[MattsAwesomeStuff]

How long is "too long"?

Your top speed will ultimately be limited by 2 things:

1 - Whether you can actually spin the tires fast enough through the drivetrain, and
2 - Whether you have enough power to actually go that fast.

#1 is trivial, and is almost a matter of convenience. On a long enough track, doesn't really matter, put it in top gear at start and your top speed isn't going to be affected any.

#2 is where you need to put all your brainpower. Figure out what the top speed is likely able to be in terms of power, not mechanically. Then work backwards to find a top gear that is reasonably within the right limit (and just a hair over so you're not leaving anything on the table).

Any time you start talking about a mixture of goals (i.e. top speed while also being X, Y or Z), then it's almost too hard to give advice because it depends so heavily on how you weigh the elements in that mixture and what you're willing to sacrifice of each to get something else.

Can't optimize for multiple variables at the same time.

 

[galderdi]

I wasn't so much looking for validation of the choice of goal.
I was more interested in any flaws in the calculations.

I believe the run up will be 1 mile. I don't have to reach top speed after a mile but being at any lower speed would hurt the average result.
I will be using either chain drive or belt drive to allow the fairly quick replacement of the sprockets to change the ratio. But realistically I will only be able to have 2 or 3 combinations.
At the most 1:1 ratio would give a theoretical top speed of 420kph / 260mph. But it would never get there.
A 2:1 ratio would give a theoretical top speed of 210kph / 130mph which I think is close to where my target should be.
A 3:1 ratio is only slightly higher than my current 5th gear so I know it will accelerate at a reasonable rate in that ratio but would only achieve a maximum speed of around 140kph / 87 mph.

So I think I am tending towards one combination with the 1.6:1 ratio. Another at around 2:1 and a third combination at around 2.3:1.

I would start with the 2.3:1 and increase the ratio if the car proves it can get to the top speed after a mile.

 

[brian_]

Just for clarification, everywhere you list "final gear" ratio you mean the overall ratio, so in the existing car that's the product of the transmission ratio and the built-in final drive ratio (of about 4:1), right?

In the new car, it looks like you are planning a single-ratio drive (by just the chain or belt, without another transmission), instead of the five-ratio transaxle in the current car. Right? If so, are you restricted to a single ratio, or can you use a multi-speed transmission?

I assume that this a straight-line speed contest vehicle, not something going around track.

 

[galderdi]

Thanks Brian, yes Final ratio equals overall ratio taking into account the gearbox gear and the diff ratio but not the wheel/tyre.

Yes the diff ratio is 4.167 to be precise.

Correct there will be no transmission as such. Just direct drive via a belt or chain. I will not be able to select gears while driving. I will be able to swap ratios between runs by changing the combination of sprockets.

I am not restricted to a single gear. But I am tending towards that setup for simplicity, strength and low weight.

The previous car was under 500kg but the new car will require a much heavier roll cage and more substantial body work. So I need to lose the gearbox in order to stay under 500kg.

Yes this will be for salt lake racing and is not intending to see corners or bitumen.

 

[galderdi]

I have done some more calculations and added the projected time to travel the first mile for each ratio. Here is what it shows:

 

[Duncan]

Hi

Those acceleration number seem a bit low to me

My car is 900 kg with me in it and I did the 1/8th mile in 7.9 seconds

That gives me an average acceleration of 6.4 m/sec2 -- 0.65G

That's with direct drive a 4.1:1 diff and 225/45/17 tyres

My problem would be top speed - at the end of the 1/8th I was doing 5300 rpm and 93 mph

You want to do more than double that
Could you get hold of a Leaf motor? - that would do the rpms

 

[galderdi]

Those acceleration figures are averages per gear. There are peaks which are higher. As high as 0.8g in 1st gear. Don't forget You are running twice the voltage and significantly more current so I am not surprised you would have better acceleration. But think about adding another 100Kg to your car in the form of the beefy roll cage and then think about what you'd be forced to remove in order to bring it under 500KG. I would be very interested to know how you would achieve it without reducing the battery capacity.
Aside from the logistics the only way this is going to happen for me is to use my existing components. Leaf/Tesla motors, LIPO batteries, Zilla controllers are not options for me. Let someone follow in my footsteps try all that, I just want to be first and at least be able to say I did it.

Of course I would like to go faster. But put it this way. The current record for the above 500kg categories is only 87mph. So if I can achieve 200kph/120mph on my first attempt in the sub 500KG category I would be happy.

Maybe after reaching that goal I could seek sponsorship to achieve a more optimistic goal.

 

[Duncan]

Hi

You have a 9 inch motor? - so it should be able to do a few more revs
I have a 9 inch I picked up Just in case if you need it

So
Not road legal
Single seater
Not worried about bendy bits

I would be looking at something similar to mine - but the narrowest rear axle I could find

Motor in the tunnel - batteries in the front

Offset the motor to give a bit more space

Bodywork to the outside (not wheel arches like mine)

My chassis is too heavy - I built it too strong - so if you start with the roll structure I bet you would end up lighter than mine and a lot stronger

Nice strong structure - a bit lighter than mine - 20% less frontal area and a much better drag coefficient

With a 9 inch motor and 7,000 rpm that would be 122 mph -

Any old british RWD wrecks lying around you could steal the axle out of ? - Marina, Ford Escort? - even a Cortina

You could just use the Subaru bits like I did - but they are wider and heavier than the older bits

Sounds fun
Is there a salt lake or something near you??

 

[galderdi]

No mine is an 8 inch. It was misrepresented in the advert. (My wife would probably relate)

Not road legal, single seater.

The cage will be heavy. Their specifications are at the extreme end of the spectrum. The tube diameter and wall thickness along with the number of tubes required is extreme. Think top fuel dragster.

There is no way to stay under 500KG if I used a car chassis.

At around 100Kg for the cage, 50KG for the motor, 50Kg for batteries etc etc etc it does not take long to start nudging the 500KG limit. Plus I can't skimp on the length. It needs to be long so it is stable at speed on the salt.

I am planning on making it FWD with no diff and just direct drive via the belt or chain. I was originally thinking shafts directly off each end of the motor but that would be 1:1 ratio and would not work. This will mean the rear wheels can be kept narrow.

The salt lake is Lake Gairdner in South Australia and is just down the road (3000km each way). Come to think of it you are probably just as close as I am.

 

[brian_]

The salt lake is Lake Gairdner in South Australia...

So presumably this will run with Dry Lakes Racers Australia, in class Special Construction / Electric vehicle - Weight I (SV/E1), for which the weight limit in that weight subclass is 500 kg.
2020 DRLA Rulebook

 

[galderdi]

So presumably this will run with Dry Lakes Racers Australia, in class Special Construction / Electric vehicle - Weight I (SV/E1), for which the weight limit in that weight subclass is 500 kg.
2020 DRLA Rulebook

Exactly.

This is the latest design on paper. Although in my head I have revised it since this. I am now going to extend the tail behind the rear wheels and add a flat panel all the way from the top of the cockpit to the rear of the tail. This will force the air to move around the sides of the cockpit rather than over the top. This should minimise lift.

 

[galderdi]

I am keeping the main body fairly narrow which is why I think the front wheels need to be exposed to keep some width for stability. But I am now thinking I'll tuck the rear wheels in under the rear panel work.

 

[brian_]

I am keeping the main body fairly narrow which is why I think the front wheels need to be exposed to keep some width for stability. But I am now thinking I'll tuck the rear wheels in under the rear panel work.

I get the logic of keeping the wheels at one end tucked into the narrow bodywork, and the others further out and separate from the body (presumably with wheel pants). It is almost like a 3-wheeler, but the rules require it to be a four-wheeler... much like an Isetta (which was narrow at the rear and available as both a 4-wheeler and a 3-wheeler with only one rear wheel). This can work with either end having the wide track: the wide front is inherently more stable, but the DeltaWing showed that narrow at the front can work, too.

The question of which wheels to drive is interesting. Acceleration will be slow, so dynamic load transfer isn't important to drive traction, and simply driving the heavy end (or making the drive end heavy, depending on which way you want to approach it) makes sense to me. I'm not sure that driving the front wheels and thus requiring drive axles that accommodate steering angles is optimal. If you don't have a differential, it will be easier to steer with the narrow axle driven. Packing a driven axle and steering into the narrow end might be a challenge. Not having drive axles hanging out in the wind on the wide end would be beneficial aerodynamically. To me, the combination of all of those factors leads to a wide front and rear wheel drive with a simple live beam axle... but it's your car.

 

[galderdi]

Thanks again Brian. I appreciate your experience and input.

Those are valid considerations you've pointed out.

Having now built a FWD and a RWD car from scratch I found the FWD much more forgiving with imperfections in the geometry.
This probably doesn't hold true for a live axle. But for Independent suspension the driven wheels tend to pull in more toe under acceleration. Not a big deal in FWD but induces instability in a RWD.
Also I am avoiding a diff to reduce weight and I need to keep the significant majority of the weight in the front half to keep stability at high speed. So keeping the motor on the front wheels gives me a head start on that objective.

 

[galderdi]

Just another point of interest:

I aim to make the new car much more sleek than the old as per the sketch. Therefore I am expecting an increase in performance above 80kmph/50mph. But it is hard to quantify at this point so I haven't bothered trying.

But I am also removing the gearbox and Diff in favour of the chain or belt drive. Most people agree transmissions cost efficiency. I am seeing some figures on line indicating 5-10% loss for a standard transmission. Obviously a chain drive is not 100% loss free. But what am I realistically gaining? Can I increase the acceleration figures by 5%?

 

[Duncan]

Hi

My tuppence worth - go RWD but no diff - I'm thinking about a "hardtail" with a go kart type driven axle
Or if the rules require it a four link rear

Cover the front wheels - the top of a tyre is going at twice road speed and contributes a LOT of drag

My 11 inch motor has a double ended drive - if 1200 amps and over 800 kg with 4.1:1 reduction gave me 6.4 meters/sec2

Then 1:1 with 500 kg would give - 2.5 m/sec2

And 500 amps would give 1 m/sec2 - or 0.1G

I keep thinking about bolting the rear of the chassis to the case of my motor and bolting (somehow) the wheels to the two ends of the driveshaft

Another alternative would be to use a VW gearbox with the motor hanging out back

I hope the problems with the organising body are short term and you can get back to the twisty stuff again!

I'm intending to a couple of events this year - see how I get on

 

[brian_]

Having now built a FWD and a RWD car from scratch I found the FWD much more forgiving with imperfections in the geometry.
This probably doesn't hold true for a live axle. But for Independent suspension the driven wheels tend to pull in more toe under acceleration. Not a big deal in FWD but induces instability in a RWD.
Also I am avoiding a diff to reduce weight and I need to keep the significant majority of the weight in the front half to keep stability at high speed. So keeping the motor on the front wheels gives me a head start on that objective.

Toe out is actually much more of a stability problem. The tendency to change toe under driving or braking force is dependent on the scrub radius. While most commonly considered as an issue in suspensions which steer, it also applies to the rear suspension. You're probably accustomed to a positive scrub radius (which causes toe-out under driving force), and it can be difficult to avoid a significant positive value with wide tires, but in this type of vehicle the tires will presumably be narrow and you should be able to design for whatever scrub radius you want.

... I need to keep the significant majority of the weight in the front half to keep stability at high speed. So keeping the motor on the front wheels gives me a head start on that objective.

Mass distribution for stability is certainly a substantial consideration.

 

[Frank]

What kind/type of motor are you considering?

Sent from my SM-T380 using Tapatalk

 

[galderdi]

Thanks again Brian. True I hadn't thought of it like that. Yes I usually start with a little neg toe and then it adds even more under acceleration. Both my cars have had steerable driven wheels (even the current RWD). Even if I did revert to RWD for this project they would be steerable rear hubs because I am using a FWD donor vehicle.

Hi Frank, It is an Advanced DC 8 inch. Yes I know there are so many better options around. But as I've said before I am working with what I have. Plus more power is only possible if the batteries are increased which would surely put me over the weight bracket.