'Falling rate' rear sway bar.....

[slug_dub]

I have a Racing Beat thin wall hollow front bar, which is as stiff as a one inch solid bar, to compensate the soft front springs in roll. As camber settings have changed, as well as some height adjustment, I have been oscillating between a 12mm rear bar and none at all. I think I have been oscillating around the ideal rear roll rate for all setups and am now finally getting around to doing something about it.

I'd be happy to run no rear bar at all if I wasn't picking up a front wheel. Of interest, Gordon Murray, famed Formula 1 and McLaren F1 designer, didn't run rear sway bars on his cars.

Keep an eye on your front sway bar mounts, they are likely the weakest link now.
I have softer springs still than yours, RB front sway and stock rear sway, and having been able to follow my car around Morgan Park I can see the front inside wheel is close to lifting off.

[Magpie]

3 pages now.

Even with my spring rates 10/9 and OEM sways I can lift the inside wheels given the right conditions.

No me but you get the idea

[plohl]

It was a few years ago now, but I spoke to signature

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[madjak]

predictability = confidence = lower track times

The spring concept in the sway bar might work or might not, but if it adds any variability to the behavior of the car you'll find yourself going slower overall.

Sway bars are pretty much the only aspect I haven't mucked with on my car. If you are serious then really sway bars need to be adjusted per track to suit the conditions. The events I drive at vary so much that you may as well ballpark it somewhere in the middle then learn to drive around it. Unless you are running on only one type of surface and building a car specifically for that track, then I see little point going down that rabbit hole.

[sailaholic]

There's also the consideration of why does it matter if you are lifting a wheel? It's not a rear, so you aren't loosing drive.


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[rascal]

There's also the consideration of why does it matter if you are lifting a wheel? It's not a rear, so you aren't loosing drive. Sent from my iPhone using Tapatalk

In the ultimate pursuit of lap times it matters.
If you are lifting a wheel then that wheel is contributing zero to the cornering power, ie you are getting less than the maximum grip that the car can offer. Therefore you will be slower through the corner than if all 4 wheels were providing grip.

Admittedly it's the unloaded wheel anyway so the difference is probably minor, but still not desirable.

[StillIC]

There's also the consideration of why does it matter if you are lifting a wheel? It's not a rear, so you aren't loosing drive.
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This is a good question.

In years past it was common to see many race cars cornering on 3 wheels; commonly Porsche 911s lifting a front or various front drive cars lifting a rear. But I haven't seen this phenomenon on any top level race cars for some time now. Aerodynamic downforce may have something to do with this.

But I believe that wheel lifting is an indicator that that end of the car is *relatively* too stiff in roll...so stiff that the roll rate becomes irrelevant as once that wheel leaves the pavement there is nothing that that end of the car can do to resist more roll. It may as well be rigid in roll. Only the other end of the car with 2 wheels planted is resisting roll at this point. I think this might indicate that more grip could be generated if the front and rear roll rates were better matched, due to reduced overall roll angle and use of the 4th tyre. I might be wrong. There might be other aspects at play offsetting this perceived disadvantage too.

[sailaholic]

I asked this question of a suspension designer / tuner last night over dinner.

His response - Theoretically yes you should have more grip with 4 wheels on ground but in the case, the most likely case for increasing load on the inside wheel is a reduction of load on the outside wheel. Given that's the 'working' your likely to find your laptimes increase by getting that 4th wheel on the ground with no other changes.

His thoughts - go back to stock bars, get the spring rates right (ie go up to get separation between your front and rear rates) then fine tune with bars. Going up spring rates and down bars will also increase the likely hood of keep all 4 wheels down while increasing the independence of each side leading to higher ultimate grip.

His comment was soft springs and big bars can work well on dirt based cars where you want lots of wheel travel without spring bind, but some roll stiffness. He's generally found it disadvantageous on smooth / bitumen surfaces.

As a reference, I found my 9/6 fine on the road, I'm down to 7/5 now after removing the ac and setting the car up for relatively smooth dirt work.


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[NitroDann]

4 tyres down will work better.

You do not get an equal increase in traction to the increase in pressure applied to the wheel, because friction is not the only thing giving a race tyre traction.

Dann

[pepejesus]

I think the question is, 'given that I'm running compromised, non-optimal spring rates for my tyres, which I won't/can't afford to change, how can I address the balance issue (oversteer) in the cheapest way?'

I would suggest leaving the rear bar and running a bigger front sway. I believe it's an RB tubular up front already, but I assume it's the smallest of the three. Get the one size up bigger one.

You shouldn't have much trouble selling the older one, so there you've got a relatively inexpensive band-aid solution, without the downsides of your current set up that others have pointed out (inconsistent handling).

[madjak]

The issue with a MX5 is that the rear suspension has very limited droop, which is made even worse with stiffer sway bars and some aftermarket shocks. If you get the body rolling too far, regardless of how soft or stiff your sway bars are it will lift the inside rear tyre. This is a big problem for MX5s because we mostly run torsens which are basically just an open diff once you lift a rear wheel. Run any serious grip and you'll encounter this at some point.

On my car, I was lifting my inside rear wheel on lots of corners which meant lost drive out of the corner. I upgraded the rear shocks to ones that had more travel and also upped my spring rate all round and it's solved the issue for 95% of the corners where I was struggling. There is just one corner on a hillclimb coarse where I still get loss of drive but it's a very tight hairpin that is off camber and I'm also skipping off the inside kerb. The inside wheel has no chance. The only thing that will help me here is a plate diff.

Have a look at it yourself. Jack the car on one side and measure how far the rear wheel can droop. This doesn't even account for loading the other side of the car from lateral Gs.

[Magpie]

As for what Madjak has said, measure the travel in the rear. For mine, the rear has 70mm or damper movement and the wheel moves 97mm (3.4° of camber gain). Therefore the motion ratio for the rear is 0.721649485.

Consider that the bump stop is about 38mm, this leaves 32mm of travel (before the bumpstops). Assuming 6 kg spring rate, there can be about 192kg of weight transferred to the wheel before you hit the bumpstops. This works out to be about 1.5 of cornering force (with a few assumptions)

Weight transfer = ( Lateral acceleration x Weight x Height of CG ) / Track width
Weight transfer = (1g x 1300kg x0.3m)/1.428m
Weight Transfer = 273kg
Assume 50/50 weight transfer F/R = 136.5kg to the rear

5kg = 27mm of travel
9kg = 15mm

To prevent the wheel unloading (excluding clutch pack diff) you need to reduce weight transfer. Assuming track width is a constant you can either reduce G's (not desirable), lower CoG or make the car lighter. Springs do not reduce weight transfer, the springs length is changed (resistance) as a result of the weight transfer. When the sway bar in introduced it has the effect of changing the weight transfer, hence you want predictability in what the sway bar will do.

Trade off with using stiffer springs is less camber gain so more camber may need to be added to maintain the desired tyre contact patch.

Sort your spring rates out then fine tune 'roll' with the sway bars. Also, what are your tyre pressures, alignment setting, tyre temps. Tyre temps will give you feedback on alignment and pressures. Understeer/oversteer can also be adjusted by changing pressure/alignment.

In the new year I will disconnect the rear sway and log the data on the rear suspension, then connect the rear sway up and see what the data says and if it agrees with the formula. Bonus is I will be able to 'feel' the change and recognise what the car is like without the rear sway.