Motoring questions you are afraid to ask.

[Alfaguy]

This was a popular thread on Octane so we should keep it going.

I have a constant drain in the battery of my Golf Gti - I have to disconnect the battery if its going to be parked for any prolonged period. Is there any way to systematically track down the source of the leak by using a multimeter etc?

[Type17]

Motoring questions you are afraid to ask.

Yes, set the multimeter to Amps, and disconnect the neg lead from the battery.
Connect the red wire of the meter to the neg lead (or the car body/earth) and the black lead of the meter to the battery neg post.
The reading you get now should be tiny (radio memory and clock, possibly alarm, if it's set).
If the draw is more than about 0.05 amps, there is a problem - you can try removing one fuse at a time to see which circuit is causing the most drain.

[Wheelnut]

Motoring questions you are afraid to ask.

Somebody over here has the same problem: http://www.kelsey-forums.co.uk/cgi-bin/ ... 29/new#new

[Deadly Dave]

Motoring questions you are afraid to ask.

Yes, set the multimeter to Amps, and disconnect the neg lead from the battery.
Connect the red wire of the meter to the neg lead (or the car body/earth) and the black lead of the meter to the battery neg post.
The reading you get now should be tiny (radio memory and clock, possibly alarm, if it's set).
If the draw is more than about 0.05 amps, there is a problem - you can try removing one fuse at a time to see which circuit is causing the most drain.

Or you could pull a fuse and use the multimeter to check the current through the fuseholder contacts. You could also have a problem in the alternator.

[Wheelnut]

Motoring questions you are afraid to ask.

How do you re-size pictures so that they fit in the avatars here? (I think I used to know but I forget)

[hi-rev]

Motoring questions you are afraid to ask.

Why do brake pedals turn soft over time, and have to be re-bled ?
The system is closed, the fluid is the same, and the wear of the brakepad cannot be that great, so what makes the pedal soft ?

[natnif]

Motoring questions you are afraid to ask.

Why do brake pedals turn soft over time, and have to be re-bled ?
The system is closed, the fluid is the same, and the wear of the brakepad cannot be that great, so what makes the pedal soft ?

my understanding is a combination of things. Degradation of the fluid over time, combined with wear on some seals and pipes gives just a little bit less pressure over time.


I posted a similar question on Octane in the same thread but never got an answer. Why do saloon cars not get rear wipers? Hatchbacks have a steeper rake on the rear screen so their run off is better, but they still get one!

[Ryan]

Motoring questions you are afraid to ask.

Why is reverse a straight cut gear?

I know why forward gears are helical (most of the time) but can't figure why reverse isn't.

[sleepy-vtec]

Motoring questions you are afraid to ask.

Why is reverse a straight cut gear?

I know why forward gears are helical (most of the time) but can't figure why reverse isn't.

I'm going to give an idiot answer here, that its so you can't select reverse instead of fourth when you're barreling along. This was one of my greatest fears when learning to drive!!!

[Ste.D]

Motoring questions you are afraid to ask.

How do you re-size pictures so that they fit in the avatars here? (I think I used to know but I forget)

On the photo hosting sites (photobucket etc.) you can change the size of what the photos will be uploaded to, through an option menu. I'm not sure if there is an easier way though.

[Huggie]

Motoring questions you are afraid to ask.

Why do brake pedals turn soft over time, and have to be re-bled ?
The system is closed, the fluid is the same, and the wear of the brakepad cannot be that great, so what makes the pedal soft ?

There can be lots of stuff. As above, the fluid gets old, the flexi hoses can bulge a bit/go a bit elastic and if the pads and disks are in tatters there can be deformation under pressure and your piston/caliper may skew a bit etc etc.

If the pedal is getting long as opposed to soft, then it can be a couple of other things. You can have contamination or leaks on seals or lines that let air into the system, hence needing a bleed. Worn wheel bearings will also give you a long pedal (too much travel before it firms-up). The slackness in the bearings will allow the brake disk will tilt slightly under the weight of the car pushing the pads/pistons back, so the first bit of pedal movement just closes up all the gaps and stands the disk back up. Two quick taps followed by a good push on the pedal sort of confirms this one but not conclusively.

Strangely when I autotested a mini, the pedal used to go long but for a different reason. The car did so few miles and the pads wore so slowly that the caliper pistons and seals would stick together. I would occasionally have to remove the pads and pump out the pistons a bit just to get things moving but only push the piston back enough to wedge the pad back in. The seal sticking to the piston would deform as the piston moved out a of the caliper by a fraction of a millimetre but being stuck it would retract the piston once the brake pedal pressure went.

An experienced hand can normally spot the problem quickly enough.

H

[Huggie]

Motoring questions you are afraid to ask.

Why is reverse a straight cut gear?

I know why forward gears are helical (most of the time) but can't figure why reverse isn't.

I'm going to give an idiot answer here, that its so you can't select reverse instead of fourth when you're barreling along. This was one of my greatest fears when learning to drive!!!

Firstly reverse gears are often straight cut because it is cheaper and easier to cut the teeth. They do not need to carry the power or sustain the wear that the forward gears do so don't need to be so refined (helically cut).

The forward gears in your car use a "synchromesh" system that equalises the speeds of the gears that spin freely on the main shaft with the synchro-hubs that carry the power onto the main shaft. As you engage a gear, a tapered cone is pressed into a tapered ring which brings the two components to the same speed using the friction of your gear oil. Once the parts are close enough in speed, the oil film thickness is squeezed thinner and thinner until the gear parts can engage a ring of teeth on their mating faces. Put a lot more simply, your reverse gear generally has no synchromesh though I believe my Land Cruisers had a syncrho on reverse.

Brain hurts now, I'm off to bed


H

[sleepy-vtec]

Motoring questions you are afraid to ask.

Terrific info and service as always Huggie, you're a credit to oct... Sorry 40aoY.

[-alan-]

Motoring questions you are afraid to ask.

Informative post on the gears Huggie. As an additional point, because of their shape, the teeth in straight cut gears are generally stronger, but also noisier in use due to the way the face of the teeth engage - hence the pronounced whine in cars when you reverse.

Why do saloon cars not get rear wipers? Hatchbacks have a steeper rake on the rear screen so their run off is better, but they still get one!

The boot on saloons acts as a massive splitter, which helps stop the low level flows from ground level recirculating back up to the window. The water/dirt particles thrown up from the road only get as far as the vertical panel at the rear of a saloon car, and don't generally have enough energy to travel a further 2- 3ft upstream against the flow coming off the roof to reach the back window. Look at the dirt pattern on the back of any saloon. Hatchbacks just have one flat surface at the rear, with nothing to stop the dirty (in every sense) air from swirling round and coating the entire rear panel and window.

[Huggie]

Motoring questions you are afraid to ask.

[quote=-alan-]Informative post on the gears Huggie. As an additional point, because of their shape, the teeth in straight cut gears are generally stronger, but also noisier in use due to the way the face of the teeth engage - hence the pronounced whine in cars when you reverse.

Why do saloon cars not get rear wipers? Hatchbacks have a steeper rake on the rear screen so their run off is better, but they still get one!

The boot on saloons acts as a massive splitter, which helps stop the low level flows from ground level recirculating back up to the window. The water/dirt particles thrown up from the road only get as far as the vertical panel at the rear of a saloon car, and don't generally have enough energy to travel a further 2- 3ft upstream against the flow coming off the roof to reach the back window. Look at the dirt pattern on the back of any saloon. Hatchbacks just have one flat surface at the rear, with nothing to stop the dirty (in every sense) air from swirling round and coating the entire rear panel and window.[/quote:p2t6t0oc]

Thanks for the help there -alan-. To show my appreciation I will add to your saloon/hatchback wiper answer too. The boot on a saloon also acts to streamline the air as it passes over this part of the car. By this, I mean it helps to prevent the air passing over the roof and rear screen from breaking out of a laminar (streamlined) pattern and into a turbulent pattern. This smooth streamlined air is separated from and sort of lubricated by the boundry layer in contact with the rear screen. This largely allows the contamination to stay in the air as it passes over the screen.

On my E36 which I never wash enough, there is a distinct pattern of spray on the rear screen where the air passing along the side windows swirls over the rear quarter panels and meets the air passing over the roof and rear screen. One look at the rear spoiler on an E36 M3 shows that the design engineers in BMW catered for these two different air-flows in the design of the spoiler which follows the pattern of soil on the car. Look at the angle on the top surface in the centre and the underside towards each side of the spoiler below:




Hatchbacks on the other hand have a big empty bubble of very turbulent air following them down the road which as you rightly point out can't support the mass or inertia of the muck stirred into it so it lands on the rear screen.

There are drawbacks to being able to touch type at speed.... sorry!

H