[Scott Oliver]

Scott Oliver  asked the Naked Scientists:
   
Hi there i wondered if you can settle an argument for me?
 
I build high performance engines and i was asked by a customer if they should upgrade the brakes on there vehicle after having the engine i built fitted.

I replied to this by saying  "if you don't drive the vehicle any faster than you did before but just use the extra power you have to get to the legal speed limit of 70mph quicker then NO. you don't need upgraded your brakes"
 
A friend of mine responded by saying i was incorrect and that if a vehicle was to have a collision accelerating up to and at the point 70mph the damage would be greater that that of a vehicle cruising at 70mph was to collide with the same object.
 
My understanding is that at point of impact the terminal velocity of each vehicle would cause the same amount of kinetic energy and therefore  the same amount of damage is this correct?
 
Thanks for your time
Scott Oliver

What do you think?

[rosy]

Provided the driver has taken their foot off the gas, the key point is the speed the vehicle is going at the moment of impact. If the driver has failed to see the obstruction and driven full into it without lifting their foot then the extra power might make things worse than if the same thing had happened with a less powerful engine.

Also, if the driver is accelerating at the point of noticing the obstruction they will carry on accelerating for the time it takes them to register what's going on and take their foot off the gas and put it on the brake, so if the acceleration was much better then they might be going a bit faster at the point of collision, but probably not any faster than the speed they were aiming to accelerate to.

If the new engine is significantly heavier or lighter then for the same speed the amount of momentum will be greater (worse for braking) or less (better), which might make a difference if it were a noticeable fraction of the total weight of the vehicle.

But the key point is, as you say, the speed. If they're sticking to the speeds they were using before, then the same brakes will be as adequate (or not) as they always were.

[Geezer]

I think it's the speed of the vehicles at the time of impact that determines the amount of damage, so in that respect you are correct. However, bear in mind that the potential for increased acceleration associated with greater power output also means that the vehicle can be travelling faster (than an unmodified vehicle) as it approaches a road hazard that the driver has not yet detected.

When the driver does detect the hazard, his higher speed means he will have less stopping distance before he reaches the hazard, and this, coupled with the fact that his reaction time will be just as long, could make a considerable difference to the outcome. In that situation, better brakes might be enough to prevent a fatal accident. So, in that respect I have to agree with your friend.

[CliffordK]

I suppose there are a few questions...

What would be the point of putting an 700HP engine in a Pinto...  unless you want to drive the Pinto faster?
(sorry, does that date me a bit? [|)])

If you have made your car heavier, for example making your Pinto 50% heavier, then you would certainly need to upgrade the brakes.  If the car remains the same weight, then perhaps not.

Are you also adding higher traction tires?  Shouldn't the brakes match the tires?

The velocity vs acceleration argument is a good argument, and to some extent there are advantages of having a "responsive" vehicle.  However, if you start out at a red light, and get up to 70MPH before getting across the interchange (or at least have your wheels spinning at 70 MPH)...  perhaps good brakes would be helpful!!!  Not all driving is on the freeway after all.

[graham.d]

If I remember correctly, wouldn't it be a wise improvement to the Pinto to have a non-exploding fuel tank :-)

From the physics point of view, and as rosy said, it makes no difference how fast you get to a particular speed; the energy you have to dissipate in the brakes is the same.

[Don_1]

I think you are right, but only in a situation where the brakes are not applied.

If a car is capable of 0 - 60 mph in 6 seconds, it must need better braking capability than one which takes 12 seconds to reach the same speed. Leaving aside driver reaction times, if 4 seconds into acceleration, braking is necessitated, the faster accelerating car may have reached 45 mph, while the slower car may still be at around 30 mph, the faster car will be nearer the obstruction and, with identical braking power, will cover a greater distance under braking.

Of course you would be right again if the driver of the faster car were to accelerate at the same rate as the driver of the slower car, but lets face it, if the driver has come to you to improve the performance of his car, the chances are that he intends to use that power, otherwise, why did he bother in the first place???

All in all, I would agree that increasing the power output of a car would necessitate uprating the braking system.

[graham.d]

It is a bit of an odd argument to base the quality of the brakes on the acceleration capability. The brakes should be as good as possible in any case, independent of the acceleration that a driver chooses to use. The corrollary to this argument is saying to the guy with the standard brakes that his car won't stop as quickly, from say 40mph, as one that has had its brakes improved. I would argue that the basic model is therefore inadequate.

In any case, modern braking systems do not generally have the old problems that you cannot stomp on the pedal hard enough, or, if you do, it locks the wheels. I would assume disc brakes all round, anti-lock systems, modern pad materials, adequate cooling and suitable tyres. If we are talking about older vehicles then improving brakes is a good idea (if remotely practical) whether the car has a bigger engine put in or not.

The idea that being able to accelerate faster increases the need for better brakes because you may have reached a faster speed before the sudden need to apply them is not a situation that is likely to occur too often. Where I agree that there may be a correlation between someone getting a more powerful engine and the need for better brakes, is in the psychology of the person who wants a go-faster car. He is probably likely to be someone who likes to drive fast and, therefore, will be in need (whether he realises or not) of a good braking system.

To answer Scott's question again, your answer is right and you win your argument. Whether you should give advice on brake improvement to your customers is up to you, but if you can do it then it's good advice that could potentially get you more business :-)

[peppercorn]

Why has no one mentioned brake fade?
That can still occur (even on a 'modern') can't it?

Acceleration in of itself isn't the issue, but as mentioned driver reaction times (off the gas, on the brake), driver personality (a granny is unlikely to hot-up her ride) and brake fade (again linked to driving style) are.

I would suggest the OP is wrong (on average) for these reasons - old or new car.

Replacing the factory engine (or at least it's bottom-end <as opposed to oversizing pistons> - where the majority of the weight is) with a larger one is also, almost certainly, going to call for (ideally) larger brakes.  The centre of gravity may be higher or lower also - brake balance issue.

[imatfaal]

I would agree with Don that whilst an accident at 70mph is the same (with respect to peppercorn the car might be heavier or lighter - we cannot know the change in characteristics) a car that accelerates more quickly will be more likely to be going at a higher speed when an incident occurs. 

In reality the answer is to find a brake specialist and set up a mutually beneficial referral relationship.

[graham.d]

A lot of modern car models come with a variety of engine powers - often done by simply "re-chipping" the electronic control systems. Brakes are usually the same and are as good as they reasonably can be - at least the brake facilities don't normally have "options" to make them better. It would be extremely bad marketing to suggest that they were less than optimum on the cheaper model.

[Geezer]

Gosh, I wonder why they put those really expensive brakes on Porsches, Ferraris and the like?  []

[Bored chemist]

Gosh, I wonder why they put those really expensive brakes on Porsches, Ferraris and the like?  []

Because they can.

[Geezer]

Gosh, I wonder why they put those really expensive brakes on Porsches, Ferraris and the like?  []

Because they can.

I'm sure there is some truth to that, but for those who must, performance isn't just about going faster, it's also about being able to slow down in less distance repeatedly without the brakes fading.

For example, according to Road and Track magazine, a Honda Accord requires a minimum of 145 feet to come to a stop from 60 mph.

A Lamborghini Galardo Superleggera (whatever that is, but I'm sure it not cheap) only requires 109 feet to stop from 60 mph.

[graham.d]

It also has a lot to do with very wide, sticky tyres (totally rubbish in the snow of course). Using slicks would be even better (except in the wet) :-) Most people can't afford the advanced materials and design methods used in a Ferrari's brakes. People who have fast, high performance cars, and are willing to pay for them, will pay for lots of other advanced features too. It is also the case, at least in europe, where if you buy a car that will do 145mph you will get it with tyres rated to that speed, even if the country's driving restrictions don't permit it. Expensive cars have generally better safety features too, better crumple zone design, tensioning seat belts, good airbag systems, directional headlamps, hands free voice activated ancillary controls, tyre pressure warning systems, radar distancing systems, driver monitoring (detects wandering) plus many others. This is all good stuff but comes at a price. Ideally these features should be on all vehicles and, indeed, you can get some of these features on some low performance vehicles too (the Prius for example). But this is a long way off the guy who just wants to get a few more bhp from his engine.

[Geezer]

Ahem!  []

I suspect it's impossible to reach a conclusion on this without a bit of quantification. If we are talking about a power increase of less than 5%, the braking issue is probably entirely academic. If we are talking about 100%, I think it would be extremely foolish not to upgrade the braking system (and lots of other things too.) So, somewhere between these extremes, the braking capability of the vehicle should be carefully considered.

If the power increase is less than 5%, I have to wonder why anyone would bother spending money on it, because the actual difference in performance will be difficult to detect. On the other hand, spending the money instead on brakes, suspension, and tyres, could significantly improve the vehicle's performance.

 

[CliffordK]

I suspect it's impossible to reach a conclusion on this without a bit of quantification. If we are talking about a power increase of less than 5%, the braking issue is probably entirely academic. If we are talking about 100%, I think it would be extremely foolish not to upgrade the braking system (and lots of other things too.) So, somewhere between these extremes, the braking capability of the vehicle should be carefully considered.

So...
If it is more on the range of a 4000% increase...
That is when you not only update the brakes.
But also add a parachute?

[Geezer]

So...
If it is more on the range of a 4000% increase...
That is when you not only update the brakes.
But also add a parachute?

If you are talking about your Pinto, I'd skip the parachute and invest the money in a whole life policy.

[CliffordK]

So...
If it is more on the range of a 4000% increase...
That is when you not only update the brakes.
But also add a parachute?

If you are talking about your Pinto, I'd skip the parachute and invest the money in a whole life policy.

No...  You'd have to go with an aircraft engine to bump the Pinto up by 4000%.

But...  The FIAT Topolino had about a 20HP engine.
Replacing it with a supercharged bigblock, one could theoretically get it up to about 800HP (actually it would be 820 HP for 4000%)

Maybe that is a bit optimistic for an HP rating.
But, I think the original HP rating was closer to 13HP (500A) or 16HP (500B/C), so your actual numbers could be in excess of a 5000% gain.