How much does wheelbase affect speed?

[NewDerbyDad]

Hi guys,

I've been lurking for about 2 years. I found you guys right after our first cub build - a little too late after I spent some money on a bunch of useless gadgets lol. We did well that first year but really crushed it last year using the info found here and some of John's products so thanks to you all!

This year my son wants to try districts and they have a rule about using the stock wheelbase. All of our other cars are extended wheelbase. So we built a car using stock wheelbase of 4.375", added some of John's fenders, and did a couple test runs on an aluminum track (our actual pack track is an older wooden one) and using my car from last year as a reference (my car was about 1/2 car length slower than his last year) and we were surprised to find that this year's car was about equal with my car from last year despite my car sat for a year and we did no re-prepping.

We did have a real bad case of the wiggles at first. We corrected that by swapping rear wheels and axles side to side, tightening the wheel gap, and introducing some over-steer. That seemed to cure the wiggles and give us consistent times - just slower than I expected.

Both cars are set up as rail runners, rear wheels canted using the silver bullet and DF axle bent using more of an old-school method.

Here are the particulars:

My car from last year:

Weight: 5oz
Weight at front axle .85oz
Wheelbase: 5.75"
COM: 1" in front of rear wheels
Wheels: Stock, mold-matched, light sanding, John's wheel bore prep method & materials
Axles: Stock, John's axle prep method & materials

This year's car:

Weight: 5oz
Weight at front axle .8oz
Wheelbase: 4.375"
COM: .7" in front of rear wheels
Wheels: Dynasty Rage, John's wheel bore prep method & materials
Axles: Stock, John's axle prep method & materials

So my question is: does it seem like anything is amiss or can this be expected with a shorter wheelbase? Could it be simple aerodynamics?

I also notice this: the DF axle isn't really canted anymore since I was tweaking it during test runs: its more angled towards an oversteer (axle bend looks more pointed forward than pointed down). When I came home from testing and measured, steer was WAYYYYY agressive - like 6" in 1' agressive. I'm afraid of messing with it and reintroducing wiggles, but my gut tells me to adjust it to be angled down and start testing again. I have only a small window to test tonight and race is tomorrow.

Any thoughts from the pros? We could surely use some right about now!

Thanks much guys!

 

[ngyoung]

How much of an angle do you think is on the bent DFW axle? If it is less then the angle that the rear holes are, maybe giving it a little steeper of an angle will help with steer so that you don't have to turn it to the point where the bend is facing so far forward that it is barely canted.

If the car is wiggling slightly but still getting consistently better times then with the steer cranked hard and stable. Is it wiggling right at the transition or does it start from hitting the track joints?

You can also effect steer biasing the weight in the back towards one rear wheel or the other. Try moving a little towards your rear wheel on the DFW side and check the steer on the tuning board. You may be able to get the wheel pitched less and get the same steer.

 

[TRE]

your scout wheel base car will be slower than extended wheel base car..with extended wheel base you can get the back wheels and weight back a little more

 

[Obsessedderbydad]

Extended wheel base allows for more stability which allows you to not be as aggressive with your steer into the rail. Stock or shorter wheel base can be faster, but with stock you have less stability so without having everything perfect you're going to have more steer on your car. How is your weight placement setup? I'm very surprised to see the extended wheel base doing so much better seeing that you have the rage on this years car.

NG allotted to the possible problem with your steer. It almost sounds like your DFW has so little of bend that you're having to over steer to get the wiggle out. Basically because the bend isn't at a good enough angle, the moves your making for steer have to be greater to see results. I'd increase the angle of the DFW and try again. Usually my DFW bend is just enough to where it'll fit into the bore without having to push on it. At that angle I apply just a tad bit of pressure to make adjustments when I tune and see results. I'm guessing the tuning you're doing now, you can see how much you're turning the DFW axle to make adjustments.

 

[ngyoung]

Good point TRE. With your rules were you able to move the wheel base back so the rear axle is as far back as your previous cars? Some rules are worded so that you can still shift the wheelbase back by cutting off one end of the block and gluing it to the other end. Others specifically state that the wheelbase and location need to be maintained.

your scout wheel base car will be slower than extended wheel base car..with extended wheel base you can get the back wheels and weight back a little more

 

[ngyoung]

Good point TRE.

NDD, with your rules were you able to move the wheel base back so the rear axle is as far back as your previous cars? Some rules are worded so that you can still shift the wheelbase back but maintain the wheel base. Some will allow for cutting off one end of the block and gluing it to the other end if the rules say to use the original slots but don't specify that they can't be moved. Others specifically state that the wheelbase and location need to be maintained.

your scout wheel base car will be slower than extended wheel base car..with extended wheel base you can get the back wheels and weight back a little more

 

[Chief]

I am not sure of your rules exactly but look at safer racers car in the pics section. If you can mimic that with the D4D wheel weight system and build your car using the tips on the forum, you should not have any problem taking home hardware...Best of luck

 

[NewDerbyDad]

Thanks guys - should have mentioned that. We are allowed to move the wheels to wherever and drill our own holes as long as the stock wheelbase was maintained.

I have the rear wheels 13/16 from the rear of the car - fairly far back I thought. All of the weight is located just over the wheels proportionately front and back. Weight at front end was .8oz. I did try moving the weight more forward which increased the front end to .85oz but it didn't really affect anything.

I'm thinking I should pull the DFW and inspect the bend on my axle, then reassemble with the angle down and try again.

 

[B_Regal Racing]

You can also effect steer biasing the weight in the back towards one rear wheel or the other. Try moving a little towards your rear wheel on the DFW side and check the steer on the tuning board. You may be able to get the wheel pitched less and get the same steer.

+1 with biasing the weight more. Weight high on the car can introduce wiggles as well. There is a lot of weight on the FDW. With that much weight, I would think there is something amiss if that much steer is needed to stop the wiggles.

 

[Kinser Racing]

Push your wheelbase back with the rear axle at 5/8". Your COM at .7 is to aggressive with stock axles and your wheelbase position. That's why you need so much steer. Fix those two things and make sure you axle is pointing down and slowly turn it to point forward for steer and your speed will pick up.

 

[561 racing]

Push your wheelbase back with the rear axle at 5/8". Your COM at .7 is to aggressive with stock axles and your wheelbase position. That's why you need so much steer. Fix those two things and make sure you axle is pointing down and slowly turn it to point forward for steer and your speed will pick up.

Excellent advice!!

 

[NewDerbyDad]

Thanks everyone. Thebes definitely some things we can work on before districts, but for tomorrow about the only thing I can do without a serious overhaul is move the weights forward a bit which will bring the COM closer to .75 0 maybe .8 if you all think that will help. Outta time to do any realistic tests at this point.

 

[Kinser Racing]

With any weight movement you want to be able to adjust steer.

 

[quadad]

With canted and well prep'd, but full weight BSA wheels, along with BSA diameter axles, my kids ran ~ 1/2" COM with full extended wheelbase and ~ 3/4" COM with standard wheelbase on their cars every year. I don't think we ever had more than 4" of drift in 4' running on a wood track. It was never the highest level of competition, but it worked for us.

 

[Knotty Racing]

I would take a look hard look at your rear wheels and axles and then work your way to the front.

With stock axles, they may have a slight bend in them and be causing alignment problems.

You should do a slow roll test down the track to make sure your rear wheels are not touching the rails.

Do your rear wheels migrate out to the axle heads quickly and at the same time with a slow roll?

You might even throw a straight axle in the front to look at the rears, once you have the rears sorted, you can put your bent axle back in and start over.

Future Stuff - push the rear axle back to 5/8 as Kinser said. Com from the rear axle is fine for tuning/setup reference, but your (PE) potential energy comes from the nose of the car to your com. Move that up the track and you have more of it.