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MadManMike

By MadManMike
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george_seamons Too Much Spare Time

george_seamons

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I'll stick to private work at the minute...I prefer pocket money + wages to just wages. I love where I work right now, so not in a massive rush to jump ship.

You're right though, its all about customer base. I'm slowly building mine up...got several customers who I do repeat work for. Only problem being, the Leon doesn't make a very good work van, haha!

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AndrewEH1 Trials Master

AndrewEH1

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Well got the new toy today not quite the r but really happy with it generally nice car to be in and to drive.

...and that's meant to save you money??!!? :wink:

Looks good, well jelly!

Need a new car the A-Class is okay but not exactly sporty...once everything is sorted in the next month I'll be looking for a nice cheapish MX5

Edited by AndrewEH1
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Greetings Post Whore

Greetings

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Bloody hell, no wonder your set-up always looks crashy, 2cm of negative travel's nothing most single-seaters will have more than that, for a tin-top driving on rough surfaces that's really not enough. In answer to your question though, unless you're rolling enough to run out of travel then it's not going to make any difference. If you are reaching the end of your travel through just roll, (which is something that shouldn't really be happening) then yes, you will get more roll with more travel. To reduce the roll you either need stiffer spring rates/shocks (shocks will only really reduce roll on turn-in/direction change, not in the middle of a long corner), stiffer anti-roll bars, lower the center of gravity, or raise the roll-center. Unfortunately other than lowering the center of gravity, they all have their disadvantages.

Cheers! I'm not sure how much negative travel the car has now, it's going to be around 4-5cm at least, I specifically set it up to have as much travel as possible. Just wondered whether it wasn't too much. The car rolls way too much in my opinion, especially considering it as very thick ARB's set at the hardest setting.

I don't think I'm reaching the end of my travel through roll, so presumably my idea won't work? With my current suspension I can increase front spring preload which should give a stiffer rate, and at the same time drop the car so the ride height is not effected. Wouldn't that help? I need new rear springs, going to go with harder ones this time. Current ones were 90N/mm, now I think they're at most 80N/mm. Getting 110N/mm. I think my rolling could partly have something to do with completely shot rear springs. Also, with stiffer ones, I could drop the car on the rear by at least 30mm which would address your point of lowering the centre of gravity.

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greg1040 Trials King

greg1040

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...and that's meant to save you money??!!? :wink:

Looks good! Well jelly!

Need a new car the A-Class is okay but not exactly sporty...once everything is sorted in the next month I'll be looking for a nice cheapish MX5

Well saves the monthly payments so can't complain and petrol the r happily drunk £10 a day lol

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RobinJI Too Much Spare Time

RobinJI

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Cheers! I'm not sure how much negative travel the car has now, it's going to be around 4-5cm at least, I specifically set it up to have as much travel as possible. Just wondered whether it wasn't too much. The car rolls way too much in my opinion, especially considering it as very thick ARB's set at the hardest setting.

I don't think I'm reaching the end of my travel through roll, so presumably my idea won't work? With my current suspension I can increase front spring preload which should give a stiffer rate, and at the same time drop the car so the ride height is not effected. Wouldn't that help? I need new rear springs, going to go with harder ones this time. Current ones were 90N/mm, now I think they're at most 80N/mm. Getting 110N/mm. I think my rolling could partly have something to do with completely shot rear springs. Also, with stiffer ones, I could drop the car on the rear by at least 30mm which would address your point of lowering the centre of gravity.

You see, this is where it gets more complex/awkward. Firstly, adjusting the 'preload' on your coilovers will not make the suspension stiffer at all. All it will do is set where the suspension sits within its travel at ride height, the spring is still the same rate, and will act as such. As the cars weight is already compressing the spring to its ride height, adjusting the spring-perch up will simply move the whole car up. If you then adjust the strut to be shorter to counteract this, the car will simply move back down to where it was, nothing to do with the spring has changed at all, you've just moved the damper down. With these sort of set-ups, the best bet tends to be to aim to set the dampers height so you've got 1/2 to 2/3rds of the travel as positive travel, and 1/2 to 1/3rd as negative (when at ride height.) What you're doing isn't actually preloading, it's simply moving the spring around relative to the damper.

In it's true sense, preload is when the spring is adjusted to the point where the damper/droop-stop is holding the spring compressed even at ride height, so you've adjusted to the point where you've got no negative travel at all, and then kept going to compress the spring. The name's literal, the spring is pre-loaded, in that it's loaded before it starts its travel. The only reason some race-cars are set-up like this is down to high levels of aerodynamic downforce meaning that when it gets up to running speed and the downforce is added to the loading the suspension ends up in the middle of its travel. Kind of like you set sag on bike suspension, you set it for the ride height in use with the riders weight, rather than unloaded.

Also in terms of dropping the car to reduce roll. Ignoring external factors, lowering the center of gravity is the best way to reduce roll, because a lower CofG will mean less weight transfer, however if the suspension's designed to run higher, then you'll often drop the roll-center by more than you've dropped the height of the car, which will actually result in more body-roll, as well as some iffy handling characteristics, so lower definitely isn't always better. Whether this is the case with an E36's back end I don't know, some cars are fine with low ride heights, but some are really badly effected.

It is a bit of a minefield, but I'd definitely start by setting your dampers to be sat somewhere near the middle of their travel at ride height. It would also be worth having a look at what spring rates tarmac rally cars run. Yours do sound in the right ball-park to me, but I'm no BMW expert. You need everything working properly at being suspension before you can really know the best way to correct the body roll.

Edited by RobinJI
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Greetings Post Whore

Greetings

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You certainly know your shit :P Your initial explanation about preload/ride height goes slightly against what I assumed by trying to work it out logically but on second thought it makes sense to me. So as long as the spring stays preloaded, there will be no difference between reducing spring preload by say 1" and dropping the car by 1" without touching spring preload?

Also in terms of dropping the car to reduce roll. Ignoring external factors, lowering the center of gravity is the best way to reduce roll, because a lower CofG will mean less weight transfer, however if the suspension's designed to run higher, then you'll often drop the roll-center by more than you've dropped the height of the car, which will actually result in more body-roll, as well as some iffy handling characteristics, so lower definitely isn't always better. Whether this is the case with an E36's back end I don't know, some cars are fine with low ride heights, but some are really badly effected.

This is the only part I don't understand. Can you explain the difference between roll centre and the centre of gravity?

I'll be looking into some proper suspension later this year, maybe in the winter so long as my current dampers don't fall apart (SO happy I managed to get hold of some springs because it was looking pretty bleak for a moment!). I'm sure Ohlins or any other high-end manufacturer will be able to work out the spring rates based on the load of each wheel and the application of the vehicle.

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AdamR28 Post Whore

AdamR28

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The roll centre is related to the angles of the wishbones, and the intersections of where they would cross if you extended their length.

Roll couple is the distance between the roll centre and the centre to gravity. This value increases when you lower the car (broad statement...).

A higher roll couple means there is more roll in the car when the cornering forces act through the centre of gravity. So, lowering the car often gives more body roll if everything else stays the same. However, most people will fit stiffer suspension or anti roll bars at the same time, so they don't notice it.

This does make the car stiffer (loose term) though, obviously, which actually reduces the amount of grip across an axle during cornering (again if everything else is the same - in reality you'll add camber, sticky tyres etc). Too soft a car doesn't give enough feedback to the driver though and it feels all sloppy, so there is always a balance to strike...

I'm on my phone so it's a bit awkward, but I'll try and find a link with diagrams explaining roll centre and couple.

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AdamR28 Post Whore

AdamR28

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Pic 1. http://photos.motoiq.com/MotoIQ/Tech/The-Ultimate-Guide-To/i-xgj2s9j/0/O/roll_couple.gif

Pic 2.. http://image.modified.com/f/17363212%20w750%20st0/0508_sccp_02_z_%20suspension%20roll_center.jpg

You also need to consider the front and real roll centres and couples in relation to each other if you want to maintain 'nice' 'handling'. They are both in inverted commas because they are fairly subjective, but the general consensus is that a small account of corner entry understeer (which is a stable condition, therefore giving you the best chance of making the apex cleanly without any drama), followed by a gradual shift to slight oversteer on exit (allows you to wind lock off to reduce tyre scrub, and means you can get power on rather than having to take it off to quell understeer) is fastest.

This is achieved by having a slightly higher roll couple at the front (I think that's the right end... Robin?!), which of course all sports car manufacturers know and build into the car, so by us messing about with it we are usually undoing their years of testing :P

As always there is more to it, but it's worth reading up on if you are serious about getting the best out of your car. Took me quite while to get my car sorted but it's completely different now just from a few relatively minor and inexpensive tweaks (tyre sizes mainly!).

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