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Discussion Starter · #1 · (Edited)
Been thinking about this today and thought you guys might be able to assist(im hoping scuffers can have an input as I do like trying to learn from him)

Ok first question is, most elise/exige have a 40/60 weight distribution, now ive read in few books that this is pretty close to optimum rather than 50/50 because when you head into a corner with deceleration the weight transfer to the front brings the car more 50/50 meaning you dont overload the front end compared to a normal 50/50 sports car.

If thats the case is that the perfect weight distribution above all else for our chassis, if you have the ability to pull alot of weight out of the car, should that be the number you should always be aiming for.

The reason I ask is im pondering my recent geo and setup.

This is with 100kg of ballast in the drivers seat and 10kg of fuel(fuel tank is only on passenger side)



this has a 58.8% rear bias.

So im working on pulling more weight out of the car, which will be again mostly from the back end of the car which will again send my bias further forward.

Should I actually be aiming to only pull weight out of the front to get me back closer to 40/60.

I understand from what scuffers said on the weight thread on lotus talk that a couple of percent wont really make any difference but since im still pulling more weight out of the car it would be nice to know what is classed as 'perfect'

If there is such a thing of course, im sure driving style, types of tracks probably play a part aswell.



Next thing would be spring rates relating to the weight distribution, now if we ignore the aero for a second and assume that the downforce my car produces on track matches identically the weight distribution the static car it(not saying it will but just to make it a little easier)

I have fitted 900lb fronts and 1150lb rears to my car, the reason for this choice is based on jamies car in which he runs 1000lb/1400lb. His car is 110kg heavier than mine with a more reward weight distribution and runs slicks, so Ive taken 250lbs off the rear end and 100lb off the front, the reason for not taking much off the front is I have a 200mm extended splitter plus big canards so I need to make sure I keep the splitter off the ground at speed.

The issue with this is that im not balanced spring wise to my weight distribution, my springs are 43.9/56.1 compared to static weight 41.2/58.8, i know its not much but again should i be aiming to perfectly match my weight distribution ignoring aero for a second.

For example if i change the fronts to a 800lb that would match my weight distribution, assuming i can keep the front end off the tarmac it would be a better choice?

If thats not possible and 800lbs do touch at say 130mph(my splitter edge will be 60mm from tarmac static) or under heavy braking should I up the spring rate at the back to 1300lb?

Finally if anyone can comment on tyre width verses weight distribution im all ears.

With a 255mm rear tyre if i match the weight distribution it would only be 178mm which wouldnt be fees able, im running a 225 tyre on the front currently, so I do wonder if you try and match the car up in a certain way other than static weight and maybe expected weight under braking etc

If you ahve got this far and have any comments I would be most grateful for your input
 

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Your weights are pretty good...you've got it to within 2-kg.
Can you get it to?
LF 163 RF 176
LR 232 RR 253
 

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Optimum is, as per Riley & Scott, 56% on rear.

They stated they didn't care where the engine is, as long at they get 56%.
 

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The issue with this is that im not balanced spring wise to my weight distribution, my springs are 43.9/56.1 compared to static weight 41.2/58.8, i know its not much but again should i be aiming to perfectly match my weight distribution ignoring aero for a second.
BTW I get 58.9% rear...
And, you don't want to match your spring rates... Your front wheels have about a 1.37 mechanical advantage (Spring-to-wheel travel) and the rear has 1.34.
 

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If you are dialing in your spring rates, you need to be looking at the natural frequency of the corner. You also want your rear natural frequency to be slightly higher...by the time it takes your front wheels to complete one cycle, you want your rear wheels to complete on cycle by the time you rear wheels cross that same bump... so take your average lap speed (from time) and calculate the time it would take to travel the length of your wheelbase at that speed and subtract that from your period of oscillation to get your rear wheel's period of oscillation to get your rear natural frequency.

And yes, you can fine tune it to each track...
 

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Discussion Starter · #6 ·
Your weights are pretty good...you've got it to within 2-kg.
Can you get it to?
LF 163 RF 176
LR 232 RR 253
We got a perfect 50% cross weight pretty easily, im planning on prioritizing pulling weight out of the rear right side above all else to try and bring it closer to 50/50 left to right aswell, but most of our tracks are clockwise so having a little on the drivers side(right) isnt the end of the world.

I know I can pull another 5kg from the front end(battery+heater) but thats pretty centrally mounted between the 2 front wheels.

For the back theres a lighter alternator(1kg) Carbon rocker cover(1kg) few brackets on engine(1kg) lighter wing uprights(1kg)

I need to lose 20kg off my belly which should help aswell

Theres things like making my carbon roof which is 5kg saving, but its pretty much the saving in the centre so shouldnt really make any difference to the distribution(apart from centre of gravity height)

So if we can remove 23kg off that weight on the drivers side it should get me closer

regarding spring oscillation, is there a way of working out what they are per average spring rate on a 2.25" spring.

What does oscillation have to do with it anyway, I assume its your damper that controls that, rather than the spring, thats just designed to resist load

Optimum is, as per Riley & Scott, 56% on rear.

They stated they didn't care where the engine is, as long at they get 56%.
Ok thats cool to know, never heard of riley and scott, so just googled them.

to get to 56% i need to remove another 54kg off the rear end, thats quite alot when ive already covered most things to save weight from, only way I could do that would be to put smaller wheels and tires on, and remove the turbocharger/manifold and chargercooler setup which would just make me to slow down the straights.

ive mentioned the 24kg off the back I know about without doing those items but the issue is that for example if I lose 20kg then all that doesnt come off the back, so will also be removed from the front, so actually I need to make it massively lighter to achieve it.

Food for thought though
 

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We got a perfect 50% cross weight pretty easily, im planning on prioritizing pulling weight out of the rear right side above all else to try and bring it closer to 50/50 left to right aswell, but most of our tracks are clockwise so having a little on the drivers side(right) isnt the end of the world.
No you did not get it perfect! Perfect are the numbers I gave you. The only thing you got perfect is the "static" cross weight. As long as your car is parked (static), you've got it perfect! Congratulations!

I don't know about you, but I like to drive (dynamic) my cars... :shift:

As long as you don't slide your tires, you can approximate the static weight distribution. Once you get on the limit (you have to cross it some times to be on it), all dynamics laws apply. If you force in a static weight stagger, the further it is off from the center of mass, the more violent your snap will be. You are only 2-kg off from your particular car. So, you would have to be a damn good driver to complain about that. But why? It is very easy to set it up right now.
 

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I need to lose 20kg off my belly which should help aswell

Theres things like making my carbon roof which is 5kg saving, but its pretty much the saving in the centre so shouldnt really make any difference to the distribution(apart from centre of gravity height)
Dynamics is in all 3 dimensions... you are only looking at the fore-aft direction. You have mentioned lateral. But your up-down center of mass is very important! It is much more important to loose 5-kg off your roof than 20-kg off of your pan...
 

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What does oscillation have to do with it anyway, I assume its your damper that controls that, rather than the spring, thats just designed to resist load
If there could be a way of removing your shocks completely and retaining just your springs, that is what you want to do to measure your period of oscillation. With just the dampers gone, push down on a fender. The time it takes the fender to go from top and back to bottom, is your period of oscillation. Your damper should be adjusted to only allow 1.5 cycles. If you get more than 1.5 cycles, you don't have enough damper capacity (under damped), if you don't get 1.5 cycles, then you are over damped. Your shocks do not hold up your car! And they are not there to compensate for poorly selected springs...
 

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Don't forget to take account of the Motion Ratio and Angle Correction Factor of the suspension.
AFAIK, it's 0.67 front, 0.72 rear. The Rear ACF is 9deg from vertical(not sure about the front). I wish to find find more info on the rest of the dimension..

For the sake of discussion, I think the whole "natural frequency" thing is a huge crock of sh%t.
Here is a reason reason why I say that:
The idea of "natural frequency" in terms or ride and handling originated from normal human walking frequency of 1~1.5Hz or roughly 70~90CPM. Older cars were boaty because of it(that, and the damping sucked).
Other than that, it only tells you how stiff the spring is after the fact, and devolves into "if you have high spring rate, you have high spring rate."
The usual example one will find is something like this:
0.5 - 1.5 Hz for passenger cars
1.5 - 2.0 Hz for sedan racecars and moderate downforce formula cars
3.0 - 5.0+ Hz for high downforce racecars
Then, whoever listens to that BS is lead to throw a dart on where their desired NF is. Why pick 1.8Hz over 2.0Hz? Damned if I know. Why choose 4.5Hz? Because RACECAR!!! Just plug in the numbers based on what your chosen NF is, and you'll get your spring rate based on equation:

K_springRate = 4pi^2*Freq_natrual^2*Mass_sprungMass*Ratio_motion^2.

I don't think that's engineering. It's playing pick-a-number.


The only thing that's useful about it is that you can calculate "Critical Damping" based off of natural frequency, and it gives you a ballpark from which you can set up the dampers. But that's for getting a ballpark figure for damping, which is useless since we're trying to get a ballpark figure for springs.
Also, having higher NF at the rear so that the rears can catch up to the fronts hitting a bump is based on "flat ride" theory, and I'm lead to believe that it's also based more on comfort rather than performance. It probably inspires driver confidence during hitting a bump at steady state cornering, though, since the driver will feel the front and rear absorbing the bump and settling roughly at the same time the bump is hit. So, I guess that's a good thing.

Based on my limited knowledge of the subject, I think suspension should be optimized around what kind of tire you're using, based on Load vs. Slip Angle vs. Lateral Force vs. Temperature. You can get that info using this rig:
Companies like MTS Systems make the rig.
The only useful tire data I was able to find was from Avon tire for FormulaSAE design students, and they go through Calspan Co. for tire testing at a couple hundred dollars.

After you have the tire data, then you optimize the suspension around getting the tire to work at its maximum potential.
And unfortunately, I don't know how to go from there. :facepalm

Carroll Smith's book advoated was something like run the softest possible setup and control the roll with a bar. :shrug:

I may have said a lot of things without contributing. Educate me. Why is X spring rate better than Y spring rate, and how do you arrive at that conclusion? ... because RACECAR?
 

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To the OP. You have reasonable questions. I believe you and Simon live on the same island, so I suggest you contact him directly for help (compensated). Further, I suggest you ignore all the presumed technical help posted so far. It varies from misinformed to misapplied to naive to complete BS.
 

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We got a perfect 50% cross weight pretty easily, im planning on prioritizing pulling weight out of the rear right side above all else to try and bring it closer to 50/50 left to right aswell, but most of our tracks are clockwise so having a little on the drivers side(right) isnt the end of the world.

I know I can pull another 5kg from the front end(battery+heater) but thats pretty centrally mounted between the 2 front wheels.

For the back theres a lighter alternator(1kg) Carbon rocker cover(1kg) few brackets on engine(1kg) lighter wing uprights(1kg)

I need to lose 20kg off my belly which should help aswell

Theres things like making my carbon roof which is 5kg saving, but its pretty much the saving in the centre so shouldnt really make any difference to the distribution(apart from centre of gravity height)

So if we can remove 23kg off that weight on the drivers side it should get me closer

regarding spring oscillation, is there a way of working out what they are per average spring rate on a 2.25" spring.

What does oscillation have to do with it anyway, I assume its your damper that controls that, rather than the spring, thats just designed to resist load



Ok thats cool to know, never heard of riley and scott, so just googled them.

to get to 56% i need to remove another 54kg off the rear end, thats quite alot when ive already covered most things to save weight from, only way I could do that would be to put smaller wheels and tires on, and remove the turbocharger/manifold and chargercooler setup which would just make me to slow down the straights.

ive mentioned the 24kg off the back I know about without doing those items but the issue is that for example if I lose 20kg then all that doesnt come off the back, so will also be removed from the front, so actually I need to make it massively lighter to achieve it.

Food for thought though
What rake are you running front to back? If you increased it a bit you would shift some more weight to the front...
 

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Don't forget to take account of the Motion Ratio and Angle Correction Factor of the suspension.
AFAIK, it's 0.67 front, 0.72 rear. The Rear ACF is 9deg from vertical(not sure about the front). I wish to find find more info on the rest of the dimension..

For the sake of discussion, I think the whole "natural frequency" thing is a huge crock of sh%t.
Here is a reason reason why I say that:
The idea of "natural frequency" in terms or ride and handling originated from normal human walking frequency of 1~1.5Hz or roughly 70~90CPM. Older cars were boaty because of it(that, and the damping sucked).
Other than that, it only tells you how stiff the spring is after the fact, and devolves into "if you have high spring rate, you have high spring rate."
The usual example one will find is something like this:

Then, whoever listens to that BS is lead to throw a dart on where their desired NF is. Why pick 1.8Hz over 2.0Hz? Damned if I know. Why choose 4.5Hz? Because RACECAR!!! Just plug in the numbers based on what your chosen NF is, and you'll get your spring rate based on equation:

K_springRate = 4pi^2*Freq_natrual^2*Mass_sprungMass*Ratio_motion^2.

I don't think that's engineering. It's playing pick-a-number.


The only thing that's useful about it is that you can calculate "Critical Damping" based off of natural frequency, and it gives you a ballpark from which you can set up the dampers. But that's for getting a ballpark figure for damping, which is useless since we're trying to get a ballpark figure for springs.
Also, having higher NF at the rear so that the rears can catch up to the fronts hitting a bump is based on "flat ride" theory, and I'm lead to believe that it's also based more on comfort rather than performance. It probably inspires driver confidence during hitting a bump at steady state cornering, though, since the driver will feel the front and rear absorbing the bump and settling roughly at the same time the bump is hit. So, I guess that's a good thing.

Based on my limited knowledge of the subject, I think suspension should be optimized around what kind of tire you're using, based on Load vs. Slip Angle vs. Lateral Force vs. Temperature. You can get that info using this rig:
Tire Force Test - YouTube
Companies like MTS Systems make the rig.
The only useful tire data I was able to find was from Avon tire for FormulaSAE design students, and they go through Calspan Co. for tire testing at a couple hundred dollars.

After you have the tire data, then you optimize the suspension around getting the tire to work at its maximum potential.
And unfortunately, I don't know how to go from there. :facepalm

Carroll Smith's book advoated was something like run the softest possible setup and control the roll with a bar. :shrug:

I may have said a lot of things without contributing. Educate me. Why is X spring rate better than Y spring rate, and how do you arrive at that conclusion? ... because RACECAR?
Like you said...all the calculations are just to get you into the ball park. They also help in figuring out what is wrong with the car...again, getting you back into the ball park. The calculations will just get you there faster, with a lot less testing...

And yes, softer with bars... but "softer" is a relative term...
 

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First of all, there is no such thing as 'perfect'. It depends on your car, the track, and (especially) your driving style.

That said, in general, I think the answer is: if you can remove weight, remove it. Making the car lighter overall will be more helpful than a small change in weight distribution.

I would in general be more concerned with left/right weight distribution than fore/aft. You can always change tire sizes a little bit fore/aft to make up for changes in axle weight, but you won't likely want to run different left/right sizes. Either way make sure your cross weights are good 9looks like you know that already).

As far as springs, your numbers don't mean much to me as I usually work with rate rather than load. The short answer is that for best mechanical grip you should run the softest spring your aero loading will allow. IMO frequency calculations are of minimal use unless you have accelerometer or wheel position data from the track you are planning to run.

Keep in mind that unless you are an F1 driver, getting all of this perfect won't make that much difference. There are people that are damn fast in muscle cars with 35/65 weight and people who are damn fast in 911s with 65/35. Set the car up so that you are comfortable with the way it is balanced, not so that it matches random numbers from the internet.

EDIT: just noticed where you said 'fuel only on passenger side'. If by that you mean that your fuel tank is on the passenger side, I would look at fixing that long before I would worry about a few kg of static weight change. You can set the car up to work well on pretty much any static weight situation with suspension tuning, but you can't set it up to work with 30kg of fuel on the passenger side and expect it to work equally well with 5kg. It is equally hard for the driver to get used to the balance when it changes all the time. Fuel tank should be as close to the CG as possible, and in any case L/R centered in the car if at all possible, unless you are running such short events that there is no appreciable change in fuel level.
 

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To the OP. You have reasonable questions. I believe you and Simon live on the same island, so I suggest you contact him directly for help (compensated). Further, I suggest you ignore all the presumed technical help posted so far. It varies from misinformed to misapplied to naive to complete BS.
I think this is good advice. I'd rather have real-world knowledge than educated guesses.

San
 

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Goodnesss...

Toaster, Forget about your corner weights on a scale or what Simon's/Jamie's TA car has. None of it is comparable to yours or anyone else's. you have a unique car with unique DF and what 300hp.. simon/jamie has like 700? His DF is X and yours is Y or who knows...

With all the downforce you have, asking to guess a spring rate is going to be all but impossible. You have to be willing to TRY different rates. Pick a set of springs off the shelf and give them a try. Perhaps start light, 700f/900r.... You don't have much power to pull all this aero through the air as I recall, so your speeds will be lower, which means DF will be lower (relative to Simon's car)-- so your springs *can* likely be a good bit lighter and still work well... Keep the valving light for starters so that the springs/shocks can work/move. From that, you'll have a place to start making observations and then changes.

If you're getting a lot of platform movement, and grip is adequate, start stiffening up. Stiffen up to the point your platform is steady and grip is still good. If you over shoot, you'll have a flat car that will slide off the track. Same thing goes for valving. There's no magic spring rate and no magic number of clicks. You have to just pick a spot and try it. The spot you pick needs to be err'd to the side of too soft, not too hard. That's important, IMO.

Also, you may find that your wing is a lot more functional on a relative basis to the splitter and your biggest problem won't be the front diving into the ground (as you're concerned) but the rear squating at speed causing the nose to the lift. I've experienced this for myself... So with the big wings, you have to start adding spring to the rear first (and possibly more chassis rake). It takes power to make a lot DF though, and I'm just not sure how much of that you really have... Again, start soft and take a harder set of springs to the track test day with you.

Don't be afraid to buy some springs either. They're $55 a corner and take an hour to swap all 4 corners--- if that much time. CHEAP! I have a bunch of springs over the years. I'd suggest getting the shortest springs that will fit your threaded area on the shocks and not coil bind. I use 5" springs on the front and 6" on the back with my penske shocks, FWIW... The shorter springs are both lighter and cheaper.

If you're at the track and swapping springs, you can do some quick 'n dirty testing just using ride height and -equal collar movements- to corner balance. That's quick and will typically get you within a couple % of perfect crosses--- and that's close enough to tell you if you're heading in the right direction at the track on a test day. Dial it in perfect when back at the shop before the next race to extract those tenths of a second a lap that level of detail is good for. You're just getting the rough draft done at the track anyway...

I like FormulaBob's post. He has some really good advice there:up:

-Phil
 

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can't disagree with most of this ^^^^

sitting here, I have no idea what your cars going to do, spring rates are very much a suck it and see for anything with Aero, hence why with Jamies's car, I went from 1,000 to 1,400 lbs as the effects of the aero on was a lot more than I had expected.

also, in the middle of this you need to think about ride heights, and how stable you can get it, under-floor and the diffuser will only work if the platform is stable, having the car bounce up and down will basically kill any effect they may have, and bear in mind once your over ~50/60mm off the floor, it's meaningless anyway.

as for weight distro, not sure what the optimum is, I have always just had to work with what I end up with, as I am not about to blast up a car just to improve the distro, that said, I think anywhere between 60/40 and 65/35 is kind of right for the car, I do however, wildy disagree with the comments that it makes no matter where the weight is as long as the distro is right, that' just bullsh1t.

think about it, if you moved all the weight to the very front and rear of the car, what's that going to do for your polar moment of inertia?

this is the same argument about if you have a 5Kg's flywheel where all the weight is in the centre vs. round the edge, both weight 5Kg's, but will be nothing like each other in characteristic.

you also need to consider the hight of any weight, 5Kg's on the roof of the car will be very apparent, 5Kg's more fuel in the tank is impossible to feel (assuming it's not sloshing about!)

should also add that whatever dampers you have will need to be re-valved as it's extremely unlikely they would be have done with 1000+lb springs in mind (it was easy for me as I had 4 way Quantums to play with so could adjust the slow/fast rebound to suit).

What you need to figure out here is that nobody has a clue what it's going to do till it turns a wheel, there are just way too many unknowns, even if your aero works the same as Jamie's, you don't have the power to push it through the air, as efficient as Jamie's aero parts were, the aero drag is significant.

BHP absorbed = (Cd x A x V^3) / 1225

A is frontal area in m^2 (we used 1.75m^2) and v is speed in m/s.

to put numbers to this, we are talking some 46Hp at 80Mph, and this increases with the square of speed, so at 100Mhp it's up to ~95Hp (and producing some 3,000+N downforce), at 125Mph this get's to ~183hp (and 4,800+N)

in the basis that you have ~300hp, you're not likely to get much over 125 (if you can even get there?)

at 160Mph, Jamie's was using some 385hp in aero drag and giving some 7,900N, that's over 800Kg's, it's not insignificant on the suspension, it's not just the spring rates that have to be thought about.

(I should mention that we never ran it at this high downforce configuration on track!)
 

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I do however, wildy disagree with the comments that it makes no matter where the weight is as long as the distro is right, that' just bullsh1t.

think about it, if you moved all the weight to the very front and rear of the car, what's that going to do for your polar moment of inertia?

this is the same argument about if you have a 5Kg's flywheel where all the weight is in the centre vs. round the edge, both weight 5Kg's, but will be nothing like each other in characteristic.
I assume you are referencing the Riley comment, and I would generally agree with you, but I think it is important to point out (again) that it appears the OP's question is in context of a small amount of weight on his existing car. We are not talking about building a car from scratch here.

Generally removing weight will reduce PMOI anyway, and if we are worried about shifting WD, then for sure it will as that weight must be far from the CG.

Even so, if I could remove 200lb from my car right over the CG, my PMOI would be much worse, but I would do it without hesitation. Assuming you are starting from a good platform (which obviously he is), removing weight will most always make the car faster, regardless of CG or PMOI changes.
 
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