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Maximising weight distribution for handling

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Re: Maximising weight distribution for handling

Post by boost panda on Sat Dec 13, 2008 9:14 am

DT, as CTWG60 said the specific gravity of petrol is only aroun 74% that of water, so 70l of water would weight 70kg, but petrol would be a smidge over 50kg.

I was doing a vehicle body report this week, and found a good book in our engineering library. It's called Vehicle Body Engineering by J Pawlowsky, it's pretty old school from 1969 but the maths makes sense. Here's a stress distribution diagram that I was gonna use in my report but didn't in the end.

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As you can see it's an Austin A35 (old book!) with all the weights coming down, and all the reactions (mainly tyre contact patches) going up. You can calculate a rudimentary "corner weight" from this. Oh yeah, in the industry back then an occupant of the car was always 91kg (200lb in U.S.) , hence the multiples of 91 on the diagram.

The graph underneath the illustration shows the bending moment and shear force diagrams for the length of the car, and shows the areas of most stress etc.

The same could be done for a frontal profile of the car, showing the weight bias from nearside to offside too. Most (maybe all?) RHD cars have the engines on the O/S and gearbox on the N/S. Add a driver into the mix (O/S too) and it's obvious that the O/S suspension is under higher load than the N/S.

Hope this makes sense.
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Re: Maximising weight distribution for handling

Post by mrbeige on Fri Feb 06, 2009 1:46 pm

Had a very interesting conversation with a colleague at work (he does lurk on here - mdecock) about general handling and suspension setups, lowering etc etc. and I thought it would be good to add this onto this thread as it was about handling.

The conversation arose from having the geometry setup on the Golf VR by a company that have several race teams (RX7s, Alfas, and now an Aston), and I'm sure to some this is trying to teach grandma to suck eggs, but it's all about roll-points and centre of gravity.

Your roll point is easy to determine. If you draw a line from the wishbones until the cross each other that is your roll point. The centre of gravity is a little more difficult to calculate but is a little academic for the purpose of this discussion (I'm sure Mike (mdecock) could explain how this is one a better than me). The idea is to ensure that the roll point is above the centre of gravity, thus after you corner the car will return to a steady state (pendulum effect). If you have it the other way up, then you end up with a 'system' all out of balance and will tend to be unstable after cornering. By means of hastily drawn diagram please see below...

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Now, it's all very well and good lowering you car, but if you end up in the state of the second diagram then you'll end up with a car that handles like funk all on earth.

BUT....

In most cases a lower car will ultimately perform better on and off the track soooo, using adapters to extend down the bottom of the bottom ball joint and thus pushing the roll point up will always help matters for lowered cars. From what Mike was telling me Brunswick actually moves the wishbone mounting points up on their Alfas to get the same effect, which for us amateurs may be a little prohibitive!

Still, tis all interesting stuff..... Smile


Last edited by mrbeige on Fri Feb 06, 2009 2:16 pm; edited 1 time in total

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Re: Maximising weight distribution for handling

Post by junkie on Fri Feb 06, 2009 1:54 pm

Can you add some text to the pics Stu, im all confused.
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Re: Maximising weight distribution for handling

Post by mrbeige on Fri Feb 06, 2009 2:17 pm

junkie wrote:Can you add some text to the pics Stu, im all confused.
Added for carity. The grey bits are the wheels and the black bits are the wishbones Wink

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Re: Maximising weight distribution for handling

Post by boost panda on Sat Feb 07, 2009 2:25 am

Good stuff Stu! Exactly what I covered last semester too. I've got an awesome handout on Roll Centres, their theory and kinematic roll centre analysis. Unfortunately it's not available electronically, but i'm happy to type up / scan in if anyone's interested now that my two mofo projects are handed in - woohoo! Here's a taster:

The motion of a racing car body from side to side is not problematic. The movement will compress the springs and twist the the anti-roll bar if fitted. It will result in a very small reduction in the centre of gravity height and is easily limited to an acceptable level by the anti-roll bar. Movement in the vertical direction is much less desirable. If the car rises during a corner, called jacking, the centre of gravity clearly rises, weight transfer is increased, grip is reduced and the peak lateral acceleration that can be obtained is also reduced. If the car moves vertically downwards, then grounding is a problem. In cars with high downforce, vertical movement is very undesirable because it will alter the performance of the diffuser. We therefore need to know firstly where the roll centre is and how it moves around during roll.
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Re: Maximising weight distribution for handling

Post by mrbeige on Sat Feb 07, 2009 3:56 am

boost monkey wrote:<SNIP>The movement will compress the springs and twist the the anti-roll bar if fitted. It will result in a very small reduction in the centre of gravity height and is easily limited to an acceptable level by the anti-roll bar.

<SNIP>If the car rises during a corner, called jacking, the centre of gravity clearly rises, weight transfer is increased, grip is reduced and the peak lateral acceleration that can be obtained is also reduced.
So, the anti-roll bar maintains a lower centre of gravity and reduces 'jacking' and hence maintains grip, and I suppose helps to maintain the same level of grip no matter what corner you through the car into?

Jon, did they lecture you on how to calculate roll centres?

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Re: Maximising weight distribution for handling

Post by boost panda on Sun Feb 08, 2009 3:25 am

Yeah, my handout has the equations and calculations in but they are B I G, like pages long.

I can scan them in if you like though Smile
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Re: Maximising weight distribution for handling

Post by junkie on Sun Feb 08, 2009 3:28 am

I say go for it Jon.
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Re: Maximising weight distribution for handling

Post by mrbeige on Sun Feb 08, 2009 3:28 am

boost monkey wrote:Yeah, my handout has the equations and calculations in but they are B I G, like pages long.

I can scan them in if you like though Smile
If it's not too much bother, it'd be interesting to look at. Maths-mungus I'd imagine study

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Re: Maximising weight distribution for handling

Post by boost panda on Sun Feb 08, 2009 3:18 pm

Yeah no probs, I have morning lectures tomorrow but should be able to scan in the afternoon. I'll speak to the lecturer and see if he has an electronic copy that I can email you also Smile
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Re: Maximising weight distribution for handling

Post by kevhaywire on Tue Feb 10, 2009 4:19 am

mrbeige wrote:Your roll point is easy to determine. If you draw a line from the wishbones until the cross each other that is your roll point. The centre of gravity is a little more difficult to calculate but is a little academic for the purpose of this discussion (I'm sure Mike (mdecock) could explain how this is one a better than me). The idea is to ensure that the roll point is above the centre of gravity, thus after you corner the car will return to a steady state (pendulum effect). If you have it the other way up, then you end up with a 'system' all out of balance and will tend to be unstable after cornering. By means of hastily drawn diagram please see below...

Can't see the image (damn photobucket and work Mad ) but I've been banging on about Roll centres on the Rado forum for ages, but it mostly falls onto cloth ears. Laughing

Basically, all these numpties with slammed Corrados think they handle great, with their roll centre's completely f'cked. What they think is good handling is purely the result of stiffer springs. The springs' primary role is to hold the car up, so stiffening them obviously reduces body roll and a lot of people think that a lack of body roll constitutes good handling, but there's far more to it than that.

Once a car leans into a roll, the loaded struts need some travel in them to absorb bumps and cope with camber changes etc etc. What usually happens with slammed VWs is the suspension probably has about an inch of travel (aswell as the wishbones pointing at the sky!) to begin with and when they enter a turn and load the strut, the only "give" comes from the bump stops, which in severe cases, can deflect the car off line should you hit a bump. And not only that, a loaded strut bends (and is why you don't see many aluminium dampers), so if your loaded strut only has the bump stops to call on for travel and you have zero wishbone arc left, guess what has to take the brunt of all the forces? Yep, the piston rods.

BTCC cars are largely to blame. They bounce and skip round corners and have the wheels buried in the arches, so people think it's cool and like to copy it. BTCC cars have no damper droop on purpose and, but you can't do that on a road car.

But you can't tell people that. So long as the arch to tyre gap is aesthetically pleasing, to hell with the consequences....

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Re: Maximising weight distribution for handling

Post by ctwg60 on Tue Feb 10, 2009 9:45 am

I agree with the above.

One of the few thing I haven't changed on my car is the suspension. I have new VAG struts and bushes purely because I wanted to see what new Corrado suspension was like and they handle the extra power and torque in corners just fine providing lots of feed back and control. I'd only change the roll-bars in the future and maybe stiffen up the rear end to stop the front end lifting on hard launches, purely for QM times.

Most people think lowered suspension is great when they fit it, what they fail to realise is that it's replacing 20 year old knackered stuff!

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Re: Maximising weight distribution for handling

Post by mic_VR on Tue Feb 10, 2009 11:06 am

So what's the answer then Kev? Is it possible to either raise the position of the top mount and lower the wishbones, or if you want it to really handle without major mods is it a case of getting a good set of coilovers but be a bit more sensible on the actual level of drop-age?

On another note, skyward wishbones will wear the hell out of your steering rack. Believe me, I've seen it!
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Re: Maximising weight distribution for handling

Post by mrbeige on Tue Feb 10, 2009 11:25 am

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^^^^^ That's what you want Mic. points the wishbones the right way and gives even more adjustment. Allows you to lower the centre of gravity, whilst maintaining handling Smile

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Re: Maximising weight distribution for handling

Post by kevhaywire on Wed Feb 11, 2009 1:56 am

ctwg60 wrote:Most people think lowered suspension is great when they fit it, what they fail to realise is that it's replacing 20 year old knackered stuff!

Yep! Any cheap old coilover kit is better than 20 yr old knackered stuff! Very Happy Well, for the 5 minutes the cheap kit will last at least Laughing

Mic_VR wrote:So what's the answer then Kev? Is it possible to either raise the position of the top mount and lower the wishbones, or if you want it to really handle without major mods is it a case of getting a good set of coilovers but be a bit more sensible on the actual level of drop-age?

What Beigey posted! You need to drop the wishbone and tie rods down to match the reduction in ride height, thereby maintaining the proper roll centre Smile But that stuff won't last long on a daily road car. The rose joints you'll get 2K out of, tops. You can get rubber covers for them, but that type of bearing just isn't designed to last a long time.

The better, more permanent solution would be to fit the running gear from the R32 or TT. They run low and have revised hubs and wishbones to suit. That way you can use normal ball joints and bushes and maintain the correct WB angle, which will last for years.

Or the other, less fashionable solution would be to stick to the standard ride height. The VR6 rides as high as it does for a reason. That reason being VW know more about Roll centres than owners of trailored show queen specials Very Happy

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Re: Maximising weight distribution for handling

Post by boost panda on Wed Feb 11, 2009 11:52 am

Laughing quality posts guys!

Sorry about the lack of scannage on this topic, my suspension design/CAD seminar was cancelled on Tuesday avo, but I'll see the matey again tomorrow. I'll go scan the pages in now (although the whole book is a good read, so I'll try to get an electronic copy asap).

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Re: Maximising weight distribution for handling

Post by boost panda on Thu Feb 12, 2009 6:50 am

Lost my roll centre handout from which I quoted earlier, and I need it for a lecture this afternoon! When I find it, I will scan that bad boy.

I am pure awesome.
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Re: Maximising weight distribution for handling

Post by mrbeige on Thu Feb 12, 2009 7:49 am

boost monkey wrote:Lost my roll centre handout from which I quoted earlier, and I need it for a lecture this afternoon! When I find it, I will scan that bad boy.

I am pure awesome.
What are you like Rolling Eyes

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Re: Maximising weight distribution for handling

Post by boost panda on Fri Feb 13, 2009 1:16 pm

Forgetful!! I think maybe one of my enthusiast housemates has wandered off with it... Kuh... Rolling Eyes
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Re: Maximising weight distribution for handling

Post by davidwort on Mon Apr 06, 2009 2:32 pm

I've noticed, over the years, that VW tend to lower 'sport' or 'performance' models by about 15mm, this seems to be fairly consistent across many of the VW family of chassis.
So would this generally be the sort of amount you can lower without really affecting the handling in a detrimental way and lowering the centre of gravity a little to help handling? or is it more of a cosmetic thing?
Presumably the standard ride height of the 'average' model is how they were designed for optimum handling (with a lot of maths and development!)

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Re: Maximising weight distribution for handling

Post by boost panda on Mon Apr 06, 2009 3:44 pm

Handling is always a trade off with things like tyre wear and comfort, to name just two.

I wouldn't say the average car was designed for optimum handling personally. Optimum comfort, perhaps?

And yeah, there's a lot of maths involved! Trying to get my head round it for my design project (STILL haven't found that handout ^^^ and i need it really bad!)
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Re: Maximising weight distribution for handling

Post by kevhaywire on Tue Apr 07, 2009 12:56 am

I changed the brake pads on my gf's Polo GTI last week, which is totally standard, and I noticed that the wishbones on that are only just below parallel at the outboard end from the factory. But you can get your fist between the tyre and arch! Seems they don't give you a lot of scope for lowering at all!

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Re: Maximising weight distribution for handling

Post by davidwort on Tue Apr 07, 2009 1:25 am

most talk tends to be of the front wishbones, but I found it interesting that one mk4 golf in Performance VW had had major rear end fabrication to prevent the rear wheels moving forward in the arch as the rear beam goes past horizontal and the wheel is pulled forward as the beam rotates on it's mountings.
That issue on lowered standard cars almost seems worse than the front wishbone thing. different setup on 4WD or mk5/6 golfs obviously as they have the independent rear susp.

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Re: Maximising weight distribution for handling

Post by mrbeige on Tue Apr 07, 2009 2:59 am

So, could you convert from the beamed rear, to that of say a synchro golf, on a Corrado to improve things at the rear?

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Re: Maximising weight distribution for handling

Post by davidwort on Tue Apr 07, 2009 6:09 am

mmm, not really, I think you'd need the synchro rear floorplan and probably the rear driveshafts to keep the wheel bearings in place lol

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