Anyone know the height of the roll center and/or center of gravity at the front of a stock MGB, MGBGT-V8 or RV8? Thx

Jim

roll center of stock mgb in the front is at ground level.

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“Better to remain silent and be thought a fool than to speak out and remove all doubt”.
- Abraham Lincoln

"Anyone with an intense emotional interest in a subject loses the ability to observe it objectively: You selectively perceive events. You ignore data and facts that disagree with your main philosophy. Even your memory works to fool you, as you selectively retain what you believe in, and subtly mask any memories that might conflict."

Thanks Lloyd. But are you certain of this or repeating what you've heard? I was just looking at the extra MG ('75) and it looks to me like the lower control arms angle upward to the outside a bit. The instant center or intersection of the lower control arm and upper control arm would have to be at ground level for the roll center to be at ground level I believe. Of course I understand that the IC will move around with suspension travel so the roll center will too. I think that's pretty unavoidable. But I'd like to try to establish the length of the moment arm at static ride height as a starting point.

Jim

Jim.....on a stock MGB, the control arms are parallel to each other and to the ground....hence roll center at ground level. Any variance to that means weak/short springs etc.

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“Better to remain silent and be thought a fool than to speak out and remove all doubt”.
- Abraham Lincoln

"Anyone with an intense emotional interest in a subject loses the ability to observe it objectively: You selectively perceive events. You ignore data and facts that disagree with your main philosophy. Even your memory works to fool you, as you selectively retain what you believe in, and subtly mask any memories that might conflict."

In theory surely if both arms are parallel then the roll center would be somewhere between the wishbones in height and as they are symetrical in the center of the car width?



The roll center of an SLA suspension can be found by drawing a line from the center of the contact patch of the left tire to the instantaneous center for the left wheel (A) and a line from the center of the contact patch of the right tire to the instantaneous center for the right wheel (B). The roll center is located where these two lines cross. If the suspension is symmetrical the roll center will be located along the center line of the vehicle. If the suspension is not symmetrical the roll center location can be located to the left or right of center (depending on the geometry).

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in the MGB forum its a 1971 MGB - pretty standard unless the PO screwed it up. if I'm posting in the 1100 & 1300 forum its a 1965 MG 1100

here is a nice animated version

http://www.racecartuner.com/03/304.html

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in the MGB forum its a 1971 MGB - pretty standard unless the PO screwed it up. if I'm posting in the 1100 & 1300 forum its a 1965 MG 1100

Thanks. Without looking, just in my head, I was using the wrong contact patch so clearly that isn't right. If the arms truly are parallel then the IC would be midway between them at an infinite distance, making the roll center ever so slightly above the road surface I would think. However, I do not think body roll would move it side to side as in the animation due to the infinite distance, but I guess that would depend on how the control arms move. Thanks guys that really helps. Any idea how high the center of gravity is at the front? What I'd really like to know how how long the moment arm is.

Jim

Butch Gilbert says - lower a-arms being angled upward screws up your roll-center and goofs up your handling

His solution to this is to install a set of spring height adjusters like the stock cars use - you can find em at Speedway motors website

see also this discussion... lower front springs

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Maybe Dave Headley will weigh in.

God I love this sh*t. Seriously. Even on street cars, I am fascinated by this stuff. Thanks for asking the Q in the first place. Most owners don't care, but it is this type of detail that makes owning cars fun.

I have a set of tube shocks and I'm in the process of making a set of tubular uppers out of some 4130 leftover from the sprint car. I should be able to lower the inboard upper arm by about 1.75 inches.

I played around a bit with lowering the inboard pivots of the lower control arms but was not at all happy with the results, as it really affected the other suspension geometry in ways I did not think were helpful. In the end I put it back where it was to start with. I think it's really a pretty good suspension, though much maligned by those who believe that new is always better. I don't really agree with that theory, but I'm willing to listen. The one thing I was able to do that made a very significant improvement was to fit air bags in place of the stock springs. That gave a better ride, a nice progressive rate, and improved the handling at the same time. Not too often you can find that combination.

Jim

This question gets kicked around every 6 months or so....Dave must be on vacation, so I'll answer.

The laws of suspension dynamics are just that...laws...givens...they are not guesses or opinions.

The roll center on an IFS system with parallel arms that are also parallel to the ground is at ground level. Nowhere else.

The MGB front suspension system was put in production in 1947 and was a design copied from the GM 'knee action' of 1934. Not exactly new stuff here. When you lower the front on an MGB by using shorter springs...the top and bottom arms both slant up at the outer ends. The roll center is now below ground level....making the problem worse. A quick glance under an MGB to look at the arms will tell you how the car is setup....no mysteries there.

The rear roll center on an MGB with stock type suspension is at axle height. Draw a line from the front roll center to the rear roll center and you have the 'roll axis'. The car rotates around this roll axis when it turns into a corner. Think of a body rotating on a rotisserie. Since that line (roll axis) on an MGB slants down towards the front rather steeply on an MGB, much weight is transferred to the outside front wheel when cornering as a result of this rotating. This is dialed in understeer and other gimmics like big sway bars as example are needed to offset this.

Other factors of the front end setup are castor angle and anti-dive. The crossmember on an MGB is mounted in the chassis at about a 7 degree angle....giving about 7 degrees of castor with the resulting camber gain. This also gives about the same amount of anti-dive. When you fit 'castor wedges' to make the car easier to steer, you lose the camber gain and anti-dive that is built into the suspension. Not good things to lose. If you fit somesort of component that gives adjustable ride height, it won't change the underlying problem of the control arm angles. Moving the pickup points is the ONLY correct cure for this.

If you drive around at 5/10s...none of this matters very much and it is all about looks.

EDIT: I think it also must be said that any MGB lowered with no regard for fixing the problems that are induced by that lowering makes it just about looks. When I glance under an MGB that is sitting pretty low....just to see how they did it....and see all the goofed up geometry, I know that car is not capable of much. Bad geometry will make the car generate less cornering force (measured in fractions of a g) than a stock car with the same tires. Don't be fooled into thinking that a 'darty' car is really a 'responsive' car. If you never try to get to the limit......well enough said.

My 2 cents.....your opinion will no doubt differ...........

MORE EDIT: ....I should have read this through. Ever wonder why a 3/4" bar works good on one car and other cars might need a 7/8" bar to get the same result? You probably don't wonder about things like that....however, one big factor is the angle of the roll axis...steeper roll axis needs a bigger bar to offset this. Bigger bars usually are masking other problems.

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“Better to remain silent and be thought a fool than to speak out and remove all doubt”.
- Abraham Lincoln

"Anyone with an intense emotional interest in a subject loses the ability to observe it objectively: You selectively perceive events. You ignore data and facts that disagree with your main philosophy. Even your memory works to fool you, as you selectively retain what you believe in, and subtly mask any memories that might conflict."



Edited 3 time(s). Last edit at 05/04/2012 10:12AM by balloonfoot.

I just got back but I'll add my $.02 to Lloyds correct assesment. The only reasonable way to raise roll center is to lengthen the kingpin and raise the upper trunnion and angle the upper arm. This creates an intersection of the arm extension lines above the ground which raises the RC. You can read more at www.fast-mg.com, click the "Grip Tech" button.

At this point in the game I'm not so much about racing as I am about enjoying the overall experience of driving the car. Of course, give me a clear mountain road and things might get interesting real quick.

However, as a starting point here,let me say that I like the original handling characteristics of the CB MGB just fine, and aside from better tires, a little shock tuning, and a little stiffer sway bar it doesn't need much and what it does is mostly a matter of personal preference. Suspension dynamics may be based on laws, but every driver has a different way of interacting with those laws. That has a lot to do with how fast he is as a driver and he will not be equally fast in all environments.

I've played with the MGB suspension a bit over the decades, and there is really quite a lot you can to with it to better meet your individual preferences. I like wide tires especially well, which brings along an entirely new set of issues. But the basic geometry I'm OK with. It's a very forgiving car when driven fast, and it is quite a lot of fun when taken well beyond the limits of adhesion.

Now as you know, a number of MGBs have been converted to Jaguar IRS. Indeed, some have speculated that the car body was designed to be able to accommodate that suspension. I've done a couple of them myself and feel the results have been quite good. But whereas the stock suspension puts the rear roll center at the level of the spring hangers, the Jag, being IRS puts it somewhat lower I believe. The control arms are not parallel however, and depending on how you set the ride height the roll center can vary a good bit. I think it'd be cool to determine what ride height with the Jag corresponds most closely to the stock roll center position, if that is possible, and then come up with a range. The usual range seems to run from level half shafts to level lower control arms, though that certainly isn't definitive. I'd like to determine if the stock MGB roll center falls within that range.

Jim

all very interesting! i read Dave's grip article - (found it by googling "grip tek mg"

unfortunately, a lot of that SCCA modification stuff is irrelevant to we vintage racers in clubs such as CSRG and VARA-group-1,

- who's got a formula for improving a car that is basically required to have oem spec suspension ?? (- essentially for improved street use, AX or maybe rallye ??)

TIA

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I've done vintage legal suspensions for a number of vintage racers.

Not dealing with exact numbers, but it looks like the Jag rear roll center is about 5" above the ground. Which I would assume to mean that the MGB with Jag IRS will have somewhere around half the incline of the stock MGB in the roll axis. For those of you who are expert in setting up competition suspensions, what could one expect as the result of this change?

Jim