# My car locks lower gears when on high speed, why?

When I'm on say 50 Km/h my first gear is not accesible and second gear is too, 3rd gear however is hard to push in but 4th gear is easy to slip in, why?

I'm assuming you have a manual transmission.

Stop doing that, you will damage your transmission.

Your powertrain can be broke down into three sections. The engine, including the fly wheels. The clutch and input to the transmission. Finally the transmission output.

When you push the clutch the engine is disconnected from the transmission input. When you shift a gear, you change the ratio between the input of the transmission and the out put. Also when you shift gears there are collars called blocking rings that slide over and engage a set of teeth to lock you into gear. The blocking ring and teeth have to be spinning at the same speed for this to happen otherwise they'll just grind. To get them spinning the same speed a synchronize is used. The synchronizer uses friction to get the two part spinning together. As you drive you accelerate the engine to gain more speed. Then you push in the clutch the disengage the transmission. As you shift into the next gear the synchronizer will slow the clutch allowing the next gear to engage. As you shift gears the engine also slows so that when the clutch is let out they are roughly at the same speed.

Now lets play a hypothetical game. Lets say you can reach 20Km/h in first gear at 6000 rpm. How fast would the engine have to spin to get the car to 40Hm/h. The answer is 12000 RPM. How fast would the engine have to spin would to get the car up to 60Km/h. the answer is 18000 RPM.

In this hypothetical if you're driving at 60Km/h and try to shift the car into 1st gear how fast are you asking clutch to spin. The answer is 18000 RPM. The synchronize just can't handle that. Because there is such a difference in speed the gear just won't engage. It's probably grinding but so fast that you can't tell.

• Thanks for the answer vini! no harm done to the engine. While I was on high speed, I have clutch floored all the way down and never had let my foot off the clutch. I assume this was safe while I was testing, am I correct vini? or would this still do any damage? – Jomar Sevillejo Feb 19 '16 at 0:31
• @JomarSevillejo The engine would be fine but the synchronizers would not be happy. As i mentioned the synchronizers use a friction type material and that can be damaged. – vini_i Feb 19 '16 at 0:32
• Thanks vini, I missed the part of the gears still spinning because the car was still moving. I was wrong to think that the gears in the transmission were not moving because the engine was detached. A big face palm. Thanks vini! – Jomar Sevillejo Feb 19 '16 at 0:37
• @JomarSevillejo Switching to a higher gear at low speed slows the clutch down. This will not damage anything. If you go into a gear that is too high you will lug the engine, meaning that the engine RPM will drop too low and the engine will have a hard time recovering while speeding up the car. – vini_i Feb 19 '16 at 0:45
• @JomarSevillejo just to add that if you drop the engine RPM too low by using a gear that is too high you can damage the engine. This lugging significantly increases the forces on the internal of the motor and can result in high wear and outright mechanical failure (stuff breaks). – Ukko Nov 15 '16 at 17:39

In Russia many pro drivers and just experienced drivers brake with transmission by shifting to lower gear in speed and letting go clutch fast no problem.

• Is letting go of the clutch not bad for the drivetrain? It's taking all the enrgy so sudden. – Jomar Sevillejo Jul 14 '16 at 10:05
• @JomarSevillejo It can be damaing, but experienced drivers will rev the engine to match the transmission speed, such that when they release the clutch, the engine is already spinning at the required rpm for the lower gear. This is called rev-match downshifting, and is the least damaging way to use the engine to slow down (engine braking). Pro drivers will use a technique called "heel-toe shifting", where they will press the clutch with their left foot and brake with their right toe as normal, then rev match using the heel of their right foot while still pressing the brake. – MooseLucifer Jul 14 '16 at 17:54