Shorter Stopping Distance for ultralight vehicles?

mheslep

The concept of ultralight vehicles is intended to allow greater fuel efficiencies in part by the use of composite structures to reduce mass by 2 or 3x. In several discussions of these vehicles I have seen and heard mention of the supposed additional safety benefit of shorter stopping distances, but I have not found any elaboration on why this is so, implying I fear that I missing something obvious.

Of course I reached for the standard stopping distance derivation: the kinetic energy of the vehicle and the work done by friction are both linearly related to mass, so that stopping distance is independent of mass as shown here:
http://hyperphysics.phy-astr.gsu.edu/HBASE/crstp.html
giving the familiar distance = velocity^2/(2*Cf*gravity)

So is there some other mass related factor here that is, say, a practical result of chassis, suspension, tires, or brake design? Reduced sway?

The ultralight vehicle article is here:
http://www.rmi.org/images/PDFs/Trans...nStategies.pdf
Is lengthy covering several disciplines and I do not mean to introduce it all here. I am referring to the safety section on pg 14:
Thanks for any comments

mheslep

Bump.

Am I missing the obvious?

Mech_Engineer

While lighter vehicles are easier to stop, technically stopping time has more to do with the capacity of the brakes, and the contact patch of the tires. You can make a relatively heavy vehicle stop very fast with big enough brakes; but big brakes are expensive, require more clearance (larger wheels on the car), and more maintinence.

Really the largest benefit of lightening a vehicle is the kinetic energy required to get it moving, reducing fuel consumption when accelerating. In the case of race cars, reducing weight increases acceleration with a set amount of power generation.

NateTG

In racing you can also use softer tires with a lighter car which improves the coefficient of friction. Lighter cars don't need the same amount brake ventilation (also power related) and lighter cars benefit more from aerodynamic downforce.

For suspension, lighter cars will have less unsprung weight, but heavier cars may have proportionally less, and I'm not sure which end comes out favored there.

mheslep

Yes. Modern brakes have ability to lock up the wheels immediately anything up to super large SUVs AFAIK, so this should give no advantage to ultralights.
Yes of course. One of the oft cited reasons for not building cars that obtain these advantages is safety - low mass losing to high mass in collisions. Now there are claims that low mass vehicles have the advantage in stopping distance which helps the safety case and could allow the efficiency savings to go forward. Unfortunately I don't see how this (stopping dist) is accomplished.

mheslep

And also increases rolling resistance, which is antithetical to the concept of ultralights. I don't think that is how they get there?

Yes I can see the reduced structural mass allows many other things like brakes to also grow smaller, but I don't see how that helps with safety and stopping distance?

Mech_Engineer

The point is that there isn't any fundamental reason an ultra-light car can stop faster than a heavy one, as long as the brakes and tires on each car are sized appropriately. Theoretically, if you are using the same size brakes and tires on two different weight cars, the lighter one will stop faster; but, this argument is not really applicable to a vehicle that is being designed from scratch and can have brakes designed accordingly.

The advertising claim that super-light cars stop faster than heavy ones is really just trying to sell them; it isn't necessarily based in fact. It could be argued that it is easier and cheaper to make a light car stop quickly, but that's about it (and it's easier and cheaper to do most anything performance-based in a lightweight car).

montoyas7940

An ultralight aircraft can fly much slower and land at a steeper angle. The slow speed means less energy to dissipate with brakes. The steep approach angle means a more precise touchdown and better obstacle clearance at the end of the runway.

This is not the same subject but I have always thought it was interesting. A lightly loaded airplane will not glide as far as the same airplane heavily loaded.

mheslep

Did a bit of surfing and collected stopping distance specs as tested by Edmunds

Stopping distance from 60mph
BMW M3: 3726 lbs, 19" tires, 100 ft (best any vehicle Edmunds tested)
Jaguar XF: 4200lbs, 20" wheels: 108 ft
Pontiac G8 GT: 4000lbs, 109 ft
Audi A6: 114ft
Lexus LS 400: 4500lbs, 120 ft
VW Golf GTI (1998): 2800lbs, 139 ft
Jeep Wrangler Rubicon (year?): 165 ft
1997 Wrangler: 184 feet (rear drums)
2003 Wrangler: 167.4 feet (rear disc)
2007 Wrangler (4 door): 4592lbs 148 feet (rear disc, larger front disc)

So distance is all over the place, with little correlation to mass. I conclude then that this is all braking system and wheel/tire related, which further detracts from the claim by car ultra-lighters that they have an intrinsic advantage in stopping distance.

mgb_phys

The UK driving test has a big list of stopping distances that you have to memorise. Of course you aren't asked what 73m looks like on the road - you just have to recite "18m reacting and 55m stopping at 60mph"
Many of the accidents on UK roads are presumably caused by drivers trying to use a theodolite to measure the distance to the car in front while driving.

A UK car show just found that the shortest stopping distance was for small sporty hatchbacks, typically < 25m from 70mph or a 1/3 the official distance.
This site lists the typical distances for lots of cars (100mkh = 62mph) http://www.movit.de/rahmen/stoptbl.htm

mheslep

thanks mgb_phys, very interesting as it lists the same vehicle 'empty' and 'fully loaded'. The fully loaded cases looks to be on average 3-4M longer and in some cases 15-20M longer!! This then supports the case of the ultralight vehicle designers: they can stop shorter. I'm at a loss to explain why!

Mech_Engineer

No, what this proves is that increasing the weight while keeping the same brakes means the vehicle will take longer to stop. This is because brakes have an associated "power rating," which can be thought of in terms of horsepower or watts.

Since the brakes at maximum clamping force can only convert a specific amount of kinetic energy per second to heat, having more weight means more kinetic energy which in turn means it takes longer to convert all of the kinetic energy to heat.

mheslep

Isn't that what I asserted, expressed as a negative?

Ah. Ok, I suppose I knew this but was skimming by it. You've expressed it clearly here and exposes my misconception: the original distance = v^2 / (Cf*g) equation, independent of mass, is not reflective of modern reality. That equation is derived assuming dynamic friction (locked, skidding tires) where the vehicle mass directly controls the stopping force due to friction. The modern reality is unlocked tires and the stopping force is due solely to the brake pressure, so that the stopping force is mostly independent of vehicle mass in the case of tire/surface static friction, rather is dependent on the brake pad pressure. In that case, for given brake horsepower and anti-lock braking, the lighter the vehicle the sooner it stops.

Edit: Mech_engineer - I see you had been saying essentially this above already; I missed the point because I was too focused on that equation. Thanks.

mheslep

I think the point of the ultralight vehicles is that they can afford to put the same HP braking system in their 1600lb vehicle (planned) as is used in say a comparably sized 3000lb vehicle and thus they'll stop dramatically shorter. The ultralight designers referenced in the OP article are keenly aware of safety criticisms in their design so they are planning to take advantage of stopping distance.

Mech_Engineer

One of my favorite articles Road and Track has ever published is the August 2003 "Power Trip" where they have a 0-100-0 deathmatch. Basically they haul butt to 100 mph as fast as they possibly can, and then slam on the brakes to get back to zero. Shortest time in each class wins.

Not only is it incredible the amount of time some of the vehicles take to do it, but lots of useful data was recorded about each vehicle, that can be used to compare them in a sort of an apples to apples test.

Here is an interesting graph from that article:


http://www.roadandtrack.com/article....&page_number=5

The Viper puts down an average of 237 hp getting from 0-100 mph, but has an average braking power from 100-0 mph of 547 hp. Looking at the graph we can see the braking curve is very linear, so we probably have a good estimate of the braking system's maximum power dissipation (taking into account traction available from the tires as well)...

But look at this next graph:


http://www.roadandtrack.com/article....&page_number=9

In the "exotic" class, the Saleen S7 is pitted against the Lamborghini Murcielago. The S7 weighs in at 3050 lb, a full 1140 lbs lighter than the Lamborghini. Yet, the Lamborghini stops 70 feet shorter and 0.8 seconds faster from 100mph than the S7. Why?

Both cars have the exact same tires fitted (Pirelli P Zero Rosso's, 245/ 35ZR-18 front and 335/ 30ZR-18 rear), so the answer has to be a combination of more traction available to the Lamborghini because it weighs more, and the fact that the Saleen does not have ABS. The Saleen should have more braking power available, since it has 1" larger discs in the front and 0.8" larger dics in the rear, but its traction is limited by its lighter weight, and its lack of ABS causes the tires to lock up easily...

The effects of no ABS can be seen in the graph, where the Lamborghini's braking curve is completely linear all the way to from 100 to 0 mph, while the Saleen's fluctuates wildly since the driver has to modulate the pedal to try and make up for the lack of ABS. Even though the Saleen was much faster to 100 mph, it ironically loses the 0-100-0 because the Lamborghini is HEAVIER (more traction available from the ame set of tires) and has ABS. The Lamborghini puts down an average of 606 braking hp, versus the Saleen's "paltry" 370 braking hp.

So there you have it, a case where being heavier means a shorter stopping distance...

mheslep

It may be important to point out that the ultralight concept car proposed in the Moore - Lovins paper not some kind of tiny toy car. It is a five-six seater roomy design and comparable in passenger room to the Ford Taurus, and thus Moore-Levins has room for Taurus sized brakes. However the Moore-Lovins design is 854kg and the Taurus is 1423kg.

mheslep

Interesting, note the stopping g's: I'll call the 911 stopping time from 100 mph ~4.4 secs so that is just over one G. I wonder if there is an upper G limit beyond which it doesn't improve safety on average to stop any faster. That is, lots of minor injuries in numerous high G stops - no impact vs severe injuries in the relatively rare impact.