I pulled off my rear shocks to take off the dust boots so I can attach some motorsport grade stroke sensors (zip ties).

The shocks only have 84 mm of stroke at full droop. The bump stop takes up 24 mm of that.

So I effectively have less than 60 mm of stroke.

Probably much less when you consider spring compression at 1G.

Should I lop off half of the bump stop, or just get better coilovers?

 

your stroke length only matters if you're using all of it. did your stroke sensor () indicate full usage? if it did then you can start by cutting the bump stop in half and checking again.

 

I haven’t actually installed the zip ties yet.

My coilovers aren’t off the shelf engineered parts, they’re custom deals from a Taiwanese brand… I think I should just buy some HKSs or Ohlins, since the damping seems way too soft, and the rear stroke seems insufficient. It takes too long too take a set, and it slides in any tight corner…

 

Limiting rear shock droop travel limits weight transfer forward under braking (rear pitching up) which keeps the rear loaded for traction on turn in.

There may be a reason even high end coilovers for the FD have limited stroke.

I have Ohlins Road & Track and the rear has limited droop travel.

 

+1 Mazda loves droop travel. the Rx8 has like a foot of droop travel, its hard to get it off the ground. but then its only got a couple inches of bump travel.

anyways, you should look at where your 60mm of stroke lays. weather its bump or droop, and then if you can, you adjust that travel where you want it.
or maybe you start backwards, and see how low you can get the rear without rubbing, and then add your suspension travel to that. in the front the tire will hit the upper frame rail, not sure about the rear
don't forget the FD has a motion ratio of like 1.7 or something, so 60mm at the shock is like 102mm at the wheels which is enough

 

Limited, sure, but the spring rate is usually high enough to keep you off the rubbers.

 

It’s 60 mm off the car! Just the theoretical max droop. So I probably have much much less bump stroke on the car with the wheels on the ground. Will have to put them back on first.

I wonder how much bump stop I can afford to remove… and whether to keep the conical side or the other side.

The fronts have plenty of stroke both ways.

Am I the only person who has to fiddle with the uprights every time I take my upper arm off?


Also, anyone know a good way to inspect the rear pillowballs?

 

The conical part is the progression. A 24mm bump stop isn't much on a road derived vehicle, you don't say whether you're monos or twins, which might have a slight bearing.

Usual way to check suspension is wheel in the air, 9 and 3 and 6 and 12, shaken not stirred!

 

I know, I was just wondering whether to keep the progressive part or not if I was going to shorten it.

I know it's not that big of a bump stop, but 86 mm of total stroke just doesn't seem like enough. I should probably go ahead and just do the zip tie test before I hack up the bump stops.
Pulling the shocks is not as big of a job as a I remembered...

For reference, HKS documentation seems to indicate their track coilover kits (16kg springs, 6 kg higher than mine) have 100 total stroke in the rear. That sounds more reasonable. If anything, the fronts have less stroke, since they don't have helper springs (well, the new model does, but it's pretty small).
https://www.hks-power.co.jp/product_...ermax/db/31754 and https://www.hks-power.co.jp/product_...ermax/db/15504 (in Japanese though)

Debating whether to spend $600 on some used ones... or $1800 on new ones. My shocks are too soft on compression for track use, and the car just refuses to take a set on tight corners. Long, sweeping corners are OK, but S-type corners are just awful.
I also get no traction on tight steep uphill corners (namely the third corner at Ebisu West). Car just slides. I think this is due to the lack of rear stroke, but it's probably in part a differential issue. Need a nice clutch pack.
It's also where I am most likely to get fuel starvation issues, or it was before I got an in-tank baffle box.

I'm actually not sure whether mine or monos or twins.

Maybe I'll just change the hard black rubber out for some softer, more progressive urethane stops...

 

Way to tell, is if there's a step in the body of the shock = twin tube.

General rule of thumb, softer you can run the car and the more droop, the better for traction. Aero, slicks v r-comp, speed of the corners all play a role, the hks rates seem on the heavy side to me @16 and looks to be 18kg - you were running a v4 oem wing weren't you on r-comps?

The bilstein mono comp bump stop is around 40 or 50mm in foam, urethane or hard rubber stops wouldn't be great if you ride them with any frequency........assuming anything is less than billiard table smooth over there. If there's anybody who revalves Bilstein locally, the stock street ones converted to adjustable seat and digressive piston have proven to be the equal of just about anything - amazing when you consider the stock damping rate on those is so shitful! No adjusters to twiddle either, either a positive or negative.

 

I have a smallish aftermarket wing and I am currently on Nankang AR-1s. Will have an aftermarket front bumper on the car again soon, but no major aero.

I also need an alignment and corner weighting, but it's just such a huge hassle to get this work done.

 

Longer stroke is helpful for on road ride quality - which is why SakeBomb did their longstroke setup: https://www.sakebombgarage.com/fpspe...dfv-fd3s-rx-7/
It's also why I did my custom Fat Cat setup and eliminated the stock hard rubber bump stops.

I'm going to say that it probably helps on track too - especially from apex to corner exit where you are trying to put the power down and fully weighting one of the rear corners. I had to raise the ride height on my Spec Miata when running the street based Bilsteins so that I had enough ride height to allow the suspension to compress and put the power down. It would want to step out in that scenario. Less of an issue with the Penskes.

 

It's not clear to me what the difference is. Is the shaft longer, or is the body shorter? Or what?

You have to consider ride height when setting up dampers...

For sure, it's generally helpful to not run out of stroke, unless you're a race car with tons of aero, in which case bump stops and packers are a huge part of suspension tuning.

I am pretty sure I am running out of stroke. Maybe I only need an extra 10 mm, though. I don't really care if these dampers break, so maybe I should just run it without bumps to see exactly what the shock is doing.

Or I'll just get some Apex'i bump stops that are about the same height, but way softer.

 

Not entirely sure either. I can speculate and make the same generalizations that you probably thought of already but I won't bother to.

 

And does it mean it's optimized for a taller ride height?

They charge a ton for their special versions, so a bit of clarity would have been appreciated... Hell, they charge $300 just for helper springs!

 

Ask Sakebomb why they did longer rear stroke and what the advantages and disadvantages are.

Surely they dont have so much hubris they will claim to better suspension engineers than Ohlins and are simply fixing a flaw in the original.

Adding their articulated top spring perch was a cost and nhv addition to the original, but the stroke values would be free to Ohlins engineers.

I suspect that the longer rear stroke (droop) is a concession to comfort over performance.

 

I seem to recall there was a thread/post about this, but I doubt I will be able to find it.

 

Yeah, they did post about it ages ago but I don't remember who the forum name was. The description on their website doesn't do justice / isn't detailed enough compared to what they designed into the setup.

 

I recently tackled this topic and wanted to share my findings. I measured the overall travels when working with Ultimate Performance Speed Shop to dial in their double-adjustable monotube JRi coilovers for the FD RX-7:

https://upspeedshop.com/product/ulti...ilover-shocks/

I wrote the following article on the coilovers at MotoIQ .com:

https://motoiq.com/project-fd-rx7-re...ce-speed-shop/


The total travels were determined by the maximum articulation of the suspension arms at full droop (at the point the arms bound) to the full compression where a 25" OD (255/40-17 tire) would contact the fender liner.

FD RX-7 Total Front Tire Travel: 6.3" (160.0mm)
FD RX-7 Total Rear Tire Travel: 6.1" (155.3mm)

Measured at the damper shaft, the above figures were reflected by:

Total Front Damper Stroke/Travel: 3.78" (96.0mm)
Total Rear Damper Stroke/Travel: 4.16" (105.6mm)

These numbers aligned with the motion ratios I have for the FD RX-7:

Front Motion Ratio: 0.60
Rear Motion Ratio: 0.68

So there is essentially only 96mm front and 105mm rear of total possible damper travel of the FD RX-7's suspension geometry using a 25" tire. The only way to get more stroke is to use a smaller diameter tire that will be able to compress the suspension further before bottoming out into the fender well.

Working with Ultimate Performance Speed Shop, we set the bumpstop heights of the double-adjustable JRi Motorsports Dampers to prevent the tire from hitting the fender liner, but to maximize damper travels. We ended up with:

UP Speed Shop JRi Front Damper Stroke (before bumpstop): 87mm
UP Speed Shop JRi Rear Damper Stroke (before bumpstop): 95mm

The car has essentially the same droop travel as stock to maximize grip, ride quality, and performance. If this was a racecar competing on billiard-table smooth tracks (which there are not a lot of in the USA), and/or it had a lot of aero and needed to maintain a consistent aero platform, we could reduce overall stroke and droop travel. For 99% of FD owners and track cars, this is not worth doing.

 

Interesting. I wonder how much of that articulation range you actually end up using during track driving? I suppose the answer to that is "it depends."

I ended up getting some clean used HKS coilovers with 16 kg springs, replaced all my bushings (except for the sway bars) with hard black urethane (and replaced my pillowballs), and installed a 1.5 way clutch pack LSD.

I am more or less happy with how the car handles. I just need more seat time and a bit of front aero.

I could probably get away with lowering the car another inch, but that would be a logistical nightmare.

I might also need a bit more rear camber, since the outside edges of my new tires turned a bit blue. That might require making other adjustments so the extra grip doesn't make the car understeer any more than it already does in high-speed corners.

 

Yeah, it does depend on many factors (spring weight, vehicle weight, ride height, tire, downforce, track, curb usage, etc...)

"ANOTHER inch"?!?! What is your current ride heights (fender heights)? I'm not sure how much travel you would have after lowering the car 2" even from stock.

I would get a probe type tire pyrometer and measure your tire temps to dial in your required cambers.

 

Yes, I also found more rear camber is needed when switching from stock Torsen to aftermarket clutch type LSD.

This is because the car now has much more drive exiting corners (Torsen only provides bias ratio torque to outside rear tire when inside rear tire unloads).

This drive loads the chassis with more rear outside weight transfer/diagonal body roll on corner exit.

 

Rear droop...

If you have full stroke shocks in the rear you still only get full droop when braking in a straight line.

Because you have a rear swaybar that connects both rear wheels.
In a corner the outside wheel being compressed pulls the inside wheel up.

Jack up your rear, loosen 1 side lower spring perch so you can get on the bump stop, put a 1ft block under that tire (ebrake on) and lower the jack.
look at your other wheel- that is your maximum available droop under full cornering. Slowly jack car back up and observe the increased droop from full compression to normal ride height then on to full 2 wheel droop.

So, do you want full rear droop on straight line braking?

No, it increases front weight transfrer which decreases braking potential, increases understeer on turn in and slows transient response.

No reason to have more 2 wheel rear droop then your 1 wheel rear droop figure...

 

It is not common for most cars to reach the suspension's (maximum suspension geometry articulation) droop limit under braking. Now, if you have a short enough damper body and have greatly reduced the droop travel of the shock itself, you can absolutely reach the droop limit under braking.

Changing droop travel does not really affect weight transfer. Weight transfer is primarily determined by the car's weight, center of gravity, suspension geometry, and wheelbase.

If the rear suspension hits the droop limit under braking, it will pick up and unload the rear tires under braking, reducing the rear tire's tractive force and contribution to braking. So braking performance actually gets worse.

Having more droop travel generally creates more grip on bumpy surfaces, curb usage, and especially on the street. Only when the car's aero setup starts to get extreme and maintaining an aero platform and aerodynamic grip start to matter more than mechanical grip is when reducing travel starts to matter.

 

I do have a pyrometer. I did use it to set the pressures. The answer is basically always "lower than you think." I think the front camber is basically perfect.

I take back what I said about the rear camber, though. The front tires have max camber and they're just as blue on the outside edges.
A bit more would probably not hurt, though.

The ride height is ~25 inches at the fenders, measured at the bend.
It's maybe 5 mm lower in the rear, but the pinch welds are roughly the same height.

It's low, but it's not racecar low. I think the spring rates would allow me to go a bit lower, but it's already kind of a pain to get out of my garage and put on the truck.

Adding a splitter might also complicate things logistically.

My biggest complaint is that the car sometimes (but not always?) pushes badly when I am on throttle. Like, the sensation of front grip drops away.
I assume that's the diff working, plus weight coming off the front of the car (especially when I am going fast enough for the wing to work), assuming I am not actually hitting the bump stops.
The problem isn't the understeer itself so much as that I don't know exactly when and why it happens.

Perhaps there's no way to know for sure without shock pots, since zip ties only tell you so much.

I previously thought I had a braking issue, but I think it was just the tires. I don't think I've managed to lock up my new 255 CR-S tires yet.
In the right corner, they generate so much lateral force it almost hurts. Haha.

A bit of front aero will probably make a big difference.

I still need to find out why my boost pressure is consistently lower than the target pressure, though. That will add more easy speed than anything else...
It's probably an intercooler piping issue, assuming it's not something inherent in the engine, the tune, or the natural pressure drop across the intercooler and piping.
I realized my hood was impinging on the cold side piping somewhat, and it's possible that was causing a leak I couldn't detect with the hood open.

I also replaced the throttle body o-ring (last time I did this was almost a decade ago) and the brake booster vacuum line (which was admittedly toast) just to make sure.