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Old 02-20-2002, 07:35 PM   #1
no_rex_yet
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Default wrx ride quality: 17" vs. 18"

There's an interesting concept being tossed around in the Prodrive spring thread that I though would be nice to bring up: 18" rims ride better than 17" on the wrx!

Simon Lines and others have pointed this out, and from my personal experience, it appears to be true. How is this possible? You would think 17's, being lighter and with thicker sidewalls would ride nicer. However, I've found that my 17" UK oem's ride very harshly. Whereas, when I sat in someone's wrx with 18's and whiteline springs, stock struts, it felt much, much less harsh.

What's up with this?
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Old 02-20-2002, 08:10 PM   #2
duckboy
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Default ride quality

the harness that you're experiencing is actually coming from the entire suspension package (tires, bushings, shock, spring...) having too much uncontrolled compliance.

when a vehicle hits a bump, the suspension (shock and spring) is supposed to absorb the bump/compression. but if bushings & tires are soft and also deform to absorb the bump, you've now added two other "springs" into the suspension equation. here in lies the problem. tires and bushings can act as springs and most often times, are considered undamped springs (relative to dampening levels provided by the shock). therefore, if they are excited by a bump and deflect/deform, they will "bounce" back and continue to resonate. this excitation is felt as a "shudder" in the suspension.

by going from 17 to 18 wheels, what you're inherently doing is increasing the stiffness of one of the "springs" and reducing the chance that it will deform/deflect from a bump, thereby removing one of the undamped "springs" from the suspension equation (nearly eliminating one of the indeterminate frequencies). this removal of excessive tire compliance now forces the shock and spring to deal w/ the bump/compression. by replacing the stock soft bushing w/ a GrpN bushing, you're effectively doing the same thing. now the suspension is forced to work even more to absorb the bump/compression. but, now controlling the suspension's response to the bump is easier since now you know that the shock is nearly the sole controlling factor in the suspension's dynamic behavior.

remember, a suspension system (tires, shock, spring...) is simply an energy dissipation system. energy is inputted into the system via a bump and needs to be dissipated some how. how and where this energy is dissipated determines the dynamic behavior of the suspension package.

duckboy...
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Old 02-21-2002, 09:21 PM   #3
scotto
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Nice and logical, a near miracle around here
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Old 02-22-2002, 03:34 AM   #4
uwrx
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Very nice duckboy.

Has anyone with 17" wheels replaced the bushings and seen an improvment?
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Old 02-22-2002, 08:30 AM   #5
Scottie
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I'll have to disagree with duckboy on several points.

This thread started out about a particular 17" wheel and tire combo riding more harshly then a 18" wheel and tire combo. I believe most people judge a particular set-up's harshness by how abrupt given impacts are with road imperfections. I would suspect that most would not consider any oscilation ("resonate") or "bounce" as harshness, but rather as bounciness. Yes, the tires do act as springs, but I would say that generally the stiffer the spring the greater the impact felt in the cockpit (all else bieng equal of course).

Part of the problem with this thread is that it's to generalized. There can be significant differences in all aspects of ride quality, including harshness, of different tires of the same size. Saying you felt a difference between 18" and 17" wheels doesn't mean much to me unless they were the same make and model tire as well as wheel, same section width, same overall diameter, and running at the same inflation pressure.

Inflation pressure plays a significant role in ride quality. If you're running an 225/40/18 versus a 215/45/17 tire, it's likely that you can run a lower pressure in the wider tire.

Now, I'm not ready to throw duckboy's greater tire deflection theory out the window. A totally unsubstantiated theory of mine is this. As your car is traveling along in the horizontal direction, the tires will encounter all forms of road imperfections. Colliding with these imperfections will create a force vector acting in some direction. A component of which will be in the upward (vertical) direction, and another component of which will be in the rearward (horizontal) direction. How much of this impact gets translated into vertical motion will depend on many factors. The shape of the imperfection ( rounded bump, shape limped pothole, gapping crack, etc.) is one. Then there's the tire.

One of the tire's attributes that weighs in on this issue is it's overall diameter. A larger diameter tire will roll up over imperfections more readily. If you have ever used a wheelbarrow with small wheels versus larger wheels, you're likely very fimilar with this phenomenon.

Then there is the tire deformation that duckboy spoke of. A taller sidewall tire (softer tire) may conform to the shape of a road imperfection more than a stiffer shorter sidewall tire, reducing the tranlation of the horizontal force component into vertical motion. Again, I go back to the wheelbarrow analogy. It has been my experience that it will be harder to get a wheelbarrow to hop up, say a inch high step, when the tire pressureis low then when it is high. You have to push harder, in the horizontal direction, to get the wheelbarrow with low tire pressure to hop up onto the step. This means the horizontal force component tends to be higher for softer tires. The springs and struts basically only work in the vertical direction, hence then can only absorb/dampen forces/movement acting in that direction. It's up to the bushing and what not to quell the horizontal component.

I'm just rambling here. It will be interesting to see if others have theories as wacky as mine.

Last edited by Scottie; 02-22-2002 at 06:05 PM.
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Old 02-22-2002, 10:23 AM   #6
Simon Lines
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Hi guys

I would say you both have good points, as has been stated the suspension package on a modern saloon (or wagon) is a very complex thing, the designers have so many ways of influencing the feel.

You can change the type of bush, the stiffness, the way it deforms, the way it interacts with it’s mounting and the way the mounting interacts with the body itself. Then you (obviously) have the springs and dampers, they will have their designated rates but they will also have degrees of compliance too, some designed in, some that just happens to be there. The hub, bearings, CV, axle, brake calliper, brake disc, etc, etc, all have greater or lesser effects. Finally we come to the wheel and tyre, arguably the largest single item in the package.

I would say however, replacing bushes is a one way street to NVH, it starts small, maybe a few roll bar bushes, you think “aaa yea, it’s a bit louder but not much” before you know it you running all polly / ally and the NVH drive’s you up the wall on long trips.

Cheers

Simon
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Old 02-22-2002, 04:39 PM   #7
UCI_Scott
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By now most people reading this know my current set up.
  • STi/SPT everything except
  • ...stock springs and struts.
  • STi top mounts.
  • 18" P1s.
  • High (42psi) tire pressure.
I work on campus at UC Irvine and there is this intersection at Campus Road with a fairly sharp dip. Everyone crawls their cars through this dip. Everyone except me, that is. I simply pretend it's not there.

Passengers cringe and tense up as they notice I am not slowing, but then the soob just soaks up the dip and settles right back down. No undamped wallowing, no jolting out of your seat. Sure, you can feel the dip, but it honestly goes though it better than most cars do at half the speed. Passengers just kind of relax after the dip and don't freak out over any more of my driving quirks.

I've got these nice brand new DMS Golds sitting in my garage for six months but I still haven't felt like installing them.

--scott

P.S. Betfore I did anything to the suspension I had Bridgestone RE730 214/45 R17 on P1s. Later I went to Bridgestone RE730 225/40 R18 on P1s -- still before any suspension work. I don't think you can get much more controlled than that. I will take the 18" any day, zero reservations.

P.P.S. re: NVH: It doesn't seem to bother this 50-year-old, but YMMV.

P.P.P.S. I'm not saying, "Hey! Listen to me! Get 18" wheels!" Get whatever you want. The reason I keep posting is because there are not that many people waving the 18" flag on i-club and I just want my data point logged and I want to offer whatever qualifications I might have for my opinions.
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Old 02-22-2002, 06:34 PM   #8
Simon Lines
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It look's like i'm going to be in Irvine over the 2nd or 3rd weekend of March, wil anybody be about?
cheers
Simon
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Old 02-22-2002, 06:48 PM   #9
WagonMonster
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If you're there, I'll be there

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Old 02-22-2002, 07:17 PM   #10
duckboy
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simon

what's your reason for visiting irvine? are you going to talk w/ Eibach to get a suspension built and cure my suspension problems?

i'd come down to drop by if you're going to be in town.

duckboy (jud)
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Old 02-22-2002, 09:14 PM   #11
UCI_Scott
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Sure! Post here, we'll track you down.

Let's see now... Goat Hill Tavern, 132 beers on tap... Hmmmm...


--scott
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Old 02-23-2002, 05:24 AM   #12
scotto
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Simon,

Yep, just let us know more when you do.

UCI-Scott,

I thought I rembered you having the DMS 40's... I have heard great things about those.. again lately. Now I don't feel so bad having some odds and ends sitting around waiting to be installed
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Old 02-23-2002, 11:34 AM   #13
Zahnster
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Perhaps we need to arrange a SCIC meet in Irvine for 3/9 or 3/16....
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Old 02-27-2002, 04:37 PM   #14
tifosi77
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Why don't you guys come karting with us at Dromo ! that evening after you guys meet during the day, Jus a suggestion since Dromo 1 is probably 10-15 minutes from Irvine. Simon YOu are welcome to come as well if any of you are interested (including Simon) email me at tifosi77@hotmail.com or you can post on our thread in the Southern California Forum under SCIC Karting Day march 9th
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