Instability under heavy braking
Discussion
Here's one for the set up guys.
Front engine, RWD, LSD, short wheelbase, relatively light car (1 tonne?) adjustable front shocks (Gaz) and brake bias control. Front suspension part polybushed.
The set up was carried out by the previous owner so no idea where we are at.
Under heavy braking, the car slows and stops but there's a feeling of unbalance which doesn't feel right. My own thoughts are that the front shocks need stiffening up. The rear brakes need additional pressure, they aren't working enough but there again many years ago I had a mini which would try to swop ends when braking hard as the rears were working overtime.
The front shocks adjustment needs full disassembly of the suspension to access the control so pull apart and adjust, try, pull apart again etc
The brake balance control is relatively easily accessible, under the drivers seat.
Need to add that the tyres are old and need replacing despite virtually full tread.
Your comments please
Front engine, RWD, LSD, short wheelbase, relatively light car (1 tonne?) adjustable front shocks (Gaz) and brake bias control. Front suspension part polybushed.
The set up was carried out by the previous owner so no idea where we are at.
Under heavy braking, the car slows and stops but there's a feeling of unbalance which doesn't feel right. My own thoughts are that the front shocks need stiffening up. The rear brakes need additional pressure, they aren't working enough but there again many years ago I had a mini which would try to swop ends when braking hard as the rears were working overtime.
The front shocks adjustment needs full disassembly of the suspension to access the control so pull apart and adjust, try, pull apart again etc
The brake balance control is relatively easily accessible, under the drivers seat.
Need to add that the tyres are old and need replacing despite virtually full tread.
Your comments please
Skyedriver said:
Here's one for the set up guys.
Front engine, RWD, LSD, short wheelbase, relatively light car (1 tonne?) adjustable front shocks (Gaz) and brake bias control. Front suspension part polybushed.
The set up was carried out by the previous owner so no idea where we are at.
Under heavy braking, the car slows and stops but there's a feeling of unbalance which doesn't feel right. My own thoughts are that the front shocks need stiffening up. The rear brakes need additional pressure, they aren't working enough but there again many years ago I had a mini which would try to swop ends when braking hard as the rears were working overtime.
The front shocks adjustment needs full disassembly of the suspension to access the control so pull apart and adjust, try, pull apart again etc
The brake balance control is relatively easily accessible, under the drivers seat.
Need to add that the tyres are old and need replacing despite virtually full tread.
Your comments please
Shouldn't the front and rear shocks ideally match?Front engine, RWD, LSD, short wheelbase, relatively light car (1 tonne?) adjustable front shocks (Gaz) and brake bias control. Front suspension part polybushed.
The set up was carried out by the previous owner so no idea where we are at.
Under heavy braking, the car slows and stops but there's a feeling of unbalance which doesn't feel right. My own thoughts are that the front shocks need stiffening up. The rear brakes need additional pressure, they aren't working enough but there again many years ago I had a mini which would try to swop ends when braking hard as the rears were working overtime.
The front shocks adjustment needs full disassembly of the suspension to access the control so pull apart and adjust, try, pull apart again etc
The brake balance control is relatively easily accessible, under the drivers seat.
Need to add that the tyres are old and need replacing despite virtually full tread.
Your comments please
Skyedriver said:
there's a feeling of unbalance which doesn't feel right.
It's really hard to know what that means. It might indicate a problem with the geometry, brake static balance or dynamic balance, spring or damping rates - or something else.I suggest you start by checking the brake balance since that is the most safety critical thing. If you are in a steady corner using all available grip and gently apply the brakes, the car should gently drift wide rather than understeering or oversteering. This checks the static brake balance ie the ratio between the front and rear braking effect for a given pedal effort. When you brake harder and harder in a straight line on a dry smooth road surface the front and rear should all lock up at around the same level of braking. That checks the dynamic balance ie how the rear braking effort is attenuated under heavy braking.
If those tests indicate the brake balance is OK but the car wanders or pulls under braking, you may have the wrong toe-in, more -ve camber than you're used to, too much steering axis offset etc.
Might be interesting to get a friend to drive your car on a quiet road whilst you watch how it behaves under braking as well as generally over road undulations, you could be stood on a convenient verge out of harms way for the braking experiment or following at a safe distance in another vehicle.
As above there's so much at play here, an overactive brake compensating/proportion valve responding to suspension dive being only one of many theories, or as said above wheel geometry could be changing dramatically under nose dive, even the wrong tyre pressures front or rear.
Too much front bias braking is as bad as too much rear, took me a number of trials and errors to adjust the valve on my first Landcruiser which was too prone to locking the front brakes (pre ABS).
Presume the brakes fitted are of similar quality and friction material and the correct size and type for the model, ie someone hasn't fitted monster calipers from some rally version to the front and left tiny pads or drums designed for the 1.3NA shopping model on the rear, obviously all brakes are serviced and free moving?
As above there's so much at play here, an overactive brake compensating/proportion valve responding to suspension dive being only one of many theories, or as said above wheel geometry could be changing dramatically under nose dive, even the wrong tyre pressures front or rear.
Too much front bias braking is as bad as too much rear, took me a number of trials and errors to adjust the valve on my first Landcruiser which was too prone to locking the front brakes (pre ABS).
Presume the brakes fitted are of similar quality and friction material and the correct size and type for the model, ie someone hasn't fitted monster calipers from some rally version to the front and left tiny pads or drums designed for the 1.3NA shopping model on the rear, obviously all brakes are serviced and free moving?
Thanks for those comments.
The car has "uprated" brakes, front & rear with discs on the rear where the car would have likely had drums. The guy who developed the car was a very well thought of engineer but is sadly no longer with us, leaving me guessing what he did. This isn't just restricted to the brakes and suspension as previous posts about the engine have similarly confused.
The car has "uprated" brakes, front & rear with discs on the rear where the car would have likely had drums. The guy who developed the car was a very well thought of engineer but is sadly no longer with us, leaving me guessing what he did. This isn't just restricted to the brakes and suspension as previous posts about the engine have similarly confused.
It's all your perception. Only you can test and change things to make you feel comfortable in the car.
Rears do little work anyway, and as you mention, work them too hard and you'll be going in circles.
But really, all you can do is change, test, change, test.....
Get a VBox or something to record decell data and datalog all 4 wheels just in case some might be locking up or something at strange times.
Rears do little work anyway, and as you mention, work them too hard and you'll be going in circles.
But really, all you can do is change, test, change, test.....
Get a VBox or something to record decell data and datalog all 4 wheels just in case some might be locking up or something at strange times.
stevieturbo said:
It's all your perception. Only you can test and change things to make you feel comfortable in the car.
Rears do little work anyway, and as you mention, work them too hard and you'll be going in circles.
But really, all you can do is change, test, change, test.....
Get a VBox or something to record decell data and datalog all 4 wheels just in case some might be locking up or something at strange times.
Thanks Steve, TBH I really need to sort the tyres out as they are rather old and may be a contributory cause. I have to wonder about the front shocks, it's a sort of uncomfortable shimmy as if they aren't evenly controlling the dip in the front end under braking but adjustment isn't a 5 minute thing as the adjusters are inaccessible without dismantling the front suspension otherwise I'd be playing with them now.Rears do little work anyway, and as you mention, work them too hard and you'll be going in circles.
But really, all you can do is change, test, change, test.....
Get a VBox or something to record decell data and datalog all 4 wheels just in case some might be locking up or something at strange times.
Update:
Tyre pressures were set at 26, as suggested by the OC although of course, they are a slightly different size.
Checked them a couple of days ago and two were 26, two, diagonally opposite were 25.
Upped the fronts to 28 and the rears to 27 and what a difference, suddenly all stable under braking. Never a known a car so susceptible to tyre pressure variations.
Tyre pressures were set at 26, as suggested by the OC although of course, they are a slightly different size.
Checked them a couple of days ago and two were 26, two, diagonally opposite were 25.
Upped the fronts to 28 and the rears to 27 and what a difference, suddenly all stable under braking. Never a known a car so susceptible to tyre pressure variations.
Good that you've found a solution, but I think the cause of the vagueness under braking was in your original post
Skyedriver said:
Need to add that the tyres are old and need replacing despite virtually full tread.
If you're impressed by the improvement with correct tyre pressures, imagine how good it will be when you've got some fresh rubber fitted...Gassing Station | Suspension, Brakes & Tyres | Top of Page | What's New | My Stuff