Lower hp classes and taking grip out with new tires

[Clint Williams]

I am new to sprint racing- currently running 4-cycle clone sr on an 04 Birel AR4X. 30mm axle, unknown stiffness with long hubs. I put new tires on for the first race and my lap times slowed down. I felt like I was owing an anchor. A buddy suggested that I needed to take grip out of the chassis as the new tires were causing it to get stuck. Does anyone have some go-to adjustments for modifying their setup when mounting new tires?

Another thing that confused me is it had oversteer on corner exit so I started widening rear track width- but that seemed counterintuitive to freeing up the chassis for the new tires?

[Gary Lawson]

What tires are they? The first issue that comes to mind is using s 30mm axle in the sr class. Is also makes me question the kart. Is it a kart designed for sr weight? Typically softening the kart would be recommended to reduce grip, but this kart may be too soft already.

Oversteer on exit seems to be the opposite of having too much grip. Widening the rear would make that issue worse but it would help if the kart was actually too stuck.
Give me a little more of the whole story and I can give you some advice on how to correct it.

[Clint Williams]

Bridgestone YHC is the spec tire for the class- WKA Gold Rush West Coast Series. Min weight is 360- I weigh 147 so I had to add a good chunk of ballast to make weight. The guy I bought it off of placed third or fourth in the series the previous season with the same setup. He weighed about 160ish.

I am pretty sure the current AR series Birel’s are made for junior classes, but this older one is I think an introductory, lower hp chassis: http://www.baileyracing.com.au/go_kart/ar4xfolder/ar4x.htm

 

[Clint Williams]

http://www.baileyracing.com.au/go_kart/ar4xfolder/ar4x.htm

[Clint Williams]

YLC- not YHC

[Gary Lawson]

I think first think would be to go to a 40mm axle. They seem to recommend the 30mm for jr and light SR. I would personally have a 40 in it even in jr classes. It’s likely it’s simply flexing too much and got worse with the stickier tires. Most times ylcs are fastest when new but certain tracks in hot weather like them to be run in a little when using a low hp engine. I think any changes aside from the axle will have minimal positive effect until you change the axle.

[TJ Koyen]

I agree with Gary. Your kart sounds like a noodle with that 30mm axle in.

Driver/Coach/Wrench : Innovative Performance/Exprit
Owner : Oktane Visual - www.oktanevisual.com
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[Clint Williams]

Thanks for the responses Gary and TJ.

Which hardness 40mm axle should I try out first?

So I understand it correctly, moving to a 40mm rear axle will flex less than the 30, transferring more weight to the rear tire and correct the over steering issue?

As for rear track width, if I want more rear grip do I widen or narrow?

And for hubs, long hubs provide more weight transfer by stiffening the axle and short hubs opposite?

[TJ Koyen]

I always start on the medium axle.

It’s hard to say what will happen because you say it’s stuck but you also say it’s oversteering, which are opposite issues.

It’s also hard to generalize what changes will offer what affect when talking about “grip”. Think about it in terms of “inside rear wheel lift”, which is what we are really affecting when we make chassis adjustments.

A narrower rear track will give you a more reactive turn-in, helping the kart tip easier and transfer weight to the outside tire more easily. This will give you more lift and generally free the kart up. However, if you already have too much lift, you’ll only be hurting yourself further by narrowing the rear track because you’ll only free the rear up more. Widening the rear track will slow down the lifting action of the inside rear wheel and make the kart more stable because it isn’t hiking the inside rear so harshly. But slow down that lift action too much and the inside rear may never get off the ground and the kart will understeer and be flat off the corner.

If the kart is “stuck”, it’s because the inside rear wheel isn’t up. The kart isn’t rotating. If it’s oversteering, it’s because you’re lifting the inside rear too harshly.

It’s possible on new tires that the increased grip has made the kart more front grip biased, causing your initial turn-in to lift the rear more harshly and give you a flat kart or oversteer when the outside tire breaks traction.

I think the issue here is your 30mm axle is too soft, causing the kart to lift, but provide no “spring” or be able to maintain the load put on the outside rear tire in the corner. This will cause the kart to either break traction and slide on exit, or cause the kart to be flat on exit when there’s not enough power/corner speed to slide the rear.

I always try to get drivers to break the corner down to 3 points; entry, apex, and exit. If they can tell me what the kart is doing at each point, it’s easier to figure out what is causing the issue.

I almost never tune with hubs because they not only change the stiffness but they change the rear track width too, which can cause contradictory results. I run medium hubs exclusively.

Driver/Coach/Wrench : Innovative Performance/Exprit
Owner : Oktane Visual - www.oktanevisual.com
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[Clint Williams]

TJ and Gary- a buddy is letting me try out a 40mm axle this weekend. I hope these pictures upload okay, but I was trying to show the tire wear.

The rear is the right rear on a counter-clockwise sprint track.

 

The front is the left front on the same track. the right front is wearing evenly across the tire. Someone at the track suggested that the uneven front tire wear might be indicative of a bent spindle or frame?

[TJ Koyen]

Photos didn’t work Clint. Try uploading at photobucket.com and copying the [im*g][/im*g] link (minus the stars) and pasting here.

Driver/Coach/Wrench : Innovative Performance/Exprit
Owner : Oktane Visual - www.oktanevisual.com
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[Clint Williams]

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[Gary Lawson]

Front tire looks fine to me. Using the whole front tire is not something I ever aim for. The only karts that appear to get even tire wear on the front are karts that run a lot of caster. The inside tire will actually wear some on entry and with all the caster it gains a ton of positive camber. The rear tires look to be wearing the inside edge hard because the axle is deflecting too much and resulting in camber gain during cornering.

High tire pressure will also focus tire wear on the inner half of each tire.

[TJ Koyen]

As usual, I agree with Gary’s points. I usually see the same contact patch you’ve shown here, where it extends to about half the outer wear indicator and I’m usually at max caster. You look like you’re actually picking a bit of rubber up on the outer shoulder.

It looks like you’re too soft on axle. As Gary said, it’s deflecting too much and wearing the inner shoulder. But it also looks like it’s a little blistered, possibly from the rear being over-stuck. Kart is a flat, wet noodle.

What’s your caster like? As Gary noted, the fronts will wear more evenly with more caster. If you’re picking rubber up on the outer shoulder and the rears look stuck, it might stand to reason that some caster will clean up the fronts and get the rear freer.

Driver/Coach/Wrench : Innovative Performance/Exprit
Owner : Oktane Visual - www.oktanevisual.com
www.facebook.com/oktanevisual
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[Clint Williams]

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Can I adjust caster and camber with this design? I haven’t taken it apart but it looks like I can only adjust ride height as the bumper mounts to the king pin bolt.

 

this is

[Gary Lawson]

Doesn’t look like you can as is unless it has an eccentric pill on the bottom. Odds are you can drill out yoke on the bottom and put a pill on though. I would also lower that front ride height. Looks like it’s full high.

[Clint Williams]

Gary and TJ, thanks for all your help thus far in helping me out. Last weekend I was able to put in a 40 mm axle and my buddy had some offset pills that come in .5mm, 1.1mm, and 1.6mm steps. I put the .5mm pill in to start with.

 

The tires all around cleaned up but now it is on the other end of the spectrum and is bogging down at corner exit and the engine started to get hot from binding.

I set the rear track width to 53″, fronts have one small spacer inside, one seat strut from left side of seat to axle carrier, .5mm pill and the plastic bumper was tight from trying to stiffen up the rear with the 30mm axle.

I asked a regular podium finisher in my class if he could take some laps and he confirmed my initial feelings of the kart and turned about the same lap times I did (which are .7-1″ off target lap times at this track.) He suggested loosening the rear bumper and possibly removing the seat strut to help free up the rear.

I was also thinking of narrowing rear track width a tad to help lift IR and moving some spacers in on the front and trying the 1.1mm pill. And maybe trying some shorter hubs. I am not sure where you take measurements from to determine hub length but they are definitely longer than the hubs than were on the 30mm axle.

 

Am I heading in the right direction with freeing up a rear a tad? I am still a little confused with the effect and relationship of front track width. Do you typically increase caster when widening the fronts or only one or the other?

[Clint Williams]

*relationship of caster and front track width

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