Is there a reason (in terms of the physics involved) why it is better to push start a car in 2nd gear rather than 1st?

Question

I am struggling to understand why it is recommended to push start a car in 2nd gear rather than 1st - in terms of the physics involved, I fail to see a reason why this would be the case. I have compiled where I am at currently with a for/against list for each option.

Reasons why 1st gear would be better

• The engine has a greater mechanical advantage, so an engine that is not firing correctly, is sputtering and not producing as much torque as a normally running engine is less likely to be forced into a stall than in 2nd gear.
• For a push speed of X, the engine will be spinning at a higher RPM than in 2nd gear and I assume a higher RPM gives the engine a better chance of starting?

Reasons why 2nd gear would be better

• It would give the people pushing the car a greater mechanical advantage than 1st gear, which may make it easier to keep pushing the car while attempting to start it. However, I'm not sure how important this is, as the aim is surely to get the engine running on its own, so continuing to rely on the force provided by the people pushing the car, even after the clutch has been engaged, already suggests the push start has not been successful?

• If the push start is unsuccessful and the car comes to a halt, it should do so over a greater distance/time in 2nd gear than 1st, thereby reducing the risk of injury to the people pushing the car, which could happen if the car stops suddenly.

Reasons why gear selection should be irrelevant

• Assuming the retarding effect of each engine stroke is a constant and the push speed (and therefore momentum) is the same in both cases, then surely the number of strokes before the car comes to a halt should be the same in both cases. As mentioned above, I assume the likelihood of a successful start is probably related to RPM. However, if it is not and is simply a fixed chance that the engine will start during each stroke, then the likelihood of the engine starting should be directly related to the number of strokes, thereby suggesting there is no difference between the two options? - While the car may stop sooner in 1st than 2nd, the number of rotations of the engine should be the same. Admittedly, higher RPM is likely to increase engine braking and therefore starting in 1st gear may mean greater retardation per stroke, possibly favouring 2nd gear in terms of increasing the number of strokes before the car stops. Having said that, given the engine does a lower RPM for each MPH in 2nd than 1st, then the car has to slow down more in 1st than 2nd to reach the speed where the RPM will be too low for the engine to run without stalling, which could in fact go in favour of suggesting that 1st gear will in fact have more potentially successful strokes than 2nd. Overall, it seems as though these benefits would approximately counteract the negatives for each gear in this case.

In terms of the physics involved, I fail to see an obvious reason why being in 2nd gear will be beneficial. Have I missed anything? Or is this more of an old wives' tale than actually being something that will impact the likelihood of a successful push start?

Answer 1

It depends on the car. If it's a big displacement high performance engine, then you may not be able to get the rear wheels to turn unless you're in the highest gear. If it's got an itty bitty engine, then push-starting in 1st may work best. While I don't have personal experience, because weight transfers to the front on deceleration a front wheel drive car probably has an advantage in this regard, too.

Back when I drove unreliable cars with manual transmissions 2nd or 3rd (in a 1969 Toyota Corolla, with 1100cc of pure power under the hood) seemed to work best, particularly in gravel.

(Note that the method is to get the car moving when it's out of gear or with the clutch pressed in, then put it in gear and pop the clutch out suddenly. Even on a little car, in second you can expect to squeak the tires before the engine catches. Note also that while starting going down a hill may seem attractive, there's nothing worse than being at the bottom of said hill without yet having started the car.)

Answer 2

A higher gear ratio means that less force is needed to turn over the engine by pushing the car. Aside from the issue of the tires slipping, humans are more likely to be able to maintain the speed of the car long enough to allow the engine to start if it is in a higher gear.

If you are tow-starting the car, the same thing applies: in a higher gear, there is less "jerk" on the tow rope when you release the clutch, which is less likely to break the rope or cause the towing car to skid or stall. Tow-starting a car at 20 or 30 mph in 3rd gear is easier than trying to do it at 5 or 10 mph in 1st or 2nd. If the towing car stalls and the towed car rear-ends it, this is not a good way to keep your friends!

Answer 3

I never heard of pushing a car is 1st; have you tried it ? In the good old days when cars were not so reliable , I started more than a few in 3 rd ( of 3 speed ). Occasionally started one in second by letting out the clutch after it was moving then quickly pushing the clutch back in and hoping it started instead of sliding the wheels. Maybe with a small engine ( 2 L) with low comprression it would be possible. In the low gears the engine turns over many RPM faster the the wheels which is very difficult , if even possible to do. In top gear the engine is only turning over at 3 or 4 times the RPM of the wheels.

Answer 4

Having pushed many cars on many occasions, on snow as well as gravel and tarmacadam. I can categorically state; on gravel 3rd best nothing lower than 2nd, on snow 4th you can try 3rd and you may be lucky, that's if you have chains or studs on, on Tarmacadam 2nd gear, no lower. Downhill you could use 1st but I would still recommend 2nd. The reason is traction, work it out for yourselves. All I can say is I've done it many many times and glad I took notice of the old chaps, otherwise I’d still be pushing now. You can start it in reverse on a slight down hill, but always remember it is always down to traction, whether or not you have any.

Answer 5

You have a better gear transmission ratio between meters and rounds per minute. In the second gear you need more meters for the same amound of rounds the shaft turns. You get therfore more power on you engine. It helps with compression of gases and friction.

Answer 6

For a push speed of X, the engine will be spinning at a higher RPM than in 2nd gear and I assume a higher RPM gives the engine a better chance of starting?

It could be possible that in second gear, since the push speed of X gives us lesser rpm of the engine providing less of a jerk when starting, causing lesser mechanical stress on the power train.

Answer 7

First a disclaimer: Bump starting a modern car is typically risky, as I have mentioned in comment above. Do it at your risk.

I believe the optimal gear depends on circumstances. For the engine, just RPM matters. From RPM PoV, you can push the car slowly at 1st gear or more quickly at 2nd gear. If the RPM is the same, it does not matter for the engine. It can however matter for the source of pushing:

1. If you push the car by a human, I would probably go for 1st gear, as people cannot run much quickly. Also, it lowers the risk of sudden change of car speed.
2. If you push the car by another car, I would probably go for a similar gear as the pushing car have.
3. If you push the car downhill, it probably depends on the declination.

From safety point-of-view, it is likely safer to bump start the car at lower speed, as you can be distracted by watching your RPM. You are less likely you will reach a curve/crossroad on the road. Also, if you are towed by a car that stalls, you are more likely to be able to stop soon enough.

Answer 8

First let's review:

• In 1st, you get small wheel revs per engine rev
• In 4th, you get large wheel revs per engine rev

Invert that:

• In 1st, you get large engine revs/wheel rev => large engine braking force
• In 4th, you get small engine revs/wheel rev => small engine braking force

All else being equal, we want the most engine revs we can get, which also translates in to faster engine rpm which will aid starting. However, we have a limit: our tires can only handle so much engine braking force before they will skid instead of roll on the pavement. For most cars (generalizing here!) first gear will make the tires skid, while 2nd gear won't.

As mentioned by @Barrie, you can vary this general rule taking into account the size of then engine (thus engine braking force) and the slipperiness of the ground (pavement/dirt/sand/gravel/snow).

Of course, nearly all cars (& motorcycles) are now fuel-injected, so if you have no juice for the fuel-pump you can't start the car no matter how well you try to bump-start it!