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GT750 Powered Race Car (D-Sports Racer)

Started by sav0r, July 22, 2019, 06:22:45 PM

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So we are like 12 days out from the race. At this point the car is basically as ready as we are going to get.

From the last test we came up a with a few issues;

  • The carb tops on cylinders 1 and 3 kept coming lose causing various issues
  • The jam nut on the throttle cable loosened cause the throttle to hang high
  • There's a significant late corner oversteer
  • Brake performance remains too low, locking the front brakes doesn't seem possible
  • We definitely weren't jetted all that great for the 90 degree heat

So we've safety wired all the carb tops and fixed the throttle cable issue. The late corner oversteer is a bit of mystery, the car also wanders a bit on straights. This could be a combination of things, there is nothing glaringly wrong with the car. We revisited bump steer on all four corners, it certainly wasn't bad. We made it better. The alignment wasn't bad, but we've adjusted it some. The wandering on the straights could be a lack of caster, we've left this alone for now. The late corner oversteer is quite puzzling, the rear of the car just gives up when the load on the rear tire should be decreasing. It's a very unusual and particularly unsettling feeling. The thoughts are that we have some mismatch in the anti-squat and that we are unloading the inside tire late in the corner as the car returns to full throttle. For now we are going to throw a little more anti-roll at the front of the car and a little less anti-roll at the rear of the car. We fabricated a stiffer front ARB and installed it. We will do some bar scans at the race, this is not something we have played with yet as other issues have been more important. The car has never exhibited any understeer, not in any phase of the corner. So less compliance at the front might help. Of course loss of grip in order to change balance generally is not the right strategy, you might gain balance but that doesn't necessarily mean faster lap times. That said, more roll stiffness at the front tends to keep the rear of the car more planted, so maybe it will work? The brakes remain a bit of mystery, we've installed a softer compound pad that will function better at lower temperature. If the pads don't help, then we will have to install a smaller master cylinder or larger piston calipers, neither of which we have. Of course the car uses an odd pull type master that mounts on a spherical bearing, it's a great design that's also a bit expensive. The calipers and uprights are off a Triumph, so not exactly super common stuff anymore. Basically, new front brakes will be a project for 2022, and will probably mean new uprights and new calipers.

Anyways, the car is about as prepped as we can make it for this years event. We will load up this coming weekend, the paddock opens the 23rd, we qualify on the 24th, and race the 25th. I will be getting more pictures and video and will make sure to post here. - for my projects and musings.


Good luck! Hope it doesn't drive like a Fiero - you had to bounce back and forth between gas and brake through a turn.
"Love 'em all.... Let GOD sort 'em out!"


Thanks! I always liked the rounded Fiero. Kind of a neat ride.

When we get this thing right I will be to 100% throttle a few feet before apex all the way to the next brake zone. We weren't there at the last test, not even close really.  Hopefully we gained on it some for the race.

I need to get my RD going again, it makes for a fun pit bike and generally I leave it over at the bike show for half a day. I bought an HPI ignition last fall and never performed the install. - for my projects and musings.


And remember, there must be pictures and video!
"Love 'em all.... Let GOD sort 'em out!"


Yeah, I am going to try to get some more detail shots of the car. It's a pretty simple setup, but I have been so lax in photos lately that I need to step up my game. I am also going to make some new camera mounts to get a little better video. I have two cameras currently but I am considering a third. It's just a matter of spending the money. - for my projects and musings.


Camera is ordered. I bought an off brand "4K" camera for $25 on eBay. Lol. If I get 1080 out of it I will be satisfied.

I am going to mount it right on the front splitter and attempt some high speed footage of the asphalt racing by. Who knows if the camera is worth a shit, but if it falls off and gets run over I won't be sweating it.

I need to come up with a video lackey. It's hard to wrench, drive, analyze data, dad, socialize, and work the video all at the same time. It's just a lot of moving parts. - for my projects and musings.



There's some video a page back or so. The car is too quiet in my opinion. I am going to talk to my dad about it. - for my projects and musings.


We qualified sixth in our field of like 25 cars. The left cylinder carb slide hung and I only completed two flying laps in qualifying. Quite interesting to drive a car that is stuck semi-full-throttle. I lost brakes at the end of that second lap, not totally, but definitely enough to just totally get out of it.

We broke a half shaft in the practice session, then the carb slide issue in qualy. Not perfect, but we soldier on. We have literally zero spares, but luckily we found a guy with a welder here at the track and my father zapped the half shaft up. It's a little gross, we will see if it holds.

Most of the issues from my last post have been pretty well addressed, we just keep running into little issues here and there. Part of that is a result of the increased pace, but also the massive stress a street course like this produces. The car is so quick, I am barely scratching the surface. We won't likely get to where we want with only one more session and the race tomorrow. Even with the continued issues, we will progress.

Here's some pics. I have video, I have more pics, but I am tired. I will get some video together in the coming days. I included one photo of my son walking on the bridge railing that we normally would never have access too. I am just glad he is appreciating motorsport.

 - for my projects and musings.


I post waayyy too much


Start looking now for a half shaft and DOJs/CVJs to handle at least twice the power. We tried indy car joints on a 400 Pontiac in a dirt track car - wrung them off constantly. Ended up with slip joints and 8 ton truck U-joints. Not ideal, but best available in the late 60s. Nowadays, plenty of higher performance independent suspension production cars provide higher strength joints. Breaking a half-shaft mid acceleration out of a turn could result in agricultural racing.

Still, you'll have a ball today if the brakes and throttle get sorted.
"Love 'em all.... Let GOD sort 'em out!"


Thanks guys!

The brakes overheated on account of the throttle being stuck, so they should be okay. The drive shafts use typical VW CV's, I am confident they are probably still used on models today, but they are shortened shafts. The weld holding the splined section into the half shaft tube broke fairly cleanly leaving us to believe the machinist who did the work just didn't get good enough penetration. We couldn't exactly pound the heat in with the little flux core welder either, but we put a nice fillet on the mating surfaces and were able to apply a nice big weld. It's not all that pretty, but I think it will hold. - for my projects and musings.


Well day 2 went a bit better, but not good enough. In the first session the same drive shaft broke, again, but this time in a different spot. So we took it off and got it welded. This time we made the short trip to our friends race shop and used the big Miller MIG machine. Keep in mind the flux core weld from the previous day did not break. The only conclusion is that we are stressing stuff more than we have ever stressed it before on the rough and tumble street circuit. Fair enough.

With the repaired drive shaft we went into the race less than optimistic, but oh well. Off the start I got a nice jump getting two cars immediately. The car in front got bogged down by a poor start from the 4th place car and I managed to overtake him on the front straight moving me to third. The car was working nicely, temps were good, and the tires were starting to come in. I started pushing pretty hard, the hardest I had pushed the entire weekend. Sure enough I ran the fastest lap I had run this weekend, lopping several seconds off my best time. I caught a Formula Ford ahead and quickly passed him putting me into second place.

At this point, first place who is many time winner of the event, was long gone. The leader has in the past come within just a second or two of the all time lap record, and I knew I wasn't really a match for him on this day. So I put down another personal best lap and gapped the Formula Ford by quite a bit, then decided to settle into the race a bit and just preserve the car. By lap five I started getting a good bit of brake fade, and with the gap both in front and and behind large I slowed my pace to allow the brakes to cool. The Formula Ford in third caught me and I actually just let him past, I knew I had him on pace but I also knew I had to make it to the end. So I hung on his tail just managing my brake fade and sure enough the pedal was coming back to me. The plan was to hang onto him, then with two laps left I would get by him and retake second, with the pace advantage it would have been easy. However, water temps were starting to creep up, it was now right around 250. Too high, but not exactly catastrophic.

Unfortunately, three cars in separate incidents caused a stoppage of the race. Race control instructed us to park while course cleanup removed the cars from the racing surface. During this time, the heat soaked engine, while not even running mind you, continued to overheat and blew the head gasket. The car only pumps water while running, and without any fans the radiators were unable to exchange air anyways. A big lesson to learn there. We had to park the car, and as a result received a DNF. We officially ran the third fastest lap of the race.

Our best time was a 2:19. The fastest lap of the race was a 2:15. The all time lap record there is a 2:09.4. Performance wise we are still barely scratching the surface with the car. Even during my best lap I was basically taking it easy, and we still have a number of technical issues to look at.

In the tightest left hand corner the car was fuel starved, I believe, and it would just shut off. I was losing 2 to 3 seconds in that single corner alone as I waited for the engine to re-fire and take off. The live axles generates so much force on the multi-link rear suspension that we are eating up heim joints like they are Cocoa Pebbles. This results in a rather sloppy rear alignment and causes all sorts of fun things. We may revise the rear spool and adapt a differential. The Ikon dampers are okay, but we can do much better. We made huge strides on the brakes, but the brake fade is unacceptable. Keep in mind the track has 21 turns and most of them require braking, so brake stress is very high.  Perhaps some brake inlets will solve that issue? Tires, well, we have been running the same set of tires since we first ran the car in 2019. They have many, many sessions on them. On a two minutes lap, tires make a huge difference. In summary, there is plenty of time left in the car. We might be behind our development curve, but we are much closer now.

Overall, we are disappointed we didn't bring home that second place finish, and the DNF stings a bit more. Had the race not been stopped we would have made it to the end. Even if I had to nurse it to third it would have been a nice cap on the weekend. However, we know the potential is there. We have so much pace left that the all time lap record is within reach. 2022 here we come!

Here are some pictures. I have tons of video too, I will get it trimmed up a bit and try to post it.

Some short video. In the second video you can see it smoking pretty nicely, a blueish-white smoke. - for my projects and musings.

sav0r - for my projects and musings.


Racing and learning. I would suggest (maybe after you decide on differential) having a couple axles turned from appropriate metal and splined as needed. We even did that on the dirt track cars in the 60's (usually from old wide track Pontiac axles).
"Love 'em all.... Let GOD sort 'em out!"