Setting Corner weights - advice needed

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davidimurray

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posted on 27/3/16 at 06:24 PM

Setting Corner weights - advice needed

I am having a go at getting the car setup ready for the summer so first thing on the list is corner weighting. I haven't got any fancy gear so I am going for the locost bathroom scale setup. I know it is not perfect but I am hoping it will be better than current setup (just ride heights set)

Car is a Haynes roadster in 'wet' trim. With no driver the weights were -
LF 148kg
RF 127kg
LR 137kg
RR 164kg
Total 576kg

I then added 80kg in the drivers seat and the weight were then -
LF 154kg
RF 143kg
LR 157kg
RR 201kg

Now I've been making a number of adjustments but I am not seeing any significant change at the wheel. If i lightly press on the wheel the scales move so they are ok. THe adjustments so far are -
LF and RF + 10mm of peload added on shocks
RR -10mm preload
LR-5mm preload
and the result is
LF 157kg
RF 142kg
LR 154kg
RR 208kg

I am thinking that reducing the spring preload should reduce the weight on that wheel and conversely increasing should increase weight on the wheel.

I appreciate that is an over simplification as all 4 interact but I am surprised to see hardy no change after such preloadchanges.

Any thoughts/help greatly appreciated!

[Edited on 27/3/16 by davidimurray]

mark chandler

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posted on 27/3/16 at 06:35 PM

Think of it like a 4 legged table, jacking up just the front will increase front ride height, not the weight balance so all you have done is add 5mm to the Right Rear.

You add diagnoals so:

LF 157kg + RR 208kg = 366
RF 142kg + LR 154kg = 296

Try twiddling up RF & LR by 10mm, and LF & RR down by 10mm and see what you get (its a long way out), this will not affect ride height, a small change had little effect = soft springs.

Unless you drive with a passenger a lot try and ballast the drivers seat, I used 3 x 25litre drums (75kg)

Regards Mark

[Edited on 27/3/16 by mark chandler]

adithorp

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posted on 27/3/16 at 07:23 PM

As mark says you have to think/work opposite corners.

The total weight is fixed (obvious) and unless you add /remove/move ballast no amount of adjustment will alter it.
Similarly the total (2 added together) front or rear or left or right axle weight can't change (again unless you add/move/remove stuff)
BUT altering one corner will affect the opposite one in the same way AND the other opposing air in the opposite way. So increase the LF will increase the RR and decrease load on both RF+LR.

You need to set it in the trim you usually run, as being so light things like driver and fuel level can have a significant effect. So I set mine with an equivalent weight to me in the passenger side (ideally you'd sit in it and have someone else do the adjustments but I use 25lt drums of water as Mark suggests) and half a tank of fuel.
Normally you'd aim for opposite corner pairs to be equal so LF+RR=RF+LR.
All pretty simple until you then have to take into account how your fiddling alters ride height... lots of trial and error... adjust, check, adjust, repeat...

Note; some people prefer to get the 2 fronts as close to equal as possible (I've usually heard that from people who had uneven locking of the front wheels, I tried this once and it handled really badly).

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davidimurray

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posted on 27/3/16 at 08:14 PM

Thanks both - that makes a lot more sense.

I've got the car in road trim with fuel and 80kg of weight in the drivers seat.

I jut tried the suggestion Mark made and I now have -

LF 135kg
RF 164kg
LR 175kg
RR 189kg

so a lot better - I make that 51%
LF + RR= 324
RF+LR = 339

Should I be bothered about getting the rear / front axle better balanced or is it a case that as long as I get the the cross as close to 50% as possible it should be ok.

I make the front to rear distribution 45% to 55% rear. That doesn't seem too bad - how des that compare with other 7's?

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posted on 27/3/16 at 08:58 PM

Thats pretty good. The closer you can get the better but it can take a long time to get there... then you stick a tool box in the passenger side and it all changes.

I wouldn't worry too much about about the F/R balance. You''d need to add 65kg to the front or loose it from the rear (moving stuff gets better results) but it's a lot to change. I'd just keep it in mind later that any weight saving might be better at the rear.

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mark chandler

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posted on 27/3/16 at 09:15 PM

You will find that it's so sensitive that if you lean towards the front then it changes, it should be a leap forward now I am assuming that the ride height is correct, square across the car and 1/2" higher at the rear when loaded?

davidimurray

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posted on 27/3/16 at 10:05 PM

Thanks both - only had a quick whiz around tonight to try your suggestions so need to go through the ride heights properly tomorrow.

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posted on 27/3/16 at 10:45 PM

I like mine to be equal as possible across the front axle! Although it's a lot easier in a single seater.mines absolutely spot on across the axle and it's stunning to drive! Although it's only 287kg with fuel and fluids

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posted on 28/3/16 at 06:37 AM

Here is a handy online calculator and info

https://robrobinette.com/corner_weight_calc.htm

https://robrobinette.com/corner_weight.htm

http://grassrootsmotorsports.com/articles/understanding-corner-weights/

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posted on 28/3/16 at 07:19 AM

quote:
Originally posted by daniel mason
I like mine to be equal as possible across the front axle! Although it's a lot easier in a single seater.mines absolutely spot on across the axle and it's stunning to drive! Although it's only 287kg with fuel and fluids

Easy when all you're weight is on the centre line. I bet you too diagonals are almost equal as well. Op has a 50kg difference between left and right sides.

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daniel mason

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posted on 28/3/16 at 09:54 AM

Yeah I noticed the large R-L difference. I did mention it was a lot easier in a single seater!
I did however only have a 1kg difference across the axle on my second radical which had an off centre driving position.that also handled very well!

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posted on 28/3/16 at 03:43 PM

How to check if the ground is level

I set up mine myself and then took it to procomp and was not a million miles off. They just fine tuned it all.

The two bits of advice I would give following both of these activities is:

Check that the ground is level. You can use a very clever and very accurate method.

1 - Mark on the ground where your tyres (weighing scales) would sit when you want to carry out the work and move your car out of the way.
2 - Fill a bucket with water and place it about in the middle of where your car will be, on top of a stool so the water level is approx 50cm from the ground.
3 - Using a long clear tube with one end placed in the bucket ( or garden hose with a short piece of clear hose attached to the end) siphon off the liquid until you can hold the end up vertically creating a U bend. The water level that is not the bucket end will match the 50cm ground height.
3 - Using a tape measure or meter rule walk to the 4 wheel locations and read the water level height by touching the rule on the floor and measuring the height. Make a not of the readings.
5 - You can then sit and see which ones are high or low to the centre (irrelevant) and each other. Keep the highest as the datum and raise the other areas up to that level. Re check after adding wood, sand under wood, blocks.
6 - Ta Da, now grab a cuppa!!

The other thing is when adjusting the preload bounce the car up and down to settle the spring and suspension.

It seem like your doing well enough. I was quite impressed that I managed to get close when doing on my wonky driveway.

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davidimurray

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posted on 28/3/16 at 05:11 PM

Hmm - well an interesting day.

Carried on working around the corner weights and with everything looking close checked the ride height ......

cock!!!

To get the weight balance near I had the front 10mm higher than the left rear and 30mm higher than the right rear!!!

I've reset the ride heights so they are 120mm front and 145mm rear. Now looking over the weights I am back where I started -
LF 154kg
RF 149kg
LR 158kg
RR 206kg

All these with 80kg in the drivers seat.

So back to a big cross weight difference. the question is where do I go next? The right rear spring collar is virtually up to the top to get the ride height, surely if i lower again it will lose the ride height and the rake i have got? at the front i don't want to go lower as i have about 100mm sump clearance.

What is the best compromise - lose the rake and improve cross weights?

mark chandler

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posted on 28/3/16 at 06:22 PM

Stick a longer Spring under your bum and try again.

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posted on 28/3/16 at 08:08 PM

So now you've discovered a whole new can of worms. There's no such thing as perfect set-up, just best compromise;

With your current/chosen ride height, what range of motion do you have on the shocks and what are your wishbone angles like? Roughly (very) you'd be looking for available droop/bump between 33/66 to 50/50 and the bottom wishbone line (between inner bush and ball joint centres) about horizontal (or maybe slightly up from the wheel to chassis).
If you've got that then you need longer springs to allow you some adjustment; If not then you need to look at shock lengths as well. Whatever, you don't want negative rake.

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davidimurray

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posted on 28/3/16 at 09:13 PM

Mark and Adithorp - thanks both for your advice and support even with my stupid questions!

I will check the wishbone angles tomorrow and see how they look.

Stupid question - longer springs at the rear - how will they help?

Based on my current setup I think need to add preload on the RF and LR to take more of the cross weight from the LF/RR, but then I need to either increase the LF or reduce the RR to try and reduce the RR weight. Does that make sense?

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posted on 28/3/16 at 10:50 PM

Longer r/springs because you've wound them until you've run out of adjustment just to get to the right height. A longer spring of the same poundage would move the platform down the thread for the same pre-load and ride height, thus allow for further adjustment; Equally stronger springs of the same length would also work but higher poundage springs might not be desirable (ie possibly too stiff).

Yes, adding pre-load to LF/RR (either or both) or reducing RF/LR will improve the diagonal balance by also reucing LF+RR. However, if you increase LF that will increase that AND RR weight.

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davidimurray

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posted on 15/4/16 at 07:21 PM

Well I'm back, which probably ain't a good thing!!

After a short delay to get some new springs I now have 1" longer springs on the back so on with the setting of corner weights and ride heights. Please note in the figures below they are not true ride heights because the car is on scales so they are higher than they actually are. The scale are all level and equal heights.

So after messing around for a few hours I now have-

LF - 143kg and Ride Height 195mm
RF - 155kg and ride height 190mm
LR - 162kg and ride height 215mm
RR - 198kg and ride height 205mm

That makes my cross weights 317 - 341 so still way out.

The RR is where I am worried, should I be worried about an almost 30kg difference across the rear axle?

Next move tomorrow is to raise the RF and RR to improve the ride heights - but in my head I am thinking this will only make the weight distribution worse.

All advice greatly appreciated - it would seem like this should be easy but I keep struggling to get my head around what to make next - it is like a game of chess!

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posted on 15/4/16 at 08:03 PM

200mm, that,s 8" in old money unless your sump is hanging low you want to get the back down to maybe 5, front 5 1/2" rear.

Mine has a flat floor so 4" at the front with 4 1\2" at the rear.

Keep twiddling, it will suddenly make sense, make 1 change then note the difference each time.

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posted on 15/4/16 at 08:31 PM

It is high as it is on scales, they are 60mm high, so the real ride height is 130mm front to the chassis rails, my sump hangs down 25mm, so I have about 105mm clearance to the sump.

Will be back to playing tomorrow. Found the spread sheet on herehttp://www.everettmail.co.uk/mods/mods_corner_weights.htm that is supposed to give you the required preload adjustment based on wheelbase and spring rates. Will be interesting to see if it works.

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posted on 15/4/16 at 08:50 PM

If you adjust both right up or both left down it won't alter the cross weights (well only slightly as you won't move then evenly).

As for the hight RR, it's kind of inevitable given the drivers weight is on there. The difference across the axle won't be noticable.

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posted on 15/4/16 at 11:54 PM

Some food fpr thought.
- You will NEVER have equal distribution unless CoG is centered sideways.
- Front to Rear bias is determined by CoG placement longitudinally.
- Nobody has mentioned to loosen Suspension bolts to remove hysteresis (sp?)
- As stated before, you adjust two wheels at a time. Shorten heavy wheels a couple of notches, jump up and down on the car to settle, measure amount of change.
- If more is needed, lengthen light wheels a couple of notches.
repeat until you reach midpoint... (I hope this makes sense)
If you are not completely happy, you may try relocating stuff like Battery, Oil tank (if so equipped), fire extinguisher, tools, (adding ballast may be useful although not ideal because of weight gain).
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posted on 16/4/16 at 11:32 AM

changing the heigt of the car´s conrers will also effect other suspension settings like camber. and camber will effect tracking.

so would camber and tracking also effect again the corner weights?

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posted on 16/4/16 at 10:07 PM

After a bit more fiddling and using the spreadsheet on the page I linked to the corner weights are now within 4kg - which I suspect is more than within the tolerance of my bathroom scales!

The spreadsheet really helped speed things up as it made suggestions I never would have come up with but it only took 3 iterations to get there.

Thanks to everyone for their help - time to get her back on the road and test the new setup.

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