Welded Differential In A Front Wheel Drive Track Car: Fast And Fun
My Civic race car came with a welded front differential. As I also own a 240sx with a welded rear differential, and that car is pretty pathetic on the track handling-wise, I was concerned what welding the diff on a front wheel drive car would do to its handling.
Note: welding the front differential on a front wheel drive car is very different from welding the rear differential on a rear wheel drive car. More on the rwd platform on the bottom of this page.
Before I even looked at the car I read a bunch of Internet discussions regarding welded differentials in front wheel drive cars. Opinions there largely fell in one of the following groups:
- People who owned front wheel drive cars with welded differentials and raced them nearly universally praised the combination and would weld their differentials again.
- People who owned such cars and mostly drove them on the street were somewhere between hating the setup for its low speed discomfort and tolerating it, presumably if they did not drive in tight spots quite as much.
- People who never owned such cars typically seemed to opine that the setup would not handle, be slow, be dangerous and/or break parts. Their recommendations are typically either "get an LSD" or "stick with an open diff".
In other words, people who actually had the experience loved it, with most nay-sayers imagining the consequences rather than basing them on experience.
This was enough for me to consider the car, but I was not ready to buy it without driving it at speed first. Luckily the previous owner generously agreed to let me test drive it, at a race track, before I bought it. Since then I have had another weekend racing the car wheel to wheel, when it also rained, and one dry day of HPDE where I started to push the car.
Below are my observations on having the welded diff in a FWD car, based on me actually owning and driving such a car. Perhaps unsurprisingly, they mirror the first group of Internet users who raced welded diff cars and loved them. "Advice" of people who have never experienced a welded diff FWD track car typically does not match reality and is best ignored.
Initial Impressions - Front Toe In
When I bought the car it had toe in on all tires. My initial impressions of the welded differential's handling thus were based on this alignment.
Corner Entry: Understeer
There is some understeer at corner entry, as one would expect. To make the car turn in a healthy dose of trailbraking is required. Drivers who do not trailbrake would likely consider a welded diff FWD car hopelessly understeering and "unable to turn".
Mid-Corner: Massive Grip, Early Throttle Application
Where a welded front diff starts to shine is in mid-corner phase of turns. As throttle gets applied in a FWD car with a welded diff, the car wants to go faster and turn more at the same time. In a typical "normal" car, going faster requires reducing steering input, but a welded diff FWD car just keeps turning the more power it is sending to the wheels.
The immediate consequence of this is I was getting on throttle earlier in the Civic than in any other car I have driven, or been in. The early throttle application is a major change of driving dynamics, big enough to require several days of simply practicing getting on throttle while cranking up steering angle. The car needs to be driven in a very different way from what is taught in HPDE classrooms.
In a typical rear wheel drive car, the line through a corner flattens as the car tracks out. In a front wheel drive car with a welded diff, it feels like the line is more of a constant radius circle segment as the car is accelerating flat out through it.
Corner Exit: Power Oversteer
The eagerness of the car to turn can become excessive to the point where the car has power oversteer with front wheel drive. It sounds rather incredible. The reason I was unwinding steering at corner exit was because I felt the car would spin otherwise.
I have already succeeded in spinning the Civic at corner exit under power, so the oversteer is not something I imagined - it is very real.
Post Alignment - Zero Front Toe
This section has been added in August of 2014.
When I originally wrote this essay, the car had about 5/8" of total toe in in the front. In 2014 I realigned the car for more camber and - importantly - zero front toe. The rear was toed in in both 2013 and 2014.
With zero front toe both corner entry understeer and corner exit oversteer became essentially unnoticeable. Civic became very much like my Miata in handling - rotating into corners well under trailbraking and very stable in both corner entries and corner exits.
I zeroed the front toe to reduce tire scrub on the straights, and as of yet I don't have sufficient data to claim that one setting is faster than another. However, my current opinion is that with zero front toe the car drives as "normally" as any other road course car. I have not yet driven in the rain with zero front toe.
As I need to replace the engine in the Civic, it is my current belief that a welded differential with zero front toe would work roughly as well as a limited slip differential would, at a significantly lower cost.
I read on a forum before I bought the car that a front wheel drive car with a welded diff grips even better in rain. The second weekend, when I raced the car, it rained. It is from first hand experience that I can say that the welded diff provides incredible traction in the rain.
All of the mid-corner properties of the welded front diff remain. The car, once it starts to turn, pretty much keeps turning and can accelerate much harder over cars which have the capability to spin one of the driving wheels. This was crystal clear in the race: I was getting on power earlier than other cars and could hold power through entire corner exit without issues.
Welded diff also helps in the braking zones, acting as a form of ABS. As stopping either of the front wheels would mean stalling the engine, as long as the engine is turning, both wheels are rotating, in other words, neither front tire is locked up and sliding.
I found it helpful to apply ice racing/snow driving technique that I call reverse heel and toe - applying partial throttle while braking to prevent the front wheels from stopping the engine.
[*] The only caveat is you need some grip for the tires to use. I spent most of the weekend racing on street tires, and while I did not consider the tires remarkable, they were not slicks. For the last race I switched to slicks, and when neither of the tires has any grip, welded differential or not the car cannot do much.
I do not yet understand the physics behind the car's ability to accelerate and turn harder at the same time, but according to Internet the outside front wheel pulls the car through corners.
I have had two front wheel drive track cars - the Civic under discussion and an Integra I bought for its driveline to put into the Civic. Let's look at the specs for both cars and the test setup:
- Weight: 2250 lbs
- Weight distribution: 59/41 F/R
- Front toe: ~0
- Front camber: -3.1°
- Diff: welded
- Tires: Toyo R888 225/45-13 (F)
- Track: Watkins Glen
- Weight: 2600 lbs
- Weight distribution: 64/36 F/R
- Front toe: ~0
- Front camber: -1.3°
- Diff: open
- Tires: Nitto NT-01 205/55-14
- Track: VIR Full/Grand, Summit Point Main
I had the Civic at Watkins Glen for 3 days, running 225/45-13 R888 on the front axle to get a gearing advantage with old 205/60-13 R888 on the rear axle. After the 3 days the tires looked like this:
I had the Integra at VIR running the full coures for 2 days, the grand west course for 2 days and at Summit Point running the main course for 2 days. The car started with brand new 205/55-14 NT-01s and after the 6 days all four tires were worn to zero tread depth. Integra went through 4 tires in 6 days whereas the Civic went through 2 tires in 3 days. Seems pretty even to me.
If we assume that the rear tires on a front wheel drive track car are basically not wearing at all, I may be able to get a bit better tire life out of the Civic by dialing out some front camber. It looks like if I can wear the tires on the Civic evenly, they might last 25-30% longer than Integra tires. If we assume that the rear tires are wearing out, the two cars would come in neck and neck since the Civic's front were never ran on the rear.
The tire wear on front wheel drive cars is much more rapid compared to rear wheel drive cars. I can easily get 10-20 track days on a set of Toyo RRs on a Miata. This is because a front wheel drive car tends to understeer under power coming out of corners, and the fastest way around the track is to push it out of corners with all of its power. This means the front tires are always slipping in corner exits, and tire life shortens rapidly as a result. However there does not seem to be much difference between running a welded diff compared to an open diff when it comes to tire longevity.
Low Speed Maneuvers: Horrible
You pay a price for the awesomeness that the welded differential is, and that price is paddock maneuvering.
At something like 90 degrees of steering angle, while driving at 5 mph or so the car unmistakenly informs you that the left and right wheels want to travel different distances, but cannot. Once the tires start jumping in tight corners I do not add any further steering angle or speed. Supposedly these low speed maneuvers are what destroys axles.
I make a lot of 3 point turns in the paddock with this car, and strategically select my routes into and out of my garage/parking spot.
The car is a blast to drive. Maneuvering in the paddock is annoying, but so what? Lots of things are annoying at a track day, like sweating in a fire suit all day (racers only) or having to arrive at the track at 6:30 am. We deal with them.
Driving a welded diff FWD car properly takes practice, unlearning the commonly taught driving style, learning new habits, a fair amount of aggression and then commitment. The reward is a car that is very fast and constantly puts a smile on my face.
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Tagged: rain driving, fwd