Bleedin' handbrake!

[APS]

Preemptive apology; This post is not about how to bleed your handbrake. Sanity has not taken total leave. No, I'm somewhat (really) annoyed and need to vent a little. Just a small rant...

My son's Aygo has failed its MOT on the handbrake and I've not been able to get it to pass. At first, when I took it for its MOT on Saturday and the chaps told me it failed on handbrake effectiveness being low and binding on the left hand side, I was a little taken aback. We had worked on the rear brakes only last year since the auto adjuster on the right hand side had broken. As such it wasn't that long ago we had it all apart, cleaned up, exercised and adjusted. But, stranger things has happened.

It was about 10.00 and the MOT garage closed at 1 so I had a couple of hours to go home, and get things sorted and come back for a retest. I did so, and after about an hour and a half I had a handbrake that could easily lock up while driving on tarmac. All good I thought. Took it back to the garage. Nope, sir. It is worse now.

Errmm, OK. That's weird. But OK. Drives home, disappointed and a little ashamed.

Mind whirring. Why is it not working properly? It was adjusted properly. It worked fine. Oh well, let's get some new brake shoes. Local garage had one set left - £40 for both sides. Not much to discuss. Went home installed the new brake shoes, adjusted, and, again, the handbrake worked all good. This is like a new car! Pull the handle while driving, the wheels go *bam*. (just like they did the day before)

This morning, I popped over to the MOT garage again, for a quick retest. And, guess what, it failed. Blistering barnacles! This time they didn't show me the meter readings. I think it was really close to passing. But I was just fuming and wanted to go home. They recommended I get "someone who knows how to do this" and someone who "can measure the brake effectiveness". Well, that went down like a lead balloon with me. 

So, tonight I have tried the handbrake again. It did work. But admittedly, I think the effectiveness on the left hand side was slightly lower than on the right. It's really odd. But I think the mechanism has just settled while driving to the garage and it doesn't balance very well. After making further adjustments, I heeded the mechanic's advice and decided to measure the torque needed to turn the rear hubs with the handbrake applied. This is where the engineer in me came out...

At four clicks, the right hand wheel turned at 20 kg with 1 metre leverage, which gives about 196 Nm of torque at the hub. The left side needed about 23 kg or 226 Nm. Translating that to the force a the wheel perimeter, which has a radius of about 28 cm (it's a bit less, but we're erring on the side of caution here) that would mean 226/0.28 = 807 N holding force per rear wheel. Assuming the car weighs 1000 kg (the actual weight is a bit less, but 1000 a nice number to use) and it stands on a 15º degree incline, that would give a  downhill force (parallel to the surface) of sin(15) * (1000*9.81) ≈ 2540 N. That's quite a lot. And my handbrake is only holding 807 * 2 = 1614 N. But, 15º is quite a lot and the handbrake lever could be pulled further. Another, click and the same measurements were more than 35 kg (outside range of my gauge), or 343 Nm of torque at the centre of the wheel, or 343/0.28=1225 N at each each wheel or 2550 N in total. That's 10 N more than the force pulling the car down the hill. This to me seemed adequate. I've not really measured handbrake torque before. However, by my reckoning, it should be fine. Regardless of what the testing station may say, I am quite happy to let me son drive this "death trap".

Well, tomorrow I shall, yet again, venture to the MOT dungeon. At least now, I have done my homework and adjusted the lever inside so now three clicks is more like 4-5 used to be. I shall update you on the outcome. Hopefully, a positive one.

 

PS - I know this post is Aygo-related, but I felt the topic was equally applicable to both mechanical and cerebral ailments. Hence, it went in the General topic. Plonk!

[APS]

The car has passed. Wohoo! 

I should be elated. I am only relieved. 

It only just passed and the braking efficiency of the car overall was only one percent (point?) over the required limit. This just does not make sense to me. The car is well looked after. Now with new rear brake shoes, front pads and disks are not that old and the brake fluid was replaced two years ago. Next year, I'll take the car somewhere else for its checkup. 

It's incredible how much the cars affect the mood. At least for now, I can relax. 

Time for a Java... ☕