The SERIOUS Wheel & Tire Thread

[Shaun41178(2)]

Thats a heavy tire at 28 lbs. Hoosier has a Dr thats 23 lbs for the same size

[Honest Don]

Whatever works. I’m just impressed with what can be done on a 235 these days

[pmbrunelle]

122 days after ordering, the Enkei wheels showed up from Japan!

IMG_5903.JPG (525.11 KiB) Viewed 1600 times

It's going to be a little while until I play with these, seeing as I'm closing on the house Monday and I'll be busy with a bunch of other stuff (house, moving-related) to do.

The lead-time on these Enkei wheels is pretty long, so if I ever damage a wheel (pot-hole for instance), my car could be out of commission for a while, but I think I'll be fine if I don't drive straight into pot-holes, and I arrange the suspension to not always be hitting the bumpstops hard.

[ericjon262]

The wheels look nice, I like them better than my kojin's based on the above picture.

[The Dark Side of Will]

It's great when the parts finally hit the dock.

[pmbrunelle]

It's going to be a little while until I play with these

So now I am brainstorming and trying to work out a rear coilover spring/sleeve solution that will work with these wheels/tires (and the car in general). Some questions/thoughts:

How much travel do I need for my coilover? From Steven's posts, it appears that keeping the car off the bumpstops in a maximum G turn is his goal. Maybe a little more travel than that in case of mid-corner bumps? How often is a car expected to hit its bumpstops, and in what circumstances?

It seems like in an ideal world, suspension would remain linear forever, but at some point, we must run out of travel. In a turn, if the outside rear suspension hits the bumpstop (while the outside front is still linear), then the load will increase on the outside rear tire. I imagine that this would cause sudden oversteer (could be considered undesirable behaviour). Should front and rear suspensions be arranged so they hit the bumpstops at the same cornering G, or perhaps with a slight bias to the front bumpstop hitting first? Assuming of course that I have a close "natural balance" via tire stagger.

[Series8217]

You really don't want to run into the bump stops on the outside wheels at maximum cornering... If your track has bumps there then avoid them or increase your ride height / add more suspension travel.

[pmbrunelle]

You really don't want to run into the bump stops on the outside wheels at maximum cornering...

That's what I was thinking, for at least for me being an automotive noob.

Nowadays, it seems like newer cars are often made to ride close to the bumpstops, the bumpstop regularly contributing a "progressive spring" effect in normal driving. The original Minis were supported by rubber donuts. A bunch of black magic going on.

Probably simpler to stick with the paradigm of relying on metal springs...

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For a rough starting point / plan, here is my thought process:

• The tires are staggered, so with the natural balance, each end of the car handles the lateral weight transfer proportionally to the axle weight distribution (assuming that roll stiffnesses are tuned as such). Therefore, I assume that the rear end can be studied independently of the front end.

• No sway bar for now (but might add one eventually, to be determined).

• I don't really know where the roll center is. I'll just assume it's near ground level and that the springs see the full effect of the lateral weight transfer. This is a conservative assumption.

• 840 lb static rear tire weight + 550 lb of lateral weight transfer (1 g) = 1390 lb. Round up to 1500 lb just because (an extra for mid-corner bumpiness).

• The KYB strut has a shock rod travel of 7". Bumpstop thickness takes away from this travel. So if I run a 1.81" (Eibach EBS770034) bumpstop, then I have 5.19" of travel.

• Trim bumpstop by 0.5" to increase travel to 5.69".

• Install spring with 0.25" of preload. This should give an OK ride height, and will keep the spring from rattling around at full droop. Avoiding a tender spring avoids extra cost / parts count.

• Choose spring with 250 lb/in rate. The softest rate that keeps the car off the bumpstop is probably the best. I also don't want to over-spring the KYB strut.

• Chose a 10" long spring, so the perch is high enough to clear the tire shoulder.

• 0.25" of preload gives 62.5 lb of force preload with the strut at full extension. Increasing the force to 1500 lb will result in a travel (delta force of 1437.5 lb) of 5.75" (just a hair over the 5.69" of available travel, so I'm pretty much on target).

• Spring deflection at initial contact with the bumpstop = 0.25" preload + 5.69" travel = 5.94"

• Hyperco 1810B0250 spring has deflection capability of 6.786" before coil bind.

• At initial bumpstop contact, spring has 6.786" - 5.94" = 0.846" of remaining travel capability before reaching coil bind.

• My bumpstop trimmed down to a height of 1.31" must squish down to 1.31" - 0.846" = 0.464" height before coil bind occurs. Coil bind probably won't happen...

So that's my attempted solution at keeping things soft enough, while working within the travel limitations of stock Fiero hardware.

[The Dark Side of Will]

I think 250 is on the soft side for a Fiero that can actually corner at 1g. The cars I've heard about which were equipped with tires that could do that were all running 300+ ppi springs.

[pmbrunelle]

The cars I've heard about which were equipped with tires that could do that were all running 300+ ppi springs.

Correlation or causation (though I do get that stiffer springs would help keep the tire more square to the road)

Actually, I don't really know how much lateral acceleration my car will do. I figure that it would fall into the range of [0.85 g, 1.00 g]. This project is more "the car the Fiero could have been" versus seeking maximum effort speed.

Since the goal of the exercise was to find a coilover solution that keeps the car off the bumpstops in a turn, I picked the upper bound value for the lateral acceleration.

A solution that keeps the car off the bumpstops for 1 g will also work for 0.85 g.

[eHoward]

agree... especially without a bar

I think 250 is on the soft side for a Fiero that can actually corner at 1g. The cars I've heard about which were equipped with tires that could do that were all running 300+ ppi springs.

[The Dark Side of Will]

Correlation or causation (though I do get that stiffer springs would help keep the tire more square to the road)

Actually, I don't really know how much lateral acceleration my car will do. I figure that it would fall into the range of [0.85 g, 1.00 g]. This project is more "the car the Fiero could have been" versus seeking maximum effort speed.

Since the goal of the exercise was to find a coilover solution that keeps the car off the bumpstops in a turn, I picked the upper bound value for the lateral acceleration.

A solution that keeps the car off the bumpstops for 1 g will also work for 0.85 g.

I was being nice about suggesting that your analysis for the 1g case may need refinement...

[pmbrunelle]

I was being nice about suggesting that your analysis for the 1g case may need refinement...

Hmmm.... well there are a few items I could add to this analysis:

I could superpose the effect of drivetrain torque (WOT, low gear), transferring weight from the front to the rear of the car. However, the traction circle of the tire would prevent maximum turning and maximum forward acceleration at the same time, so I should not consider this.

Driving in a banked turn (or in a valley) increases the weight of the car, so this eats up some suspension travel.

I roughly know the corner weight of my car, but I don't know how much of it is unsprung. I bundled the unsprung mass with the sprung body. Modeling this more accurately would reduce the required spring rate I calculate, not increase it.

I don't know what the motion ratio is between the spring and the tire. I (implicitly) assumed that they moved together 1:1. When the snow melts, I would be able to measure this with a pair of dial indicators. Blooz made some suspension drawings (to scale) if I wanted to work this out on paper, but they're for an 88 Fiero, while mine is 84-87.

I am not asking to be spoon-fed at this time; I only started thinking about this coilover spring selection during the weekend.

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On Sunday, there were three of these threaded sleeves remaining in stock:
https://www.summitracing.com/parts/qa1-ck6007

Sooooo, I got excited to order things ASAP, before the threaded sleeves became unavailable. Naturally I added 250 lb/in springs to the order

It's not because I ordered springs that I am closed to making changes. Anyway, the idea with the 2.5" ID standard springs is that they can easily be swapped out with each suspension iteration...

[The Dark Side of Will]

IIRC, Steven Snyder has measured the motion ratio in the low 0.9's.

I was thinking more that your statistical assumptions about suspension excursions from the steady-state cornering condition (which never actually happens in the rear world) are way too mild.
If you're at limit cornering just off the bump stop and you hit a bump, what happens? The bump stop makes contact, rates change wildly, contact patch loading changes wildly and the car experiences a yaw (+roll) excursion. The more often the bump stops touch, the more frequently this happens, making the vehicle difficult to control at limit cornering. Increasing the available travel at limit cornering so that the car can take some bumps without hitting the bump stops helps that considerably.

[pmbrunelle]

I'm kind of wondering how the stock Fiero was supposed to work with ~150 lb/in rear springs, though maybe it shouldn't be counted as an example of "what to do".

However, other passenger cars have soft rides, and don't have the suspension travel of a 4x4? Maybe that's why almost riding on progressive-rate bumpstops seems to be a trend.

I was thinking more that your statistical assumptions about suspension excursions from the steady-state cornering condition (which never actually happens in the rear world) are way too mild.

So, I'm scouring the Internet for different ideas regarding the necessary suspension travel on a car.

Some keywords in a google search may yield undesired results:

Bottoming.jpg (101.63 KiB) Viewed 914 times

[draven]

I'm kind of wondering how the stock Fiero was supposed to work with ~150 lb/in rear springs, though maybe it shouldn't be counted as an example of "what to do".

Yeeesh on the search...

Ride compliance for the average american driver... (no correlation between your search and that statement )

As Steven calculated and Will mentioned earlier, the motion ratio of the rear strut layout is in the low .90's and can even hit and exceed 1.0 that given higher aftermarket wheel width and subsequent higher offset, i.e. usually 45mm E.T. for a 8-9" wide wheel. If memory serves correctly the improved geometry/stability of the 88 rear setup allowed GM to lower the rate of the 88 springs even further to give a softer ride and maintain the same handling.

[The Dark Side of Will]

Such a sheltered life you've lived, Pat! I could have told you that search was going to give undesired results before clicking go. At least you didn't "feel lucky"

Perfect limit cornering isn't as big a deal for GM as good ride compliance the rest of the time. GM makes up for it with progressive bump stops and keeping good suspension travel. That means that lowering the car without dramatically increasing the spring rate results in hitting bump stops far more often.

Also, the tires on the car were 0.85 G rocks, not modern performance rubber.

[eHoward]

I don't know if 150lb/in is a good assumption for the original spring rates. I agree 1,000% on Will's point on rubber in the 80s vs modern tires ... not even current summer or max performance tires.

Fiero Spring Rates

Stock

4 Cyl. 6 Cyl.
FRONT 1984 - 1987 179.5 lb/in 208.1 lb/in
REAR 1984 - 1987 228.0 lb/in 250.8 lb/in

WCF performance
FRONT 1984 - 1988 300 and 400 lb/in 300 and 400 lb/in
REAR 1984 - 1988 300 to 400 lb/in
300 to 400 lb/in

per WCF: https://westcoastfiero.com/products/wcf ... iloverkits

[pmbrunelle]

For the front, I ordered a set of Moog 5576 springs from Rock (last pair in stock).

Catalog values:
Rate: 385 lb/in
Free length: 13.69 in

I'll probably have to trim the springs so they're short enough to be installed, and to adjust ride height/preload.

Since the front spring is less standardized and more of a PITA to work with (less choices, harder to make adjustments), I figured that I should set my front spring first.

I would not attempt to predict the wheel rate with the variables of spring trimming, active coils being affected by the perches, and motion ratio. To cut through the noise, I'll do a direct wheel rate measurement like I did last time:
https://www.realfierotech.com/viewtopic.php?f=3&t=21738

Once the front spring is installed and its wheel rate is measured, then I can select a rear spring to match the front. I may want to follow the "flat ride" paradigm, where the rear ride frequency is 10-20% higher than the front ride frequency.

Working with springs from the 2.5" ID family is a bunch easier...

I wouldn't want to go the other way, choosing a rear spring and then trying to find a match for the front from a Moog catalog

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For tires, I'm looking at the Firestone Firehawk Indy 500. A sort of well-reviewed moderate-performance summer tire.

I'm mostly considering these sizes:
195/55R16 (will work with my 16x7)
255/40R17 (will work with my 17x9)

My car (with me inside) is kind of light up front:
Front weight: 510 kg
Rear weight: 730 kg

I also like the idea of narrow front tires to lighten the steering effort, and to reduce tramlining weirdness.

Sorry for the threadjack, this is more of a moderate tire setup versus a SERIOUS setup

[draven]

What is your target suspension frequency?