Bad DI, now what?

[Sam@TDi]

The new TDi project car has a Dynamic Index of around 1.7, which is a very long way from ideal.

 

I think we have a bit of a challenge with this project

Can we play a theoretical wim game? I have a front engined rear wheel drive car with a poor manufactured index of 1.7, how would you suggest improving this?

 

Yep, ok

 

Option one would always be to shed some weight! having a DI soo far above 1.00 can really only be caused by a mis-match in wheelbase to the mass, basically the chassis has far too much yaw inertia for the short wheelbase to influence easily. If we could take just 10% weight out of our demo car we'd have a DI of 1.39, much better!

 

Unfortunately to take 10% weight out we'd have to seriously comprimise the car's very comfortable characteristics so for some (most) people this would defeat the object.

 

The other option is to simply accecpt that the car will as a result of it's high DI always make poor use of it's rear axle in terms of producing lateral acceleration and then to try to work around it.

 

A good example would be the Mitsubishi Lancer Evo-5, it's actually a car with a DI of 1.35 so this would make it pretty vectra/modeo'ish to drive... But Mitsubishi's chassis dynamics team have cunningly swerved this potential marketing disaster by employing an overly pointy geometry and suspension sollution giving the car the ability to change direction light a housefly, then in order for the average joe customer to be able to control and exploit this very odd and naturally quite unbalanced set-up they've been forced to add an active yaw control system in the form of an electronically controlled torque biasing center diff. This solution works so well that not only is the car acceptable to it's intended market it's actually now considered as one of the worlds best handling cars.

[Tony]

Geometrically most 4WD cars including the Evo-5 have next to no dynamic Geometry? Static front camber and toe read 0 +- very little... Some - rear camber and + rear toe to induce some under-steer, a very deep KPI and medium castor. This distinctively suggests superior suspension set-ups with a conclusive chassis...

 

Anyway my theoretical dinosaur of a car is in the shop to improve the dire DI, I'm not concerned with adverse changes in the comfort. As you said shed some weight, my question is where and how?

[Sam@TDi]

Well shedding meaningful amounts of weight can actually be pretty tricky if the car was designed as lightweight or was very basic spec. And it totally depends on the vehicle, in the case of our demo car the fully electric and heated front seats alone weigh a great deal so replacing these and removing the rear seat would definately save 80-100kg, replacing the thick window glass with plexi-glas would be another good step... it all depends how serious the cars diet is, in the past i've spent 2days or so removing all the underseal and bonded sound deadening from a totally stripped body shell before the cars construction properly began

 

It is worth remembering that it's completely possable to go the other way and be too light

[Tony]

Can we assume a manufactured car cannot be modified unless some serious compromises are dealt with or intended for the track? I am looking to evolve the handling on my MR2 SW20 for fast track days.

[Tony]

Can we assume a manufactured car cannot be modified unless some serious compromises are dealt with or intended for the track? I am looking to evolve the handling on my MR2 SW20 for fast track days.

Thread slightly modified..... Can we continue with this "theoretical" evolution Sam@tdi

[jon]

It is worth remembering that it's completely possable to go the other way and be too light

 

I still don't understand this. Providing you kept the balance optimal and if you lost a lot of weight, resprung and redamped as appropriately surely a 600kgs X would be faster than a 1600kgs X?

 

 

I hear you when you say it's tough trying to lose weight (in the car). I saved about 40kgs doing the seats, wheels and suspension but much after that is proving really difficult. Maybe 20kgs for all of the inside and another 10kgs for various bits I've cut off the car. Next big drop is having you guys making a custom light weight exhaust, but that'll save 8kgs if i'm lucky. Then putting the cage in will add 15kgs

[Tony]

It is worth remembering that it's completely possable to go the other way and be too light

 

I still don't understand this. Providing you kept the balance optimal and if you lost a lot of weight, resprung and redamped as appropriately surely a 600kgs X would be faster than a 1600kgs X?

 

 

I hear you when you say it's tough trying to lose weight (in the car). I saved about 40kgs doing the seats, wheels and suspension but much after that is proving really difficult. Maybe 20kgs for all of the inside and another 10kgs for various bits I've cut off the car. Next big drop is having you guys making a custom light weight exhaust, but that'll save 8kgs if i'm lucky. Then putting the cage in will add 15kgs

Welcome to the realm of chassis calibration! Dynamic indexing, correlation and balancing of the coil/ damper ratio, sway reaction and aerodynamics all in one, oh and Geometry. As Sam@tdi says "you can be to light" unless you adopt a global compensation for the lighter car... Then there is still a need to address the cars intentions... Road, track. trials, drift.. each intention adopts different requirements. And this doesn't even scratch the drivers requests into the equation.

[Sam@TDi]

It is worth remembering that it's completely possable to go the other way and be too light

 

I still don't understand this. Providing you kept the balance optimal and if you lost a lot of weight, resprung and redamped as appropriately surely a 600kgs X would be faster than a 1600kgs X?

 

 

I hear you when you say it's tough trying to lose weight (in the car). I saved about 40kgs doing the seats, wheels and suspension but much after that is proving really difficult. Maybe 20kgs for all of the inside and another 10kgs for various bits I've cut off the car. Next big drop is having you guys making a custom light weight exhaust, but that'll save 8kgs if i'm lucky. Then putting the cage in will add 15kgs

 

I totally agree with you Jon that this concept as with so many aspects in chassis set-up is not at all intuitive. You are absolutely right in your observation that the lightest car should always be the fastest, and in terms of power to weight ratio, braking to weight ratio, grip to weight ratio you are of course correct, on paper there should be no question ..... but

 

The point is that as humans we're actually pretty poorly equipped to drive fast cars fast!... only being able to make around 2 adjustments per second based on optical information and if very skilled around 4 adjustments per second based on inner ear information (g-forces, yaw rate gains etc) ..... and so performance car designers / chassis set-up engineers are faced with 2 major area's of concern.

 

1/ Firstly the main question is the vehicle suitable for it's intended application?

 

2/ Secondly is my human driver going to be able to exercise command and control over this chassis?

 

Often in motorsport less experienced designers, owner/drivers and set-up engineers put far to much emphasis on question-1 and pay little no attention to question-2

 

My point is that having a car that is way too light for it's wheel base will emphatically tick all the boxes for question-1... BUT along with other problems the DI will be so poor that the answer to question-2 is likely to be NO (even if the driver doesn't admit it) and if this is the case the car will turn in poor lap times and no one will know why (unless the driver admits he can't drive the car... but in my experience this almost never happens as a matter of pride)