Brake proportioning?

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EldRitch

Well-known member
Joined
Jul 4, 2016
Messages
137
I'm wondering if the programmed front-to-rear brake proportioning is non-adaptive, and is perhaps biased to expect larger tires at the back end.

What's got me wondering is that, thinking that more traction in front would be a good thing, I mounted my 205x50 on the front (Rota Slipstream w/ 5mm spacer) and the 195x50 at the back. Tires are at 37 lbs cold, about 40 when driving.

This seems fine for cornering and acceleration, but with rainy weather, I'm finding that the rear tires come loose well before the front under braking - today I even got a message on the display "Low Traction" - when the back came unstuck during some moderate braking. I'd have expected more balanced braking, with all four coming loose at about the same time.

I'm starting to think this reverse stagger might not be such a great idea on the Spark EV. Anyone got in-depth knowledge about the EV's brake management?
 
I think it's a leverage (diameter) problem, not width. Narrower tires are usually slightly better at braking than wider tires, all other things equal.

But the brakes were balanced for rear tires with 101.4% of the diameter of the fronts, and now you're running rears with 98.3% of the diameter of the fronts, and the back brakes have more mechanical advantage over the tread than the fronts do, the opposite of factory tire sizes.

It's surprising that 3% change in front/rear brake balance would be that noticeable though.

Either that, or your current tires perform quite poorly in rain, or you just encountered a first rain after a drought and the rain brought an accumulation of oil out of the pavement.
 
It's surprising that 3% change in front/rear brake balance would be that noticeable though.

Agreed.

It's been raining here for a week or more, so not much fresh oil on the pavement.

The tires should be decent: 1200-mile Dunlop Direzza DZ102: https://www.tirerack.com/tires/tires.jsp?tireMake=Dunlop&tireModel=Direzza+DZ102&partnum=95VR5DZ102

I'm left with brake proportioning as a guess...
 
Taxman said:
Narrower tires are usually slightly better at braking than wider tires, all other things equal.
Do you mean in the rain? If so, things are "not equal" since the rears would presumably have a better time as they're generally in the wake of the front tires. So narrower tires that also have a better chance of cleared road would have an advantage.

EldRitch--you could look up the part numbers of the brake system online and see if there are any differences between the Spark ICE and Spark EV. You might be able to conclude that part number differences would change the bias. Obviously the EV is heavier, has a rearward weight bias and comes with 10mm more rear tire width. Are the friction brakes the same between the ICE and EV Sparks? If they are different, and you're really motivated, you might be able to swap parts with an ICE version and get a different bias.
 
Narrower tires are usually slightly better at braking than wider tires, all other things equal.
That is because the narrower tire has less tendency to hydroplane, having a longer, narrower contact patch.

...you might be able to swap parts with an ICE version and get a different bias.
I don't think I'm that highly motivated, unless some brave pioneer wants to do it first and document the change: easier to just swap the tires front-to-back to see if that doesn't address the issue first.
 
Zoomit said:
EldRitch--you could look up the part numbers of the brake system online and see if there are any differences between the Spark ICE and Spark EV.

I think all the ICE Sparks have rear drums and all the EV have disks.
I'd trade four wheel disks for a real emergency brake, but I really prefer a mechanical emergency brake operating front disks, like my 40 year old Saab has. That thing can do 60-0 on the handbrake in about 25 feet more than it takes with the foot brake.
 
Besides, a good pit crew can do this in, what, 2 seconds?
For me, that's more like 45 minutes, but I'll give it a try over the weekend after the centering spacers arrive.

Last night I tested the issue again, and now I'm wondering if I was just being fooled by the very short wheelbase of the car and the resulting quick weight transfer under braking. We'll see.
 
195 55 r 15 BFg comp ta 2 in front 185/55 r15 Firestone 500? Original on the front. Front Anti sway bar disconnected. Reverse staggered help make the car rotate in sharp turns and in spinning circles.
 
ayebah said:
.. Front Anti sway bar disconnected. Reverse staggered help make the car rotate in sharp turns and in spinning circles.
That is some mod,,, there.
Not sure what you are describing. :roll:
How does the car handle on the skid pad?
Lateral G's?
I suspect it's way worse than the stock set up....

You know drifting is just for show. It's not the fastest way around a race course.
 
Brake proportioning? Maybe I'm being naive or missing something obvious, but do the rear brakes EVER engage during normal driving? I mean, regenerative braking is from the front wheels only, and the friction brakes only engage during panic stops or the very last few mphs before a full stop, right? (at least the way I drive)
 
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