Project log: Prius base suspension upgrades

[galownia]

Hi all,

I recently purchased a 2008 base pkg 6, and, as I do with all cars, am in the process of improving the handling capabilities of the car. I've owned a series of race cars over the years, and driven many more than I've owned. I do almost all work myself.

Objective:
With this car, I want to create a competent handling daily driver which remains comfortable for passengers and retains stock utility and maintains or improves upon gas mileage

With my other cars, I've gone farther towards ultimate handling, which has created extremely competent, exciting cars, but cars which our guests don't enjoy riding in and require disclaimers before I let anyone drive them (tendency to oversteer, so be careful on corners; lower (but retain suspens travel with matched springs), so watch out for curbs and speedbumps; super stiff, so try not to drive over large potholes, etc)

Main complaints:
1) Excessive lean in corners - causes slower corner entry, mid, and exit speeds overall
2) Mushy turn-in response - results in uninspiring corner entry and evasive maneuvers
3) Extreme tendency to understeer - limits corner entry and exit speed, can cause wash-out in mid-corner

Plan (to be revised as I tune the car):
0) Touring wheels/ tires (DONE - see report below)
1) Stiffen the rear sway bar (DONE - see report below)
2) Add BT chassis stiffening brace (on order)
3) Convert to touring rear struts/ spring (received, to be installed)
4) Alignment (to be planned)
-------optional if still not satisfied---------
5) Strut tower braces
6a) Stiffen links/ bushings on front bar (e.g. urethane)
6b) Stiffen front sway bar

Results

0 Touring wheels/ tires:
-Marginal increase in steering response
-No noticable change in ride quality in normal driving conditions

Verdict: worth it, but not a big bang for the buck. Do only when you need to change tires anyway, and can find reasonably lightweight 16" wheels at a good price

1 Stiffer rear sway bar:
I replaced the stock 19mm hollow bar with the solid 24mm TRD Matrix bar (PTR11-02060) available for $170 shipped at Carson Toyota (TRD source - do a google search). This is a 15 minute job - everything you need is included with the bar you order.

- Greatly reduced corner lean. Car retains balance towards understeer. No throttle lift oversteer at reasonable to quick cornering speeds
- Improved turn-in response
- No noticable change in overall ride quality (not harsher)
- UPDATE: I had a chance to drive in the rain - the car maintains its balance towards understeer under normal driving. I tried several abrupt lift-offs at moderate (but not autocross) speed mid corner in the rain (after the car had set up) and did not induce a loose condition.


Verdict: Worth every penny to improve handling while maintaining a safe and comfortable ride. Excessive lean is gone. Easy enough to do on your own, even if you have only basic mechanical skills.

I will continue to add to this as I execute my plan.

[Boo]

Hi galownia,

Welcome to PriusChat. Looking forward to your log entries.

I was wondering, are there any negatives (even small or slight negatives) to putting in the larger TRD Matrix rear sway bar?

[Rokeby]

+1

I've been considering a larger rear sway bar for some time now.
There have been two or three posts scattered about reporting good
things handling-wise from a modest up grade. (I'm alredy running
with the BT plate and strut tower brace.)

Is there any reason to expect accelerated tread wear or uneven,
inside/outside, tread wear in normal driving?

[drees]

Sway bars shouldn't affect tire wear significantly. By keeping the car flatter, wear should be more even. By adding a stiffer rear bar, the wear rate between front/rear should even out a bit (but probably not significant)

[brick]

Alright, I give. I want the bar!

[galownia]

I had tried to respond earlier, but my computer froze.

Drees is correct - under normal driving, there should be very little to no change in tire wear. I've put stiffer bars on all of my cars, and have had them for over 100k miles combined. No tire wear difference on any of those, so I am confident that there should be none here either.

As for ride comfort, I reiterate nothing perceptibly different was noticed in normal driving conditions with this bar. In theory a stiffer bar does add to the spring rate slightly, which could cause a slighlty stiffer ride. However, in this application, neither I nor my wife have perceived this.

Jonathan

[ursle]

Originally Posted by brick

Alright, I give. I want the bar!

Me to, I already have one on the way...
Stuck 17" 45 series tyres on and it handles almost as well if not better than my Audi A4, have upper front strut bar and chassis stiffening plate.
Still getting 51.3 after 20 tankfuls
My motor is actually discernible in an auditory way with a K&N air filter

[2maples49]

Hi Jonathon,

Very interesting changes you've made.

I bought my 2005 a few months ago and it had "H" rated fleet tires. My highest priority was improving the handling because mine did not have enough grip nor did it feel safe. I tried several air pressures going up as high as 42 PSI in the front while keeping 32 PSI in the rear. It made little difference in the serious understeer.

I made one change and the car's understeer is dramatically reduced and it feels much safer. I replaced the tires with Goodyear Eagle GT HR high performance all season tires. I'm running 38 PSI front and 33 rear. Of course the car still has a lot of lean. Maybe I don't push the corners as hard as you do because of the lean, but I'm so surprised and pleased that the understeer is dramatically reduced at the speeds I run. And this is with the stock alloy wheels and stock size 185-65/ 15 Goodyear Eagles. This was lots of bang for the buck for me. Search the forum for "Goodyear Eagle GT" for a more detail on my experience with the Eagles.

Your experience with reduced lean and the rear sway bar sounds very tempting. Going from a hollow 19mm to a solid 24 mm sounds like a lot of extra roll stiffness in the rear. I would have guessed you'd want to split the roll stiffness increase between the front and the rear. I'd be concerned about oversteer in wet or other slippery conditions. Keep us posted on your experience.

Thanks, Dan

[galownia]

Dan,

Tires do make a huge difference in turn-in, overall grip, cornering stability, and balance. I'm glad you found a set that you are happy with. I run a set of high performance summers on my other cars, and swap out with winter tires in the cold season.

As for the oversteer - it does seem like a big change to go to the larger bar, but the suspension is still very soft overall. While lean is reduced, it is certainly far from eliminated. I have not yet tested in wet conditions (I will post if I get a chance before it snows), but I doubt to experience oversteer. In fact, even on my much more aggressive setups, oversteer on front-wheel drive cars has never been snap - I tune for lift-off, controllable oversteer, which only really appears near the limit (e.g. in autocross events). The setup I've got now on the Prius feels nowhere near what would be required to achieve consistent lift-off oversteer, and certainly not snap oversteer.

I'll continue to update as I learn more about this setup on this car.

Jonathan

[2maples49]

Hi Jonathon,

You have much more experience with high performance vehicles than I have, so you comments are much appreciated. I'm considering buying the much debated BT chassis stiffening brace. I'll be very interested in you evaluation of that part as it relates to cross wind performance. In a strong cross wind, when I pull ahead of an 18 wheeler, my car gets moved to the side more than I like. My new tires have reduced this problem, but not cured it. Maybe my my expectations are too high for a car like the Prius that is short, tall, and fairly flat sided (providing a lot of "sail" area).

Thanks, Dan