Not trying to start a BECvsCEC debate, this could be two CECs we're discussing...
What, in quantifiable terms if possible, do you consider the difference in handling between a 250kg car engine&tranny, 150kg lower power engine
and tranny. I mean it's 100kg, which is about 20% of the weigh of the vehicle, give or take. So straight line performance you only really need to
make up 20% power on top to get similar accel forward, and braking is limited by tyre reaction force which increases with axle load (not
proportionally because the tyre friction coefficient drops a little but fairly close to silly loads (IIRC it was about .85% of max friction at max
load on most car tyres)). Cornering is limited by the lateral tyre forces which again vary by that 85% limited proportion to weight but have to
counter the increasing cornering forces due to the added mass.
It would seem to me that the difference in handling could be as small as to be lost by minor suspension changes?
I'd say the chief argument is balance.
Having an extra 100kg mainly over one axel will up set the handling, but as you can say, can be partically dialed out with a good setup, but it would
still be a compromise.
Is this not more about inertia than weight? OK in a constant load situation but when you are changing direction quickly the weight is effectively multiplied by the g force. Hence light equals nimble.... Hard to quantify though.
More power only improves your in gear acceleration (initial acceleration is traction limited) and top speed (if not gearing limited).
Lower weight improves the following:
Initial Acceleration
In Gear Acceleration
Braking distance
Cornering speed
Load transfer rates
Tyre Wear
Transmission Wear
Braking Wear
Fuel Efficiency
And nearly everything else.
Its a bit of a non-debate really, unless you only want to accelerate from 50-70 on the motorway or break the land speed record then lower weight is
far more beneficial/
apart from crashing into a Volvo
Yes, indeed I'd left rotational inertia out of the equation, which increases with mass but, for arguments sake the rotational inertia of a small
I4 at the front of a seven type vehicle is likely to be notably more than a big V8 at the back of a middy due to their relative distance from the axis
of rotation (somewhere near the back axle). Of course comparing two cars like for like, twice the weight woudl mean twice the inertial load (but how
much is that on top of the primary steady state load of the mass in the corner.
<EDIT to catch up with other posts>
Yes it improves braking etc, but by how much in comparison with engine power. What I'm trying to get at is there must be some sort of line of
compromise. If going for the best physical cornering speed possible you'd go with a 50cc motorbike engine, take ages to get up to speed and then
waz round the corner at twice the speed possible with a car engine. But if you realistically want to accelerate quickly between corners the tradeoff
lies the other way. Forgetting wear rates etc and assuming your box/engine etc are sized for their power and mass already by the OEM and assuming
different engines in the same chassis/suspension arrangement so only engine mass is important.
Clearly its a non-debate if you have inifinte (or at least identical) power capabilities from either engine, but the likelyhood is that the heavier
engine is more capable power-wise. Even without wanting to break land speed records, different teams in the same race would choose different weights
and powers to get very similar times.
I think the debate is a fairly interesting one seeing as we have combinations ranging from mentally overweight V12s to mentally underweight I4s on
this board and everyone seems fairly happy with their vehicle. You see people claiming lighter cars with less power are running rings around larger
engine'd cars, hence the Q's. As an eng I understand the physics behind it and the compromises, but comparing one vehicle to another
different vehicle is fairly obvious, it's the optimisation of one particular vehicle that's quite interesting. My internal questiosn are
what loss of mass would compensate for a 50% drop in power in a general use vehicle. That answer is fairly hard to identify.
[Edited on 17/2/11 by coyoteboy]
At 150kg for a car engine and transmission is probably = 1.6 alloy engine.
At 250kg for engine and transmission is probably = 4.0 alloy engine. (assuming you're not talking of a cast iron lump)
The second will have somewhat more torque, probably save quite a bit of acceleration time from never changing gear.
The handling of the lighter car will be more responsive with higher cornering speed available on the twisties, but the exta weight and torque from the
bigger engine means it could well be more stable on the straighter sections and have better top speed/ acceleration at the top end of the performance
curve.
An example -
caterham superlight 1.6 = 515kg, 150bhp 4.7 sec 0-60, 125 top speed
caterham csr 2.3 = 575 kg, 260bhp, 3.1sec 0-60, 155 top speed (60kg higher weight)
You may be able to find people opinions of the handling of these two models on the interweb. I know someone who raced in the caterham race series
including in a R500. When he got 'old' he changed down to a lower spec/less powerfull car and said he actually did better on average over
the season - something to do with being able to plant the throttle out of every corner, rather than having to be circumspect with the throttle
(especially if its damp).
With another 40 kg to play with you could put a supercharger on the CSR and have even more power.
I don't know if any of that really answers your question though.
Regards
Hugh
I think to some extent it would be relatively easy to quantify empirically.
If you look at formula type cars the difference in lap times between a full fuel tank and a lesser full fuel tank would give you some indication of
the difference, percentage wise, that weight makes without variation in engine power or - because of the usual location of the fuel tank close to the
CG - significant variation of the CG. That difference could be carried across to any car.
Changes in CG will generally worsen the impact.
Some fairly nice points and data from those two posts, cheers, more to chew on. I also wonder if powertrain weight is less of a disadvantage to a
mid-rear configuration than a mid front due to your centre of inertia being closest to centre of rotation, throwing a spanner in those works.
I'll see if I can find any formula lap times/fuel quantities to correlate but I suspect they hide that info fairly well? Although it would have
to be mid-race as tyre temp and wear would factor into it at either end.
I'm really enjoying the research side of this process, it's nice to talk it over with like-minded people.
I haven't got any other useful scientific facts but it all seems to me that it depends on what you want from the car.... Are you talking about 2 different weight CEC engines in a kitcar??
it's much the same as compairing your cars handling when it's you in the car or you have a fat passenger
interesting conversation but consider this. with a bike engine the car weighs 500kg, with a car engine the car weighs 600kg. regardless of these
facts the thing still weighs nothing and will go like stink in what ever direction you want to put it.
unless your racing you will never get near the cars full potential. the skcc bunch go out on weekly runs there are v8's zetecs crossflows pintos
busa's r1's zx10/14's etc etc and it makes naff all difference because what do we do? follow each other in a line
^^^^ very well said
Andy
OK my over-riding reason for doing this (besides loving quick cars) is to surpass myself engineering-wise and design and build the best I can build
on my budget. Clearly "the best" is subjective as it depends on my criteria. From a vehicle I like cornering and acceleration more than
ultimate speed. From a vehicle dynamics point of view I think mid rear is optimal engine placement. I'd hate to drop in a bike engine and then be
gutted at an ultimate lack of power when a passenger was added, but likewise I'd hate to drop in a V8 and find myself unable to give it any beans
in corners because I've got 250kg swinging about at the rear. I'm trying to identify which direction I'm going to go, where the
compromise lies etc. Mechanically I don't really care - I'm confident I can make either work in that sense. I'd love a 6 speed
sequential paddle shift, but I'll forgo it if ultimate power/accel is limited without huge budget etc. Not too worried about comfort, I'm
planning on going down the general shape/size of the Warner R4, I'm just narrowing the powerplant down. As I said at the top, a quick spreadsheet
shows I can get highest easy power to weight from a 3SGTE, followed by a v8, then a bike engine of some sort, but the 3S is an iron horse of portly
stature, it just wins by easy 300hp.
[Edited on 17/2/11 by coyoteboy]
If i was you, i'd go for something in the middle range of engines. So a forced 2ltr powerplant.... Weight is in the middle of the lot and you can
get some considerable amount of power out of them, you can also get a transaxle alot cheaper than a big old engine and top end power just isnt
available about of a bike engine reliably.
EDITED to add: U2U sent coyoteboy
[Edited on 17/2/11 by eddie99]
I suppose this is why Ariel went down the route of the engine they did, I suspect they had similar aims to me but maybe without the giddy 5 year old irrational kid-like draw towards a longitudinal mid rear V8 arrangement. Though their choice of engine is outside my finances and a bit lighter.
quote:
Originally posted by coyoteboy
[Edited on 17/2/11 by coyoteboy]
Ultimately while you can design to help reduce the effects of added weight, you can't get round the fact that the lighter car will corner better,
while the heavier car would out accelerate in the straights, regardless of how good your suspension design (i.e. supposing perfect everything else).
I'm not making the mistake of thinking I can design it all out with my meagre experience, so I'm trying to start with the "best"
of everything I can afford/do/make so that my compromises dont end up with a wet fish If my meagre skills are going to struggle to work around a
boat anchor out back, I'll not take that route. Roll will be more of a concern with a heavier, higher engine, but thats a known known I can work
with.
And at the end of the day, if I'm talking 0.5 of a second a lap difference, I might as well take the cheapest route, but if I'm talking 5
seconds a lap, or 2 seconds extra to 60, I'll invest a tad more.
[Edited on 17/2/11 by coyoteboy]
In some circumstances, i think you are talking 2-3 seconds a lap.... In others it might only be half of a second, But you need to give us specific engines to compare really.... U2U sent by the way
I really dont think you can possibly hope to quantify all this and get 'the answer' you desire. Just way too many variables, including what
kind of driver you are and what kind of track you're talking about. You may as well just browse the Top Gear Test Track times - probably about
the most comprehensive set of data on this subject you can possibly get - and see if you can make any sense of it. You'll quickly find pretty
much any size/layout can be made to 'work' - how different can a Veyron and a Caterham R500 be in the parameters you're considering,
yet how similar the times. You'll also realise, I dare say, that simple old power-to-weight ratio is a surprisingly good and reliable performance
indicator - no real need to complicate things much further than that.
So if I were you I'd just generate some power-to-weight estimates for the layouts you're considering. That will give a good enough idea.
In fact if I were really you I'd just choose whatever I liked the sound of! You're building a car to enjoy owning and driving (most of the
time nowhere near ten tenths I suspect) - not to answer theoretical questions. Use the in-danger-of-becomming-slightly-OCD search for the best
possible laptime per £££ to guide your decisions for sure, but there are much more important criteria for deciding what kind of car to spend 1000s of
hours building from scratch and hopefully years owning/driving.
[Edited on 17/2/11 by Liam]
Your last paragraph is really playing on my mind though. I'm happy with numbers, not so happy with emotional drives. IF I ask myself "why
did I build this car as it is?" I don't want my answer to be "because I like the giggles a V8 gives me, even if it handles like poo and
can be out-sprinted by a car with 150 fewer ponies
As before, it seems to be power to weight that has the biggest factor as mentioned. Just make what you're happy with and don't worry about the last 1000th of a second per lap. I'm sure for all on here its their skill thats the limiting factor not the car.
Also, dont forget ultimate traction you can get from your tyres. I am also designing (slowly) a rear engined CEC using the zetec powerplant from a
mondeo. I have all my donor parts and have weighed and dimensioned them. The 2.0 zetec engine I have , with standard flywheel /clutch, heavy duty gear
box, drive shafts, starter, alternator, induction manifold+Cast iron exhaust manifold comes in at 186kg (so under 180kg if you ignore the drive
shafts, but add the air box). If I was starting again I'd use the 2.0 duratec from a mondeo. Since the literature says the engine is 18kg lighter
than a zetec, I believe you will have a engine/gearbox/driveshafts/throttle bodies etc coming in ready to run at under 170kg. This engine can easily
and cheaply be tuned to over 200 bhp - nearer 300 if you spend some more after reading flak monkeys stickied thread on supercharging. You have the
advantage of being almost at the bottom end of your specified weight range, relatively cheap, easy to get good power from, with good headroom to
improve the power later.
The caterham top models are about 250bhp and will spin their wheels in most situations if you provoke them, so in my opinion not worth going for a
great deal more power than this, although more power does mean more top end speed (I know with RWD you can get a bit more power down because of the
rearward weight bias). Also they achieve 0-60 time of about 3.1 seconds, 1G =2.7 seconds so you won't get much quicker!
If you are thinking of a V8, I would imagine it'll be quite hard to fit in the back of such a small car!
Regards
Hugh
When compairing power of differant engines you also need to look at the torque curve and gearing.
an engine producing less power but at much higher rpm will use lower gearing to improve performance.
ie an engine with 300foot/pounds at 3000rpm would be lose to an engine with 150foot/pounds at 9000rpm due to a 3 to 1 gearing advantage for the same
road speed. If you see what I mean.
http://www.datsuns.com/Tech/whentoshift.htm
http://www.datsuns.com/Tech/torquehp.htm
The above 2 articles make interesting reading.
My choise would be the smaller engine which revs high.
large (heavy) engines tend to rev lower. good for towing but prm wins for speed.
F1 do 18000rpm .
The problem is that with the correct gearing, any of the engine/box combos can produce the torque required to do the acceleration wanted (and more),
so what you want is the torque curve to be flat and near max. This isn't the case with most small revvy engines, they usually require upper end
revs to get any guts at all.
The trouble with having only a fixed few donors is that you gets what you get. Pick a light engine and you have one with a fairly peaky torque curve
which doesn't feel as nice to drive.