50/50 weight distribution

Non-engine, eg: aerodynamics, gearboxes, brakes, suspension
Post Reply
cantfindausername
Posts: 306
Joined: July 19th, 2006, 6:07 pm
Location: Crawley, West Sussex
Contact:

50/50 weight distribution

Post by cantfindausername »

Hi all,

As some of you know already I have a Toyota MR2, now, being mid engined they can be a bit lively. And I have found in the past can understeer somewhat.

This got me thinking about weight distribution. I think stock its about 40/60, or possibly more rear bias than that.

As such I've looked at losing weight from the rear of the car over the front by means of carbon engine lid, boot lid and anything else i can shed weight off.

Should I be looking to achieve as close to 50/50 distribution to increase handling abilities on track?

Thanks,
Ant
Julian
Posts: 181
Joined: June 22nd, 2006, 6:45 pm
Location: Manchester, UK
Contact:

Re: 50/50 weight distribution

Post by Julian »

This is a very tricky topic, the understeer you mention is not really down to weight distribution, if anything the low front bias should seriously reduce the understeer but because of the natural tendency to oversteer with a heavy rear and rear wheel drive the designers have configured the front geometry to give the driver balance. For a comparison consider the stratos, similar weight distribution but it is exceptionally twitchy and very direct. On the road it is a real handful and needs constant driver attention. The real difference is in the suspension, *not* the weight distribution.

I am not intimately familiar with the MR2 from an engineering perspective so please forgive any mistakes or false assumptions. I am going to guess that the problem is that the front suspension is too stiff on your car, possibly with an anti-roll bar. The rear a bit too soft and with a "stable" rear toe alignment. Changing all of these at once is a recipe for disaster so don't jump in and change all of them. Make small progressive changes and you will find you can make good improvements. The caveat as always is that everything is in good working order to start with, particularly springs, dampers, bushes and balljoints. If not you cannot hope to build a good setup.

For a mid-engined car with the centre of gravity quite high and to the rear it should feel like the car is "pivoting" around the driver's seat as you steer, changing the weight distribution can alter this and lead to some quite strange sensations once you are used to the original format. Your best option is simply weight reduction rather than focusing on one end of the car.

My normal train of thought now would lead to persuading you to try playing with your tyre pressures as a starting point as typically this is all that is wrong in the first place but I suggest you do a search on my posts to find all that advice so I don't have to type it all again. Mid-engined cars particularly can afford to run quite low pressures up front with much less weight when static and under acceleration the shift to the rear means the extra grip from a low front tyre pressure can make it more comfortable. Under braking you have extra mechanical grip too allowing you to brake harder. Don't neglect the rear pressures though and don't have too much of a difference front to rear. On track with pukka track tyres you can probably afford to go down to the low 20's psi but ideally you should check with the tyre manufacturer to find out what the limits are as it changes from one tyre to the next (brand and model).
Guy Croft
Site Admin
Posts: 5039
Joined: June 18th, 2006, 9:31 am
Location: Bedford, UK
Contact:

Re: 50/50 weight distribution

Post by Guy Croft »

Effect of vehicle weight distribution F/R and polar moment of inertia is worth researching also, Ant. There is a fair bit written on the net about this and its relevance to car handling.

GC
Julian
Posts: 181
Joined: June 22nd, 2006, 6:45 pm
Location: Manchester, UK
Contact:

Re: 50/50 weight distribution

Post by Julian »

Now that I have a bit more time and a semi-decent internet connection I can elaborate a little.

Guy has a good point about the polar moment of inertia and if you are preparing a proper track-day car or race car it is an avenue worth persuing as it really makes a difference to how sharp the steering feels and how quickly the car responds. For a road car it is much harder since nearly all of the weight you can safely remove is already well inboard and unlikely to make much difference. At the front end the radiator is a serious contender, possibly a light weight front bumper and front wings (not too hard with an MR2). Replacing the radiator without moving it isn't going to achieve much unfortunately. You can shave off some weight but ideally you want to move it back and that messes with too many aspects of the design - in terms of performance/reliability upgrade it is still worth considering though. Taking a similar approach from the rear can help with your quest for balance. A lightweight exhaust system, and rear bumper can help to remove any excess weight behind the rear wheels.

If you haven't noticed yet but there is a huge wealth of options but they tend to be small details and potentially a lot of hard graft, even with serious aforethought and planning. Your biggest hurdle in my opinion is achieving something coherent. Without a way of measuring what you are doing (corner weight scales) you can only really guess. Ultimately when finished (or at least reaching a milestone) you need to corner weight the car to fine tune the suspension and/or move some ballast around to balance things up. You aren't trying to achieve perfection, just avoid significant imbalance and any subsequent bad behaviour.

You really can go the n'th degree here but you have to understand the effects of what you are doing, some of them are not obvious and can achieve the opposite of what you are working towards. Taking any weight out of the car is going to affect the suspension damping (it will feel stiffer) and in extreme conditions you can find it oversprung which will make the understeer worse and the suspension response poor as the sprung/unsprung ratio narrows.
Last edited by Julian on September 23rd, 2008, 10:05 am, edited 1 time in total.
Guy Croft
Site Admin
Posts: 5039
Joined: June 18th, 2006, 9:31 am
Location: Bedford, UK
Contact:

Re: 50/50 weight distribution

Post by Guy Croft »

As usual some very well-considered and relevant things there Julian.

Well done indeed and thank you.

GC
cantfindausername
Posts: 306
Joined: July 19th, 2006, 6:07 pm
Location: Crawley, West Sussex
Contact:

Re: 50/50 weight distribution

Post by cantfindausername »

Thank you both for the information! Some very good avenues to look down and definately ones that I will be looking at once I get the car back.

Thanks again to both of you.
Ant
Post Reply

Who is online

Users browsing this forum: No registered users and 30 guests