I've been doing a little off-the-cuff thinking about weight balance and grip. I've not put any numbers into it yet, but it's a fairly
widely held belief that having a heavier engine near teh rear makes the car much more re-through-a-hedge happy than a front or front-mid engined
setup. That's fairly understandable - the main pendulous mass is going to attempt to take the rear with it in a straight line, stressing the
tyres more. The question for me is how much does that relate to the weight of the engine.
Since we rotate about the rear diff when turning, an engine mounted at or very near that point gives a rapid turn-in with very little load on the
front tyres from the polar moment of inertia. However as the car starts to turn the heavier rear end will attempt to continue straight on, increasing
likelyhood of oversteer. But that mass also increases grip to some degree.
Without digging out the old suspension design books and looking at the maths involved, it would seem that a heavier engine might help by allowing more
traction. But if the weight/traction relationship is not linear, the higher mass eventually will increase the force on the tyres faster than
increasing grip from the tyres. But when it lets go, the larger mass is going to be slower to change directions, meaning it's possibly more
predictable when it's let go?
Is this the case in reality? i.e. if I drop a heavy-ish lump just in front of the rear axle, how is the handling going to change - am I going to go
from a light and skittish car to a heavy but controllable one, or does the mass/traction relationship mean I go from light and skittish to giant
swinging pendulum of death that has no recovery?
(For reference, I'm still attempting to hone my design criteria to something I want to drive. I want something that's got oodles of grip,
hooks up nicely in corners but can be a bit hooliganish if required. I don't think want something that needs constant correction at every stage
as I've driven cars that felt like they were on tip-toes round corners despite having good grip, and that's not fun, but likewise I
don't want a car that the other half will plant into a hedge backwards as the grip goes from hero to zero in a split second).
It's boiling down to bike or car engine as usual, but I'm not sure which best provides the characteristics I desire.
Not really a very scientific statement here but at North Weald we found that the Xtreme hung on a bit longer with two fat blokes in than it did with
one
Excellent test-work... why didn't you go for three-up!
quote:
Originally posted by scootz
Excellent test-work... why didn't you go for three-up!
It's not as simple as how much weight is on the rear. It's about the proportion of weight between the front and rear and also how much
weight transfer there is under braking and cornering.
With a light front you don't naturally get a sharp turn in, as the weight is trying to push the whole thing in a strieght line. So initially it
tends to understeer. To compensate you trail the brakes into the corner so the weight transfer from braking makes the front end grip. Then as you get
into the corner the pendulum effect of the rear comes in and you get oversteer.... Different balance and a different set of problems and driving stlye
required...
Yep, I'm aware it's a tad more complex than that but I'm looking at general rules of thumb to make an initial decision on direction I
suppose - I'm attempting to identify if I want a light machine with not much power or a heavier/more unbalanced machine with gobs of power.
Naturally I'd do my best to balance up the distribution as best I could but it's never going to be ideal and I'm not keen on the idea
of trailing brakes into corners - my general driving style is used to a grippy 4wd where I brake before the bend, ease off into the bend and as the
car settles after the initial turn in and mild understeer I can give it almost full beans and drag the thing through neutral into oversteer on the
exit. I like that tendency. I'd build a 4wd machine if I could package it correctly (see earlier threads by me!) as I believe it's truly a
better method and feel, but that's subjective I know.
Kermit - cheers, helpful to know that my extra pies might help keep things stuck a little better
Might be worth a U2u to 'cloudy' or 'nitram38' on this forum, that have built and are running midis. I seem to remember one of
them quoted a quite exteme weight distribution - something like 70% to the rear???
Regards
Hugh
I've certainly read about their builds and IIRC one was 345kg with a 70:30 weight distro. That's seriously light and if the car engine I was looking at putting in was there it'd be something like 90:10 (the engine/box im looking at weighs about 225kg and sits ~150mm forward of the rear axle line). Otherwise my build would be very liket he warner machine.
I think anything as unbalanced as 90:10 or even 80:20 is gong to be a pig to handle in nearly all circumstances. The tail will almost definitely wag
the dog - with anywhere near a reasonable power to weight ratio wheelies will be the order of the day when accelerating and no steer (ie terminal
understeer) when exciting the corner under power - assuming that is that you get anywhere near the exit of the corner in the first place.
Extremes seldom work well in nature or mechanics so always go for balance.
The only way it will be even remotely workable is if down force can balance things out and then you are looking at higher speeds than I would like to
be driving such an unbalanced car.
Oh - I forgot to say - the height of the Centre of Gravity of the engine etc. will also have a big influence on the above comments.
[Edited on 6/9/10 by Ivan]
MMmm I know, I'm trying to avoid tail wagging but I'm struggling to see how the Atom achieves its supposed 65/35 ish rearward bias - the
engine and box from that is of similar portly proportions (not quite as bad) the same and sits right over the rear axle. It's an alu block IIRC
but they're not normally that far off the same mass after the additional material is taken into account, and even if it's 175kg that's
still a significant proportion over the back axle.
quote:
Originally posted by hughpinder
Might be worth a U2u to 'cloudy' or 'nitram38' on this forum, that have built and are running midis. I seem to remember one of them quoted a quite exteme weight distribution - something like 70% to the rear???
Regards
Hugh
Yeah, I suspect it helps with launching too, keeping the weight rearward, but how does the atom achieve that with a car engine of significantly more mass than your engine?
I have caught mine a few times in the wet (Mid engine RWD) and it is very predictable. However, im not sure how it will go with an additional 100 - 150hp......
Something else to consider - the weight of the engine is not entirely on the rear axle. My design (See avatar) has a mondeo
engine/box/starter/alternator (=185kg) tilted forwards at 22degrees (standard install in a mondeo is tilted forwards 12 degrees anyway). This means
the COG of the engine/box is 245mm in front of the centreline of the rear axle.The other major weight (me, 90kg) is balanced at 980mm from the rear
axle. Fuel tank and battery are at the front.
My wheel base is 2400 mm, and I've weighed everything I'm going to use. Total weight including driver will be approx 690kg. My weight
distribution should be about 62 % to the rear when the fuel tank if full, 66% when its almost empty. This assumes 1.5mm aly for all the bodwork, but I
will almost certainly only use that thickness for the nose - all the rest will be thinner/lighter, which will move the weight distribution slightly
forwards.
If you went for a lighter engine (duratec is 18kg lighter, and I believe taller), you would be more like 60/40 weight distribution.
Regards
Hugh
Yes, you're right, I'm almost undoubtedly under-estimating my other masses and their locations. I think it's time to plot out some
approximate masses and positions for more detail.
Interesting to have the fuel tank at the front, wasn't sure of the legality of that (though I've just printed out the IVA manual for evening
reading!).
[Edited on 7/9/10 by coyoteboy]
The tank isnt right at the front - its effectively the front of the passenger footwell (but with an alloy sheet between to keep it out of the
passenger area. I've assumed its ok there as thats where the sylva riot/spectre ones are!
Also I'm using mx5 uprights/discs/calipers at the front which are quite heavy - 21kg for the pair (discs 7.2 kg/pair and calipers 8kg/pair!), and
also using the standard donors battery - keeps costs down. If you are worried about the weight being too far back in the car, remember to spend your
money on lightweight bits for the back and not worry too much at the front (unless you're going for the untimate lightweight design that is).
The braking calcs I did give 50/50 weight distribution at 1G braking, allowing for a passenger though!
Regards
Hugh