Lowered Prime Users, Question!

I have a few thoughts on this, but before I continue can you please post up your current tire size.



Rob43

 

OK, got it.

Here's the main problem, it's the 65 series sidewall of your tire that creates almost all of the effect that you speak of.

The moment you add coilovers, they will actually exacerbate this effect. The stiffer coilover suspension will highlight all of the weaknesses of the 195/65-15 tire. Your current soft-ish OE suspension is helping to mask this issue.


Rob43

 

First, you can't compare a Subaru with a 225/55-18 (Heavy Carcass) to a Toyota with a 195/65-15 (Lightweight Carcass) with their different suspension systems, IMO this would be like comparing Apples to Softballs; I guess they are both round.

I don't like putting percentages on this stuff, but I'd go with at least 80% that a plus 2 or 3 wheel n tire upgrade will solve ~almost all of your porpoising issues at highway speeds. I've said it before & I'll say it again, from a handling / higher speed driving standpoint, the OE 195/65-15's feel like marshmallows.

So to directly answer your last sentence, my upgraded plus 2 wheels n tires "virtually" solved the issues you speak of.


Rob43

 

Prius & Forester suspensions aren’t horribly different, the Forester’s independent rear and more travel being the principal differences.

I’ve run into this in other cars. They spec one spring/shock for the car, then change the design to add weight without revising the shocks, and you get a floaty-boaty ride out of the result. I’m not confirming that this is true for your Prime, just pointing out that your theory is plausible, it has happened before.

 

What's the F & R spring rate difference between the two chassis as you see it ?


Rob43

 

That's fine, thank you.

I suggest that you put a coilover on it right away, but when that does not solve your issue to your satisfaction, remember this thread.


Rob43

 

I’ve never driven a Prime, so I’ve never felt what you’re working on. I will give you more details on my earlier experience, though. I bought an older generation Outback secondhand, years ago. I didn’t like the handling. I learned that the car had been spec’d for a certain grade of rear shock while they were designing it for target weight x. It would have been a reduction from the preceding generation of the sheetmetal. Then something went wrong late in the process and they blew the weight target for most trims. They already had a zillion lightweight rear shocks on order. It wasn’t unsafe, just a bit hovercrafty.

So I put on a set of shocks meant for the previous generation of the car, before the weight reduction program. The handling cleaned up and the car was much more fun to drive.

@Rob43 I never changed the spring rates in that car, so I can’t even offer a guess as to an appropriate spring hack for the Prius.

 

Mine does the same and I don't think it is tire flex. The shocks have low rebound damping.

 

I'm not saying an upgraded rear shock wouldn't help because it might help a bit, that's why I left a ~20% margin for that.

The absolute REAL culprit is the OE 195/65-15's, they are the farthest thing from a performance size & carcass in design.

When trying to address a problem like this, you always tackle the biggest item first. The reason there's push back from the OP is because he has already invested $$ in 15" aftermarket wheels, he would need to start all over if he moved up to a 17" wheel n tire package which would of course be a financial loss from his perspective.

OP, I will offer you this:

1) As a Suspension Builder I solved this exact issue by upgrading to a lightweight 19.2 lbs Ultra High Performance non-LLR A/S tire in a 205/50-17 size on lightweight 17 x 7" 17.0 lbs wheel.

2) I could've installed my OWN brand of adjustable rear shocks on my Prime,....but decided against it once I got a great result from my 17" wheel n tire upgrade. Would my adjustable rear shocks add an even better outcome, yes, but realistically it's a Daily Driver & I'm not attempting to set any lap records with this chassis.


Rob43

 

The rear weight difference Prius vs Prime is less than 2 Americans in the back seat, maybe 1 after Thanksgiving dinner. But it is significant, about 300 pounds, farther back than the back seat, and even a 300 lb guy in the back seat could definitely make a Prius bounce differently than empty. You could take a cheap and crude crack at the spring rate question by stuffing some of the rubber puck type things in the springs. Not sure if you can get ones that have choices about damping. I know there are blue air-adjustable (Firestone?) shocks (or struts?) for earlier generations that might be cheap enough to treat as a testbed rather than a solution. No idea if they have them for the Prime. Most people who get them are looking to compensate for sag caused by heavy stuff like a receiver hitch with 4 bikes on a rack hanging off the back, not looking for handling characteristics like the lowering people are. Just trying to keep their bikes from hitting the ground on driveway entrances.

 

I've replaced my OEM 195/65/15 wheels with lightweight Enkei RPF1 wheels on 225/45/17 tires. I noticed that the ride is much better now.

I went this route mainly for aesthetic reasons and also to keep the wheel size similar to the OEM wheel size. About 25 inches in diameter. This keeps my speedometer roughly the same. And my tire contact patch went up from 7.7 inches to 8.9 inches. It doesn't sound like a whole lot, but I think that matters. It definitely hurts the MPGe but I am already getting great mileage. I prefer the new look and the slightly stiffer sidewalls. It gives me a more confident handling ride.

It is still soft compared to many many other rides out there. But now the car feels "normal" rather than soft. I want to also look into lowering my Prime and getting a stiffer suspension, but I've already spent too much on car mods this year. =P

Maybe next year!