FD was designed for Drivers, not John Q..

remove the shock, spring, disconnect the bar. set the hub on a jack at ride height. attach a plate to the hub. use two dial indicators. raise the hub in one inch increments and you will generate a camber curve. you will be impressed w the gain per inch. the FD has racecar suspension dynamics... that's why if properly set up and driven it is a killer on a road course.

that's why i own an FD. that's why you love to drive it.

spindle height, of course, has nothing to do, per se, w the camber curve. the key would be maintaining the length and angle of the A arms so as to maintain the perfect FD camber dynamics..

i will say that when Rob started cutting/welding on the aluminum spindle i sort of broke out in a sweat. i would have delivered the pieces to my favorite circle track shop and asked them to fab them for me in real metal. certain items need to be superlight but spindles on a 500++ big tire heavy-ish road racing chassis aren't on my list.

that said Rob was no doubt in a time thrash and it was a reasonable roll of the dice that worked out.

one item that appears to be missing, i haven't seen all of Rob's videos, is a PYROMETER.

simply stated, if you are running a road course you are wandering around if you don't know your tire temps. tire temps will determine tire pressures and camber settings, and both will be different at all four corners of the chassis. you will not only knock seconds (literally) off your lap time but will end up w a much easier car to drive.

finally, Congrats to Rob for being out there and getting it done... and of course sharing w the community.

 

Nice.

 

Thanks man! Yeah i have IR temp sensors on the rear tires but i fully understand the surface is not the same as the material temp. But on tire warmers they basically are. so i set the tires to 31psi hot 160f and went from there. the surface stayed a consistent range of 67-81C not sure what that is in freedom units. but with how hot the track was it was super greasy. Definitely need to qualify eariler so i get a colder track for all of the benefits

 

All i did was took the stock rear hub and shifted the center of the tire down 1.5 inches on all 3 fixture points. A true drop spindle. So my camber curve would be close to stock sitting 1.5 inches lower give or take. With how well that worked im doing the same with the fronts.

 

Cutting and welding a cast aluminum upright is not safe and I wouldn't recommend anyone to consider that as an option. I'm glad you didn't have a failure or ramifications from that; but I would highly recommend installing those billet machined spindles asap.

Lowering that engine and moving it rearward also made a huge difference in your center of gravity, roll couple, and feeling of reduced roll

 

I’ve seen a few FDs with tube frame rear ends that moved up the suspension pickup points. Same for the front. But that’s likely to influence classing and probably isn’t as reversible. But it does accomplish the same thing without welding cast metal or machining a new spindle. Comes with other problems of course.

 

On 116 Alfas, one of the occasional more involved mods involved cutting the front cast steel upright and adding ~50mm to get away from parallel arms.....that was dubious enough, with preheat, then wrapping in a blanket or burying in sand. Welding the rears on an rx7, I wouldn't want to test them too much, a shock load - pothole, kerb, etc, would have me checking the rearview for errant wheels and sparks!

For a likely low volume part, a sheet steel fabricated upright would probably be more rigid and maybe even lighter than some of these, you beaut, CNCd aluminium ones.

 

The thing to do would be to 3D scan Rob's welded versions, make a CAD drawing and have new ones CNC'd. I've been in the process of having the stock ones scanned/drawn, but that would be potentially faster. 1.5" is about what I was thinking.

For the front, you'd ideally need a seperate steel stub axle bolted on to an aluminum upright, as it's less ideal to either machine the whole thing out of steel, or have an aluminum stub axle. I looked for a bit at using an RX8 bolt-on front hub assembly, but that won't be compatible with my MK60 setup because the sensor is built in.

 

I'd rather just scan a known good knuckle and do the offsets from there, which will be more accurate than cut and moved arms.
The front probably needs to be steel, or just cast them.

 

Most people who wants a drop spindle for FD probably would also like to increase the rigidity of the unit.

it would be sweet to have the axle and upright forged out of steel as a unit and then machine the stub axle.

Light weight is excellent, but not as excellent as having brakes.

 

I have calculated it some years ago. Maybe I still have the spreadsheet, I will have to look.

If memory serves, the instant roll center on my FD is about 1.5 inches underground.

The front springs are 1200 lb, rears are either 1000 or 1100, I cannot recall at the moment.

This FD is pretty much a dedicated autox car, definitely not street driven.

It is lowered about as much as I feel comfortable lowering it.

It would be cool if the roll center were a little higher, and the really stiff springs were not necessary,

Even with these springs though, and a sort of midlevel adjustment on the front tri-point sway bar (0.120 inch bar), there is more body roll than you might expect,

 

A 1,200lb front spring on a 0.6 motion ratio gives you a wheel rate of 432lbs, which isn't crazy.

What was your process to measure and calculate the roll center? I measured the suspension pickup points of an NSX and my roll center calculations were 1/16" off compared to the numbers when I had the NSX chassis CMM measured with a roamer arm.

 

I really don't remember just what the process was, it has been a few years ago.

If I can locate the Excel file used (most likely in a spreadsheet) I will share it.

 

Just looked for this, and cannot find one that has the roll center calculation, so I have no way to back that up.

I do still have the spreadsheet for calculating effective spring rates as determined by the wheel rate.
Front Wheel rate is .43, so 1200 lb springs yield ~ 522 lb
Rear Wheel rate is .47, so 1000 lb springs yield ~ 470 lb

That sheet is attached
Edit: the attachment is failing for some reason, no error given but the file does not appear here.

 

This is really what I was looking to make—these are by Keisler Automation for the Miata, and you'll note the four-bolt pattern on the front to convert to an RX8 hub assembly. That could also be a bolt-in steel stub axle for our OEM hub. I spoke to him years ago about building them for the RX7, and help with development, but he wasn't interested.

I would suggest that, in addition to a 1.5" drop, you might want to build some additional camber into it, as you won't be as far into the camber curve at ride height, with less aggressive camber change.

 

Where did you get those motion ratios? I see 0.6 front and 0.68 rear quoted a few times here. I have not gone down the path of suspension analysis on the FD like I have on the NSX. I will once I get the powertrain finished on my car.

 

@jkstill FYI you can upload the excel to a google sheet, make it public and post a link to it

 

I should have thought of this

FD RX-7 Wheel Rate