Chassis that flexible?

I wonder how different things would have been if you had the BT
plate under there.
.
There are some great articles kicking around about people testing
chassis stiffness by jacking four corners of the car equally, and
then letting one down again and measuring the deflection. Sags on
the order of 4-5 mm are not uncommon. Scary, huh?
.
_H*

 

<div class='quotetop'>QUOTE(wpavalko @ Nov 21 2006, 03:12 PM) [snapback]352765[/snapback]</div>

Lots of cars which aren't torsionally stiff do that. My Jeep Grand Cherokee does it. Whenever one corner is jacked up (I use a floor jack under the axle, not the frame lift points as I'd need a 20" jack), the front doors make a distinctly different sound when opened and closed. Then again, this is the same vehicle which some owners say "body twists like it's falling apart" whem climbling boulders on the Rubicon trail.

So I guess the lesson is, don't try to climb boulders on the Rubicon trail with your Prius. :lol:

 

Totally normal for cars like this. I recommend NOT opening/closing doors when the thing is tweaked like that. Oh, and try to park with an even number of wheels on the curb next time. That'll solve your problem.

 

In most cases, if it doesn't bend, it'll break. Car frames, rollercoasters, semi trucks and trailers.

 

Is this plate used to fix a problem? The chassis is a system and changing one part may cause the rest of the chassis to react differently. How the chassis bends and twists is part of the suspension design. If I installed this plate, what would I expect to experience in the way of chassis dynamics? Would the frequency of twisting, hogging or sagging either in front or behind the plate be change in a way that would not effect the springs and shocks? Is a higher frequency less noticable to the human bottom?

 

<div class='quotetop'>QUOTE(kDB @ Nov 22 2006, 07:20 AM) [snapback]352945[/snapback]</div>

Does a ladder get shorter when you step on it?

The holy grail of racing car design, I've read, is chassis stiffness. Current Lotus cars (Lotuses? Loti?) are glued together, and not just to save on the weight of the bolts. There's more than enough 'give' in the suspension, especially in a road car, to keep anything from breaking.

 

Seems normal, and a big reason Prius handling will never be in BMW territory. Actually, the Prius is a little worse in this regard than most mid size cars.

By comarpison, it takes 14,500 lbs. of twisting force to deflect a BMW Z4 (convertible!!) chassis by 1 degree. The Land Rover Freelander (small SUV) requires 11,400 lbs of twisting force to ensure the same 1 degree deflection.

I'd guess the Prius is around 2,000 - 3,000 lbs for this same level of deflection.

 

<div class='quotetop'>QUOTE(Bryanmsi @ Nov 23 2006, 10:59 AM) [snapback]353531[/snapback]</div>

Might be a bad guess. The Prius chassis flex is part of the design. This car is not meant to go canyon charging. Part of the reason that these latest BMWs are so stiff is because they are so heavy. The Z4 is a big boy in the weight department. So are most late model converts. There are just not too many serious sports cars to be had. People want their comfort and the Feds want bags and crumple zones.

 

<div class='quotetop'>QUOTE(LongRun @ Nov 23 2006, 04:51 PM) [snapback]353558[/snapback]</div>

That has to be the one of the stupidest things I've ever read in this forum.

Chassis flex is always a bad thing, regardless of whether the goal is comfort or handling. Any source of compliance other than the suspension itself, such as tire sidewalls or chassis flex makes it harder for the engineers to tune the suspension. Almost every time a car is redesigned, a goal is higher torsional stiffness with little or no increased weight.

You think the Prius is purposely flexible so it has a more comfortable ride??? :lol:

You can make a car torsionally rigid and still have plenty of crumple zones. Take for example oh...every four door German sedan sold today.

 

In chasis design, there are torsional rigidity and bending rigidity, meansured in lb/degree. Torsional rigidity is especially related to suspension/handling (imagine what happens when one tire hits a pothole). My other car is a '98 BMW 540iA. Prius is no match in rigidity department. Most people love folding rear seats w/o knowing that torsional rigidity would suffer if they have to take out an steel X-member behind the seat. That X-member imrpoves torsional rigidity greatly by about 5-10%. I know because my 540iA has folding seat as an option and I found the torsional rigidity data online for both w/ and w/o folding seats. They differed by 5-10% (forgot the exact numbers).

 

LongRun
If you want to know all about the plate there are other threads
Here http://priuschat.com/BT-tech-plate-review-...ing-t26239.html
Here http://priuschat.com/BT-Tech-stiffening-pl...ion-t25483.html
And here http://priuschat.com/BT-Stiffening-Plate-Review-t17018.html

Then you'll have to make your own mind up

 

I have to agree with priusenvy on this one. The frame should not twist enough to rack out door allignment and in a hard top no less! I could <maybe> accept this in a convertable but not in a hardtop. A BT plate would help but I think that not allowing a door to retain alignment with one tire not in plane with another is totally wrong. That means under normal driving conditions it is tortionally flexing all the time and you just can't tune a suspension system for optimum performance with that happening. No, the frame should always be as ridged as possible and all give should be only in the suspension system.

I put on a BT plate on my 06 and found the road manners so much better after the installation of the plate. No wonder!!! I'm now ready to look for other ways to stiffen the flex in the chassis. Weld in a couple of pipes between frame rails etc.
-Paul R. Haller-

 

I think one reality being left out of the discussion so far is "sufficient rigidity with light weight". Prius handles well enough for a car with its' power once the tires are improved. Making it more rigid would increase weight substantially and that would impact mileage (and everything else in the design). It's not a sports car. It's not a "sport sedan". Comparing it to those is rather silly. How about comparing it to a Camry or equivalent mid-size sedan?

 

Let us not forget that a hatchback (like Prius) is also not as rigid as a sedan if everything else being equal. I was not bashing Prius on rigidity. BMWs (like MBs, and Audi and most high-end German) are heavier than their Japanese counterparts. Cases in point ('07 Acura MDX at 4500lb vs. '07 BMW X5 3.0L at 4950lb - equivalent in most dimensions). There is always trade-off in design (weight and rigidity, for example).

 

If the door sticks when you jack up one corner, does the plate prevent this?

 

<div class='quotetop'>QUOTE(LongRun @ Nov 24 2006, 11:57 AM) [snapback]353729[/snapback]</div>

good question...probably not. the car was not under any circumstances, designed to react to that kind of torque. the BT plate most likely (have no real idea to be honest with ya) changes frequency vibration more than anything else.

 

No, it's not a sports car. Yes, there are things that could and should be done to improve the handling and safety. I don't see any correlation between weight and stiffness - a little more metal in the right places doesn't mean the car has to be any heavier overall. The BT plate, for example, weighs next to nothing, yet makes a significant difference in the handling. A Prius will never be a Lotus, but who says we can't learn a few things from the experts?

 

Chassis flexibility is an earmark of small economy vehicles. By having the flexibility a manufacturer can use cheaper light weight components and keep the costs down. When you begin to stiffen a vehicle you affect the entire vehicle which equals a higher cost for the entire vehicle.

If they ever make the Volta a production vehicle I can pretty much bet that the chassis will only flex when needed!

I had the same type of experience with a Celica that I owned. If I parked in the driveway at our apartments I could not open the doors. The blasted thing flexed too much!

After installing stiffening plates on the strut pillars, undercarriage, installing lowered heavy duty springs, aftermarket struts, shocks, replacing the rear axle with a Supra unit that included disk brakes, going plus two on the wheels and tires, polishing and balancing the engine, installing Mikuni side drafts, a new free flow exhaust, headers, and replacing half the support bolts I could follow mountain roads at high speed and never feel any body roll or flex..... I could have opened the door on that driveway but the airdam wouldn't allow the car UP the driveway. So you have to think about what you really want your vehicle to do for you....

The BMW's reflect the German design philosophy of strength+weight+rigidity=better driving. Not necessarily a good or bad philosophy. But they turn their philosophy into reality VERY well! Mercedes shows the same style of thought.

Your experience is the same as mine with the Prius flexibility. I have now driven my 2005 for 30K in the last two years and I am back on the 'fix-it' bandwagon. I just changed jobs and now have an occasional trip which includes 50 miles of twisty roads. I can't stand the flex!

My car's birthday was November 20th. I am giving her lots of gifts including the BT plate and a few goodies which I will order once monetary negotiations are complete with "She Who Must Be Obeyed".

This flexibility gets worse as time goes by and is truly the only complaint that I have about the Prius. A simple 360 degree on-ramp and the best I can do is 44MPH because of that flex! Pitiful...... My Ranger does 40!