I was looking at getting a spare set of rear hubs, and was considering going shorter to my current ones so I have options. My understanding was shorter hubs are generally used on a grippier track.
Found some OTK options on the DPE website but their description of short rear hubs and their effects has me got me wondering if I’m completely wrong. What have you guys found when using different length rear hubs?
Rear hubs are going to change the relationship between the wheel and axle and how the flex on the end of the axle works. In turn, this will change how the tire’s contact patch interacts with the track surface.
I have generally found on OTK stuff that the longer hub will give the tire more capacity to handle load. So for example, there is a specific track we race at that really works the right rear tire pretty hard, and on a standard hub we often get aggressive graining on that outside rear tire. Putting the long hub on eliminates that graining because the stiffer axle end gives more ability for the tire to take load before the axle/hub/tire relationship gives up and starts to slide, causing the graining.
Much like axles, different kart brands do different things with rear hub tuning. The conventional theory is that a longer rear hub will provide more rear “grip”. The shorter hub allows more flex on the end of the axle, so it absorbs some of the load transferred to that tire and therefore doesn’t dig the outside tire into the track as hard.
All that being said, I RARELY change rear hubs on the OTK stuff. I much prefer the feel and performance of the standard hub, and never really feel a need to change rear hubs unless we have a specific case like I mentioned above where the track provides odd tire wear or something. Some drivers like to play with rear hubs a lot, but my personal feeling is that I prefer to run the standard hub and play with other adjustments. I don’t like how it changes the feeling of the rear tire at max load when we move the flex point around on the end of the axle.
Maybe @DavidSera can provide some insight specifically on how DPE uses the rear hub to tune, as it could be different on the DPE karts.
The standard OTK hub that you run, what length is that? The rear hubs that came with my kart are 93mm long and the shorter OTK ones I’m considering are about 70mm long.
The stock hubs are 92mm I believe, so sounds like you have the standard hub.
Thanks for the reply. Everything you say makes sense, the only issue I face is trying to apply it to situations I find myself in at the track when I’m attempting to understand what my kart is doing.
Everything I’ve done to my kart over the past year has been aimed at “removing grip”. I was sold a new kart (Parolin) from a local shop and from their base set up I’ve changed:
The big ones were removing the torsion bar and 3rd bearing at the same time - it was like a flipping a switch and I dropped 4 tenths from one session to the next and was like driving a different go kart. The colder and earlier in the day the track is, the faster I go, but as the track gets hotter and stickier in the heat of the day my pace relative to my competitors who I am faster than earlier drops off slightly.
I run a restricted X30 on LeCont LH03, my thinking is that the lower horsepower karts at 180kg min weight get “bogged down” or “stuck” on tracks that are hot, rubbered up and have stickier tires running on them all day. So my line of thinking is to “remove more grip” which circles back to why I’m looking at shorter rear hubs.
Your thinking sounds right and the changes you are making are going in the right direction. 180kgs is pretty heavy and on sticky tires that definitely will be difficult to keep the kart free.
Just remember that you aren’t always trying to remove “grip”, but also trying to keep the chassis from flexing too aggressively in the high grip situations. The more rubber on the track or the softer the tire, the more load the kart is going to be under. It can be possible to take too much jacking out of the kart in an effort to reduce perceived “grip” and make the kart sit flat and not unload and then you are right back to square one with a kart that is stuck to the track.
Some negative camber or reduced caster can also lighten up the load going through the chassis as well.
Yeah that makes sense. So far the majority of changes I’ve made in this direction has improved my pace but I’ll definitely be aware of going too far.
I have found 2 boxes of negative camber and neutral caster work well under most conditions for me. If I try to dial any positive camber into it on a race day the front end becomes heavy.
Taking the front torsion bar out seemed to increase jacking in the sense that with it in, it was picking up but setting back down way too early. No bar and no 3rd bearing rotates my kart through the apex and sets it down on exit rather then pre apex/initial turn in.
I’ll try the shorter hubs when the track gets grippy, it just threw me off reading DPEs description of what shorter hubs actually do.
To me the shorter hubs give more dig middle to exits, and longer hubs more stable in faster corners.
The medium is best overall
Great hub/axle discussion! If everyone had your level of feedback tuning would be a piece of cake! Lol
A sidebar, just out of curiosity - have you tried adding the bar back in as the heat/grip increases and your delta to the others changes? Being OTK primarily in our work I just have such a hard time wrapping my head around no bar on a grippy track, particularly with something like X30.
I haven’t put the steel bar back in, but I do use the nylon bar now sometimes instead. The nylon feels closer to having no bar in at all than it does having the steel one in. As I said before to TJ, the steel bar in seems to stiffen my chassis too much to the point that the inside rear wheel always drops too early, regardless of the conditions.
I tried many different set ups while having that steel bar in, but since having it out a lot of the changes I’ve made (as listed above) has seen me improving.
I am running an X30 but in a Restricted class at 180kg. Which equals about 2 seconds a lap slower (at our local 50 second a lap track) compared to the regular X30 180kg class. I just feel with the weight and reduced horsepower of my class, that the steel bar and tight 3rd bearing bind the kart up too much.
on a medium rear axle, would putting in a short hub simulate a soft axle?
Not exactly because you are really only affecting the stiffness on the end of the axle. The center of the axle between the bearings will still be basically the same regardless of hub.
Have you found it kind of simulates a cut axle with the added end flex though TJ? I’ve tried that theory in the past with drivers that maybe didn’t have the best feedback, or enough tools in their toolbox yet, and never could reach a conclusion that was satisfying.
Honestly I couldn’t give a solid answer. I just don’t run other hubs outside of the mediums enough to say for sure.
Pretty much the same here - tried them for giggles a few times and to see if I could find value in them. If we’ve turned a million OTK laps, 999,000 have been on medium hubs. Lol
I personally prefer aluminium hubs over the standard mag hubs on greener tracks, its leave the back end more stable in my kz, subtle change but enough to make me more consistant, maybe not outright quicker. An easier drive.
