Wheel size maths
craigdiver - 11/9/16 at 10:34 AM
For the good at maths out there, does my calculations make sense and will this translate to reality.....
Donor car had 18" alloys with 40 profile and I'm planning running 15" wheels with 60 profile, I reckon this will decrease top speen but
increase acceleration, is this true in the real world?
Original wheels
BMW 650mm dia (245/40R15)
Circumference 2042mm
Top speed 142mph, when it was new ;-)
New wheels
Ford 600mm dia (195/60R15)
Circumference 1885mm
Decrease = (2042-1885)/2042 = 7.69%
Therefore 142mph less 7.69% = 130mph
Happy with >100mph top speed.
Should have a, theoretical, 7.69% increase in acceleration.
Slimy38 - 11/9/16 at 11:16 AM
Unless you're maintaining the donor car weight, I would expect your acceleration to improve anyway due to the reduced weight. On the other side,
top speed may be limited by aerodynamics rather than gearing.
In other words, yes your maths makes sense but no it won't translate to reality.
craigdiver - 11/9/16 at 01:08 PM
quote:
Originally posted by Slimy38
Unless you're maintaining the donor car weight, I would expect your acceleration to improve anyway due to the reduced weight. On the other side,
top speed may be limited by aerodynamics rather than gearing.
In other words, yes your maths makes sense but no it won't translate to reality.
Thanks Slimy38,
The donor car, BMW 5 series weighed a flipping tonne. It is also apparent that for BMW to get their 50/50 weight distribution they added weight rather
than shaving it off. The rear subframe - holy crap, must be an easy 200kg!
So yes, I do expect my space frame concoction to have rapid acceleration. My point was more wheel choice based on the acceleration/top end
trade-off.
I was concerned that by fitting smaller diameter wheels I may end up with an unacceptably low max speed.
coyoteboy - 11/9/16 at 05:04 PM
In the first order, yes dropping the wheel size will scale speed (based on circumference) and increase acceleration. It will be notable too, just a
change from a 55 to a 50 profile notably changed my tin top handling and accel. But everything else will dwarf it.
craigdiver - 11/9/16 at 05:21 PM
quote:
Originally posted by coyoteboy
In the first order, yes dropping the wheel size will scale speed (based on circumference) and increase acceleration. It will be notable too, just a
change from a 55 to a 50 profile notably changed my tin top handling and accel. But everything else will dwarf it.
But everything else will dwarf it???
britishtrident - 11/9/16 at 07:08 PM
Often shortening the gearing on roads cars actually increases the top speed.
craigdiver - 11/9/16 at 07:37 PM
quote:
Originally posted by britishtrident
Often shortening the gearing on roads cars actually increases the top speed.
I suppose the original torque curves were designed for a 1500kg car, hopefully I can half that weight, should make an exciting drive!
snapper - 12/9/16 at 05:47 AM
There are gear/speed calculators that you can input wheel/tyre size to find your speed in gear
Whilst they only give a theoretical top speed you'll get an understanding of achievable speed in all the gears
For me hitting over 30 in first and over 60 in second was the target for good off the line acceleration
In truth 40 in first would be better
I was also looking at 4th gear top speed and a cruising 5th ratio
mcerd1 - 12/9/16 at 08:42 AM
what snapper said ^^
these kind of calcs are only good for finding out revs you'll have for each speed / gear combination.
I like this one, but there are loads of different ones avalible:
http://locost7.info/gearcalc.php
for example using the gearing calcs alone mine should be good for well over 150mph - but obviously my 2.1 pinto has no chance of actually achieving
that 
most se7ens have worse aerodynamics than your average brick ! so that limits your top speed more than anything (probably just as well for a bog
std. se7en shape as its also not the most stable at high speed either...)
a rough and ready way of guessing your top speed is:
V = cube root of: P / (0.5*d * Cd * A)
or rewrite for power required to achieve a given speed:
P = 0.5*d * Cd * A * V³
where P is the power in Watts, d is air density (1.3 or 1.22), Cd is the drag coefficient, A is the frontal area in square metres, and v is the
velocity in m/s
0.5*d = ~0.65 (or typically taken as 1.22/2 or 0.613 in the UK)
of course these calcs assume that you'll have gearing suitable to achieve peak power at top speed (or very close to it at least)
So my dax will have an approximate area of 1.85m² (full windscreen etc...)
the estimates for Cd on a se7en are mostly around the 0.6 to 0.7 mark (sometimes more 
- my very ordinary tin-top is ~0.29)
so based on that I'd need in the region of 320bhp at the wheels to be able to hit 150pmh - I know cossie powered ones that get close to 150mph
with a little over 360bhp at the flywheel so that seems about right...
or a bit more realistically the ~140bhp at the flywheel I'm aiming for could be ~120bhp at the wheels and with a middle of the road Cd around
0.65 that would make it about a 110mph top speed
the gear calcs tell me that 110mph in 5th gear gives 4816rpm or 5873rpm in 4th which should be about right (I hope !) or hard on the rev limiter in
3rd
but more importantly I know that I should manage just over 40mph in 1st gear and about 70 in 2nd with 3rd being happy between 50 and 90mph
on the other hand I do plan to use the car on the road so cruising at 3000rpm will still give me ~70mph in 5th
[Edited on 12/9/2016 by mcerd1]
craigdiver - 14/9/16 at 08:20 PM
I thought the sevenesque shape looked aerodynamic but a Cd around the 0.6 to 0.7 mark is poor. What can be done to improve this? I don't intend
to fit a full windscreen and will make the front grille large and free flowing as possible. Anything else to improve the aerodynamics?
bi22le - 14/9/16 at 08:36 PM
quote:
Originally posted by craigdiver
I thought the sevenesque shape looked aerodynamic but a Cd around the 0.6 to 0.7 mark is poor. What can be done to improve this? I don't intend
to fit a full windscreen and will make the front grille large and free flowing as possible. Anything else to improve the aerodynamics?
Flat bottom
vent the back of the engine compartment
half tonneau for passenger area
Or
Rebody to LM car like spire / sabre
craigdiver - 14/9/16 at 08:53 PM
quote:
Originally posted by bi22le
Flat bottom
vent the back of the engine compartment
half tonneau for passenger area
Flat bottom? Engine bay, transmission tunnel?
vent the back of the engine compartment? Large grilles in side panels towards rear of engine bay?
half tonneau for passenger area - should be easy as was thinking of a full tonneau to keep it dry if parked up outside.
Thanks for the info.
Slimy38 - 14/9/16 at 09:21 PM
quote:
Originally posted by craigdiver
Flat bottom? Engine bay, transmission tunnel?
vent the back of the engine compartment? Large grilles in side panels towards rear of engine bay?
half tonneau for passenger area - should be easy as was thinking of a full tonneau to keep it dry if parked up outside.
Thanks for the info.
I believe the flat bottom needs to extend all the way to the back of the car. The rear panel of an average 7 works like a parachute if it's not
sealed, then again you can't completely seal it as you still need rear wheels attached and driven.
Making the front grille free flowing doesn't help the big lump of engine behind it. As a comparison, I believe some Ford's actually have a
grille that closes completely to improve aerodynamics (not that a seven can share that concept).
(Just checked, it's the Ford Focus active grill shutters that I'm thinking of)
[Edited on 14/9/16 by Slimy38]
mcerd1 - 15/9/16 at 08:42 AM
quote:
Originally posted by Slimy38
Making the front grille free flowing doesn't help the big lump of engine behind it. As a comparison, I believe some Ford's actually have a
grille that closes completely to improve aerodynamics (not that a seven can share that concept).
(Just checked, it's the Ford Focus active grill shutters that I'm thinking of)
Thats not exactly a new idea, but most do it using aerodynamics rather than moving parts...
even the original fiesta did this in in the 1970's
Mk1 Fiesta Grill
in general you only want enough air going through the nose to cool the radiator/inter-cooler/oil cooler and feed the air filter if its not drawing
from elsewhere..
but I wouldn't go blocking up the nose either, most sevens have a fairly small opening as it is so cars with forced induction or some bigger
engines can struggle to get enough cold air as it is...
as above getting a good air flow out of the engine bay is important for this, especially if you want to make it flat underneath
also ducting the nose can go along way to making sure that any air going in actually does useful work
BTW what are you planning to do with the car once its built ?
Are you actually going to be able to get to anything close to the kind of speeds above ?
[Edited on 15/9/2016 by mcerd1]
craigdiver - 15/9/16 at 08:09 PM
quote:
Originally posted by mcerd1
BTW what are you planning to do with the car once its built ?
Are you actually going to be able to get to anything close to the kind of speeds above ?
As long as can get to 70 swiftish I'll be happy, may take it on a track day for a play but will not be booting it, not my style. The aerodynamics
issue was more out of intetest and if I can make some minor changes at this early stage of the build then that would be good.
Thanks for all your input, much appreciated
Cheers
Craig :-)
