MKII Suspension Analysis: Front Suspension - Page 3 - MR2 Owners Club Message Board
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post #41 of 98 (permalink) Old February 11th, 2009, 00:59 Thread Starter
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After many hours spent googling and reading I have come to the conclusion that I have no idea if this would be beneficial or not...

One one hand, I have read a lot that says that kinematic roll centers (which I am able to calculate with this program) arn't really worth much (something Randy had suggested earlier). I have read that lateral roll center movement isn't something to worry about (although I doubt they are considering movement an order of magnitude larger than the wheel base when they say that). On the other hand, most of my research indicates that they are still worth SOMETHING, conceptually if nothing else.

Now on a car lowered 1" my current model shows a rear roll center about 1" above ground level, and a front roll center very slightly below ground level. Adjusting the front roll center only, as I suggested above, results in the front roll center being about 2" above ground level. This gives us a roll axis that points up toward the front of the car.

Now we run into the question of the roll axis. It seems VERY rare in my research to find a roll axis that points up toward the front of the car (although a presentation by Chrysler indicates an "Acura" (no model specified) with this condition. I wonder if maybe the Acura with the higher front roll center is the NSX? Its the only Acura I can think of that is significantly different overall configuration than the various Hondas, Toyotas, Fords, ect that they have data for. Just a wild guess there. Most of my research indicates that an upward angled roll axis like this will increase understeer, such as this http://pdv-motorsports.com/Mark%20Or...nclination.pdf

One other thing to note, changing the front roll center like this not only drasticaly reduces the magnitude of its lateral motion, but also makes it move in the same direction as the rear, rather than spearing off in the opposite direction. Also, they now both move in the same direction they do on a stock car, although they move quite a lot more.

In addition to the pages linked to above I found a lot of seemingly useful information / discussion on http://www.eng-tips.com/. I may try posting a version of this post there tomorrow to see if I can get any input from that knowledge base.

Last edited by Alex W; March 10th, 2009 at 18:08.
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post #42 of 98 (permalink) Old February 11th, 2009, 05:34
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You're confusing pitch axis with roll axis. The line connecting your front and rear roll centres is the roll axis, not the pitch axis. A pitch centre is found from your side view kinematics, and it has similar effects to a roll centre but in pitch rather than roll, typically a pitch axis will run parallel with the vehicle's axles unless you have a assymetric suspension set up.

Anyway, technicalities aside - inclined roll axis
Yes, inclining the roll axis so it's higher at the front will stiffen that end and hence reduce grip on that end in cornering, however that is assuming the car is balanced with a horizontal axis. What this doesn't consider is that the roll stiffness can then be shifted back toward the rear by stiffening the rear springs/arb or softening the front. Because ride & pitch stiffnesses are controlled by the springs, this can allow you to soften the front springs if you wish to reduce stiffness in pitch and/or ride.

The above assumes steady state cornering, something else that's worth considering is the effect in a transient situation. Weight transfer of the sprung mass can be split into 2 - the geometric weight transfer and the elastic weight transfer.

The geometric weight transfer can also be considered kinematic weight transfer, and is that caused by laterally accelerating the roll centre by applying forces at the contact patch- a simple free body diagram shows that with an above ground roll centre you get geometric weight transfer to the outside wheel, with ground-level roll centre you get no geometric weight transfer and with a below-ground roll centre you get geometric weight transfer toward the inside wheel.

Elastic weight transfer is weight transfer due to the centre of gravity being accelerated by a force applied at the roll centre. Except for the perculiar case of the roll centre being above the centre of gravity, the elastic weight transfer is always toward the outside wheel. The best way to think of elastic weight transfer is that it is weight transfer that is transmitted through the springs.

Now, it's important to note that the total weight transfer never changes (with constant track width, centre of gravity position, and lateral acceleration), but the ratio of geometric to elastic changes with roll centre height. The important thing about this is that geometric weight transfer occurs instantly, whereas elastic weight transfer occurs over a finite time period, because the force is being transfered via the springs - think about dropping a weight onto a spring that is sitting on a scale, the full force of the weight wont be seen until the spring has compressed as far as is required to support the weight.

Anyway, Claude Rouelle teaches in his seminars that weight transfer is a bad thing, but it's a bad thing that has to happen (furthermore, it has to happen at the front first). Having a high geometric roll stiffness (high roll centre) gives you good response but at the expense of stability. Claude suggests that generally your roll-stiffness distribution should be further forward than your weight distribution (ie, if weight is 45%/55% then stiffness should be 48%/52% F/R), so that the front reacts quickly to steering input.

Perhaps having your roll axis inclined upwards at the front will give you faster response at the front, while allowing you to have a car which is balanced mid corner?

Anyway, lots of stuff to think about, and not a lot of answers - hope it helps!
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post #43 of 98 (permalink) Old February 11th, 2009, 09:56 Thread Starter
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Quote:
Originally Posted by Driftin_AW
Anyway, lots of stuff to think about, and not a lot of answers - hope it helps!
Actually, I found that very insightful. Hopefully we can kickstart a good discussion here. You are right, I had swapped pitch and roll in my post (I did mean and was talking about roll, I'm glad you were able to figure that out). Thats what comes of posting at midnight after a couple of 11 or 12 hour work days.

Geometric weight transfer vs elastic weight transfer was another item that I had read about. Essentially, this change would increase geometric transfer (instant transfer) and reduce elastic transfer. I suppose this could be a good thing, if it provided better turn in while also improving mid corner grip by reducing body roll. But I also envision an alternate scenario where that instant weight transfer "instantly" overpowers the outside front tire, causing understeer.

Going back to earlier discussion about cars with the tendency to lift a wheel. Now that I think about it in these terms, thats probably a case of too much elastic transfer, not enough geometric transfer.
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post #44 of 98 (permalink) Old February 11th, 2009, 14:46
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I feel like I should prefix anything that follows by pointing out that I'm by no means an expert, like yourself I just have FSAE experience (although I've been in a team for 4 years now ), most of what I 'know' comes from passed on knowledge that may or may not be correct.

Anyway, I feel that increasing your roll centre height may have the opposite effect if you're trying to prevent a wheel lifting. Above-ground roll centres create a 'jacking effect' on the chassis - that is that the line of force from contact patch to roll centre points upward and therefore has a vertical component as well as a horizontal one. While the effect tends to be reasonably small, it is still present and something worth bearing in mind. I have more thoughts on this, but I'm currently working (well, supposed to be working), so I should cut this short for now.
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post #45 of 98 (permalink) Old February 15th, 2009, 05:03
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So, my extended thoughts on lifting a wheel.

What causes a wheel to lift? The way I see it there's 3 mechanisms
1 - Excessive chassis roll causes the suspension to reach a point at which there is no longer enough droop available in the suspension to allow the wheel to remain grounded
2 - droop is being artificially limited to be less than the shock allows. This could be due to the effect of an anti-roll bar providing 'negative springing' (ie springing that is resisting droop). Preloading the springs would have some effect on this too, as the amount of suspension deflection statically would be reduced (as in, the amount of deflection from full droop to statically loaded with the vehicle mass), allowing less droop from the static position
3 - I would imagine this would never be the case in an mr2 on road tyres: lateral acceleration reaching a point where the overturning moment created due to centipedal acceleration of the centre of mass cannot be counteracted by the moment created by the gravitational force on the centre of mass.

So, number 1 can be fixed by virtually any mechanism that decreases the roll gradient - stiffer springs, higher roll centres, lower centre of gravity, all of them having associated drawbacks that remind you how much fun it is playing the comprise game on suspension design

Number 2 can be easily fixed too, by reducing the rate of the ARB, unfortunately this is going to create more of number 1, and I'm not sure what the relationships would be to know whether less bar is going to make the car more or less likely to pick up a wheel, but my bet is that it would be less likely.

Now the last thing to decide is whether picking up a wheel is even that big of a problem. It seems it's often almost inescapable on some vehicles where you need a certain amount of roll resistance at one end of the car to create neutral handling (or whatever handling characteristics you happen to desire), and if the roll stiffness distribution is significantly biased from the static weight distribution then one end is going to be doing a disproportionate amount of the work, and it is then almost inevitable that it'll pick up.

Maybe another way to approach it would be to look at tyre choices that provide a better balance that enables you to run a roll stiffness distribution that is more closely matched to the weight distribution? I really don't know.
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post #46 of 98 (permalink) Old February 15th, 2009, 05:17
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Oh I also forgot - caster. Caster effectively causes the chassis to lean toward the front outside tyre, and tends to lift the inside rear
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post #47 of 98 (permalink) Old February 19th, 2009, 13:42 Thread Starter
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I found this post on the Grassroots Motorsports forum in a thread about McPherson strut roll centers:

Quote:
Originally Posted by Tyler H
My experience with coilovers has been that they require almost constant tweaking. Take your base guess at a baseline and start driving. Then tune them where they feel best to you. On an MR2, the front is about 1" higher than the back. If you adjust it level, then the front roll center is messed up and the car understeers worse. I didn't give the roll center much thought until I experienced this first hand.
When I asked for more info regarding the actual handling improvements and explained my findings. His responce was:

Quote:
Originally Posted by Tyler H
I'm no suspension expert, but here is my seat-of-the-pants experience. I initially set the coilover ride height (via shortening the length of the whole assembly) to put the car at about 1.5" lower than stock, and at an even ride height. I assumed that the OEM rear springs were sagging with age and the added weight of the Gen3 swap, hence the rear was lower than the front and I 'corrected' it.

The car handled well under most situations, but seemed to have less mechanical grip in the front at the limit, no matter how I set it up.

I was complaining about this with one of my autox buddies who has a lot of MR2 seat time and he explained the roll center thing with these cars and told me to jack the front of the car up. I used the spring perch adjustiment to raise the car ~.75~1" in the front and noticed an immediate improvement. It would grip better in sweepers and I could get on the gas sooner coming out. Once on the gas, the car also kept a tighter line. (BTW, this car had about 300whp.)
From the sounds of it this may be a very nice improvement. It does sound almost too good to be true (better turn in, better mid corner grip, able to put down power better), but it does sound like its worth a try.
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post #48 of 98 (permalink) Old February 19th, 2009, 14:04
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Quote:
Originally Posted by Driftin_AW
So, my extended thoughts on lifting a wheel.

What causes a wheel to lift? The way I see it there's 3 mechanisms
Lifting a wheel just happens when an axle is applying all of the roll moment that it can. Consider a two-axle analogy: if a big giant bus turns hard and flips over, both axles have applied all of the roll moment that they are able to, and it is not enough to resist the roll moment applied by lateral acceleration. When one wheel lifts, that axle has applied all the roll moment that it's able to, and any increase in lateral load transfer must be applied to the other axle.
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post #49 of 98 (permalink) Old February 20th, 2009, 00:02
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Quote:
Originally Posted by bentheswift
Lifting a wheel just happens when an axle is applying all of the roll moment that it can. Consider a two-axle analogy: if a big giant bus turns hard and flips over, both axles have applied all of the roll moment that they are able to, and it is not enough to resist the roll moment applied by lateral acceleration. When one wheel lifts, that axle has applied all the roll moment that it's able to, and any increase in lateral load transfer must be applied to the other axle.
Yes I realise that lifting a wheel occurs when an axle has applied all of the roll resistance that it can, I guess I was trying to make suggestions as to things that will limit/effect the amount that an axle can apply, which is the important part if you're trying to design to prevent/reduce wheel lifting.
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post #50 of 98 (permalink) Old February 20th, 2009, 15:27
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Quote:
Originally Posted by Alex W
From the sounds of it this may be a very nice improvement. It does sound almost too good to be true (better turn in, better mid corner grip, able to put down power better), but it does sound like its worth a try.
There could be lots of things at play here. It's really hard to isolate something like roll center and talk about only it's effects.

For example, let's say in Tyler's case he had -2 deg of static camber and a 1.5" drop. Most likely at that point, any additional bump travel is going to cause a LOSS of camber, so when he leaned into a corner the tire most likely went positive relative to the ground.

He raises the car back up 1", now he's probably got slightly less static camber, maybe -1.5 deg, but as the car rolls, it's actually GAINING camber for the first inch, then starts losing it, so if he rolls over the same amount he's probably got a better contact patch than he had in the first place, because the wheel is probably closer to zero camber relative to the ground. The front of the car will probably stick better.

Did he change roll center height, yes. Did that alone help his cause, maybe. But one could argue that just as important (maybe more) was the fact that he got a better contact patch when he was leaned over, and that made the front end stick better. This is just an example, and it's possible he already had dialed in ideal camber when leaned over--just trying to illustrate some of the contributing factors.

Another thing to note is that what he really did was change RIDE HEIGHT and the result was a change in roll center height (and CG height). If he used RCA's and tie rod spacers it would be a better test case for arguing the effect of roll center height because CG/ride height could be held constant.

It all goes back to the idea that it's really hard, especially on a strut suspension, to change only one thing without a bunch of other things changing.

I do think, Alex, that raising the roll center at the front of the car would help. Most springs/coilovers are too soft in the front relative to the rear rates. I'm sure there are things I'm not thinking of (or don't understand), but raising the front roll center will increase the roll stiffness and probably make the car work better. It will also give a better camber curve on a lowered car.
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post #51 of 98 (permalink) Old February 20th, 2009, 17:27 Thread Starter
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Quote:
Originally Posted by rnoll98
I do think, Alex, that raising the roll center at the front of the car would help. Most springs/coilovers are too soft in the front relative to the rear rates. I'm sure there are things I'm not thinking of (or don't understand), but raising the front roll center will increase the roll stiffness and probably make the car work better. It will also give a better camber curve on a lowered car.
As always, your input is appreciated. You are right, there is a lot more going on in tylers case, and a lot we don't know, it just happened to be one of the pieces of data I was able to find, and all the better that it was someone else with an MR2. Searching google for data on roll centers gives you tons of info for circle track cars though.

Unfortunately I don't think we will know anything for sure untill someone takes their car and runs it back to back with and without roll center adjusters / tie rod spacers and with no other changes made. However, sofar I think theory is pointing to it being a positive change.
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post #52 of 98 (permalink) Old February 25th, 2009, 12:14
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Randy, You said you moved the top of the strut inward 2" to 3".

I'm not sure if I'm even close here but, wouldn't it be more beneficial to lengthen the lca 2" to 3"?

Wouldn't it have the same effect? Only easier, cheaper, and it would give you a wider stance.

Like I said maybe I'm just plain wrong.


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Last edited by Turbomits95; February 25th, 2009 at 12:21.
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post #53 of 98 (permalink) Old February 26th, 2009, 12:33
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In autox, narrower is better, much better. Making the car 4-6" wider is very bad.

Otherwise, what you are saying is mostly correct, though if you lengthen the LCA you may also need to add more caster to compensate for the same increase of SAI that I experienced.
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post #54 of 98 (permalink) Old February 26th, 2009, 16:35
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Ah, ok. That makes sense.
Yeah, I would need an after market tension rod and longer tie rod.

Hmm, this is gives me an idea.
A longer lca with a built in bump steer spacer. For us road racers.

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post #55 of 98 (permalink) Old February 26th, 2009, 16:53 Thread Starter
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You can't build the bumpsteer spacer into the LCA, as that won't move the ball joint. You have to space the ball joint down relative to the hub.
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post #56 of 98 (permalink) Old February 26th, 2009, 17:01
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Quote:
Originally Posted by rnoll98
In autox, narrower is better, much better. Making the car 4-6" wider is very bad.
We were all surprised how much we gained by taking an inch out of each track of our '08 SAE car. The new car was about 10% down on power and was running a full second quicker than the old car on a sub-30-second course, both with the same competition driver. It's like getting free lateral acceleration.
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post #57 of 98 (permalink) Old February 27th, 2009, 15:58
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Quote:
Originally Posted by bentheswift
We were all surprised how much we gained by taking an inch out of each track of our '08 SAE car. The new car was about 10% down on power and was running a full second quicker than the old car on a sub-30-second course, both with the same competition driver. It's like getting free lateral acceleration.
Another way to imagine this. If your car is 6" wider, it's like offsetting the cones in a slalom that far the 'hard' way. Road racers don't have this issue because they have so much more space between turns.
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post #58 of 98 (permalink) Old February 27th, 2009, 16:17
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And that's a big reason why your average FSAE car and driver will set FTD at any local autocross; slaloms for street cars are straightaways for SAE cars.

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post #59 of 98 (permalink) Old February 27th, 2009, 16:23
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When we were trying to decide on track width for our new FSAE car, I did some quick & dirty calcs in Matlab for a steady state corner with varied track widths, basically just seeing whether the benefits of the reduced weight transfer from a wide track outweight the benefits of making corners effectively a larger radius by having a narrower car. In a steady state corner it showed that wider was better, but I haven't yet been able to work out a good way to model the effects on something like a slalom. Regardless, I went with my gut and decided to reduce the tracks by a reasonably significant amount, so it's good to hear some anecdotal support for this
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post #60 of 98 (permalink) Old February 27th, 2009, 17:53
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Edit: Nevermind. I thought the ball joint bolted in the other way.

Last edited by Turbomits95; February 27th, 2009 at 18:15.
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