Re:Add-ons and GVM

wrote:

Jimny can tow 500kg unbraked. So the vehicle is already rated to tow the weight being discussed. If the GVM upgrade simply takes from the towing payload I can’t see it being an issue. Which is likely why the GVM upgrade does not come with different brakes.
 

Yes, the vehicle is rated to brake GVM +500kg, but there is a difference between a) having to brake GVM, b) having to brake GVM +500kg occasionally, and c) having to brake GVM +500kg, each & every time you need to stop or slow the vehicle.

Which of these three scenarios do you think Suzuki engineered the brakes to cope with?

For me it's a concern, I've learned to have respect for braking systems.

In the UK by law overrun trailer brakes are required to operate at 50% efficiency which means that a Jimny is still going to experience an additional 650kgs worth of force at anything up to 60mph on a dual carriageway. This obviously assumes that the trailer is in a functional condition but as they are not ever inspected unless by a random roadside check or after the fact in the case of a collision it is not unreasonable to expect that Suzuki in line with every other manufacturer applies a factor of safety to the design and specification of the braking system, they have to assume worst case scenario so it's not unreasonable to expect the brakes are capable of retarding the entire train weight, maybe not a dozen times back to back from full speed but certainly once or twice.

wrote:

 it's not unreasonable to expect the brakes are capable of retarding the entire train weight, maybe not a dozen times back to back from full speed but certainly once or twice.

You seem to be reinforcing the point I made above - that whilst the stock brakes can stop the weight "once or twice", they're not designed to do it repeatedly - that is the essence of the problem, braking transforms the kinetic energy into heat energy, the solid rotors on the Jimny are likely to have trouble dissipating the heat if they are called on to repeatedly stop loads over GVM.

Ventilated discs (and calipers, etc.), from a Vitara, the diesel Jimny, or the five door JB74 are one possible solution.

In a previous response I mentioned warped rotors on a hatch I had in my youth, the fix was swapping in ventilated from the sport model - for some reason Suzuki saw fit to equip the model of Swift I had with solid rotors, but the same model in most markets had ventilated rotors, the judicious selection of off the shelf parts allowed me to significantly improve the brakes on that vehicle.

If you are traveling at 60mph with a full trailer on and have to stop emergency hard it's unlikely that you are then going to immediately accelerate as fast as you can to then do another emergency stop. Doing that multiple times in anything less than test conditions is showing a lack of road awareness. Under normal circumstances the brakes have at least some opportunities for cooling. Mine is an automatic so limited on engine braking and yet I have so far not even come close to cooking the brakes and I live in a region dominated by hills.

I've always been puzzled by this "automatic so limited on engine braking", I'm a "flatlander", but I have, so to speak, done my time in the hills, and down shifting in an automatic, just as I would in a manual transmission, has always worked for me.

I'm quite sceptical about hill descent control as that uses the ABS system to control the descent speed, until it can't because it's overheated them.

Ventilated disks will cool faster, and that fact that the new five door has them to cope with extra weight (110kg more on the GVM) suggests they're needed.

Wether it is the actual ratios being higher than the manual but with my old manual dropping down the gears the engine would hold the revs and eventually decrease them on its own. With the automatic locking a gear will only hold the revs at best but it's more likely to climb especially if the gradient increases, you can obviously help with the brakes and it saves you have from just riding the brakes to boiling point but the manual is definitely better. That said in every other respect the automatic is far superior both in town and country and off road.

I'm well aware of the advantages & disadvantages of automatic transmissions - my dislike for them stems from personal experience with the old "vacuum modulator shifted" three speeds of the '80's & '90's, and whilst I'm willing to admit that the newer electronically shifted automatics are a huge improvement, the automatic on the JB74 is an "old school" four speed especially when compared to today's eight, nine & ten speed automatics.

I've driven the JB74 with the five speed manual (which I own), and with the four speed automatic (the dealership loaned me the Jimny their General Manager drives to do a comparison), the automatic is sluggish beyond belief - we were comparing noise levels, the automatic is much quieter, but I'd d rather live with the noise of the manual than give up the "zippiness".

wrote:

wrote:

 it's not unreasonable to expect the brakes are capable of retarding the entire train weight, maybe not a dozen times back to back from full speed but certainly once or twice.

You seem to be reinforcing the point I made above - that whilst the stock brakes can stop the weight "once or twice", they're not designed to do it repeatedly - that is the essence of the problem, braking transforms the kinetic energy into heat energy, the solid rotors on the Jimny are likely to have trouble dissipating the heat if they are called on to repeatedly stop loads over GVM.

Ventilated discs (and calipers, etc.), from a Vitara, the diesel Jimny, or the five door JB74 are one possible solution.

In a previous response I mentioned warped rotors on a hatch I had in my youth, the fix was swapping in ventilated from the sport model - for some reason Suzuki saw fit to equip the model of Swift I had with solid rotors, but the same model in most markets had ventilated rotors, the judicious selection of off the shelf parts allowed me to significantly improve the brakes on that vehicle.

Vented discs aren't necessarily a magical fix though. The might have more heat dispersion, but total braking force may not be any different, depending on many other factors.

My 200Tdi Discovery 1 had solid discs front and rear from the factory and weighed a lot more than a Jimny and it was rated to tow 3.5T

I'm also not a believer in warped discs either. Every time I've done research on the subject it is always either uneven carbon build up or related to run-out.

And while I have a lot of respect for braking systems and have made major changes on other vehicles. The Jimny brakes have not yet given me any concern that they cannot cope with the job at hand, including towing duties or thrashing the vehicle round the local B roads. The op (who may have done a runner....) also made no mention of issues or concerns with braking efficiency. But they did say they were already running over weight. So the solution they are looking for here isn't to carry more weight. It was to enable to continue carrying the weight they already have, but legally for their country.

wrote:

I've always been puzzled by this "automatic so limited on engine braking", I'm a "flatlander", but I have, so to speak, done my time in the hills, and down shifting in an automatic, just as I would in a manual transmission, has always worked for me.

Engine braking will depend on a couple of things with an auto.

Some autos when you lift of the throttle will drop the revs to near idle. I don't know if the Jimny does as I haven't drive it, but it is a semi modern electronically controlled one, so I'd guess it may do. I think this is done for efficiency reasons. Some gearboxes will have sport modes or similar that will use more engine braking.

The other thing is torque converter lockup, unless it is locked you'll never get the same amount of engine braking as there will still be slip in the fluid torque converter. Most gearboxes do no lockup in anything but top gear.

This doesn't mean you won't get some benefits of engine braking by down shifting, but it will still be different to a manual. Many newer auto gearboxes have tow modes to induce more engine braking and the use of paddles can also be helpful.

The jimny auto just gives some engine braking when you lift off.

Above the lockup speed it is the same as a manual, below the lockup speed there is less engine braking.
Forcing a lower gear spins the engine faster and provides more fluid engagement at the torque convertor as well as more resistance because of the lower gear ratio.

I was surprised how much engine braking there was in my RangeRover auto- I was tracking down a missfire at idle (which of course was ignition) and I disabled the vacuum switch that cut off the fuel on the overrun. Without a dead engine to slow the car down you really had to use the brakes hard to stop it!

It was about 3 years ago when I had a test drive in a Jeep Wrangler auto. I am not used to autos but I imagined that a light foot would maintain the gear I was in and that a heavy foot would drop a cog or two and get on with it. To my surprise a light foot did very little and a heavy foot not much more. Sluggish and un-responsive is the best description I can give.

Roger

wrote:

It was about 3 years ago when I had a test drive in a Jeep Wrangler auto. I am not used to autos but I imagined that a light foot would maintain the gear I was in and that a heavy foot would drop a cog or two and get on with it. To my surprise a light foot did very little and a heavy foot not much more. Sluggish and un-responsive is the best description I can give.

Roger

Not sure what Jeep you drove, but it should have gone quite well.

An auto won't necessarily maintain a gear with a light foot, depends on many many factors. But they will usually try and run at an efficient rpm for a given amount of throttle.

Don't recall them being sluggish either.

I used to have a 4.0 litre Jeep Cherokee with a 4 speed auto. It absolutely flew and was very smooth. Same engine and gearbox would have been in a TJ era Wrangler.

The JK that replaced it used the VM 2.8 litre diesel. Earlier models used the 4 speed auto, which would have been more sluggish, but still probably faster than a JB74w Jimny from a roll. They later got a 5 speed version of this box and more power, so should have gone even better.

The current JL version I think uses an 8 speed box with either a 2.0 litre turbo petrol unit, which is genuinely rapid. Or a 2.2 diesel. I've driven the latter and thought it went very well.