Rear bumper -- tight or loose?

TJ has a video on set-up that ends with something like this: “Never tighten the rear bumper.” After years of driving my Rotax CRG’s or Zanardis with my bumper tight, I’ve recently experimented with it loose, now on a CRG KT2 with VLR 100. While trying to keep apples to apples (obviously same day, same track conditions, same fuel level each run, no change in tire pressures, short time between runs, etc,), I’ve gotten inconsistent results at best. Sometimes tight wins, sometimes loose wins. As I examine my Micron 5-2t data at home, I cannot find a pattern (speed, rpm’s, braking, lateral acceleration) that gives me the answer. Best I can tell, it is usually my driving that is the variable (we are talking no more than 1 tenth between the fastest lap in the 2 configurations, although time differentials vary throughout each lap).
If requested, I can give much more set-up info.

I’ve run the rear bumper loose 99% of the time for the last 11 years. The same probably applies for the 5 years before that, but we didn’t really start using the current style of plastic bumpers (KG/OTK) on a regular basis until 2010.

Personally, I would run with a loose bumper. I think the reality is the rear of most karts is already fairly stiff across the back. With the seat strut bar and axle I don’t think there is a lot of flex on the back half of the chassis unlike the front end of a chassis that flexs qute a bit which may explain why your not seeing much difference, especially in a lower HP class. I would include side pods too, they should not bind and be easily installed or removed. Let the chassis be the spring.

My mounts are bent from a wreck, is it worth replacing them?

Personally I run mine loose, because no matter how many times I tighten it, it just become loose anyway. So I feel like the decision has been made for me, lol.

When you say mount do you mean the slide part or the bolts into the chassis? If its the slides can it be bent back into shape?

One reason I went tight on the bumper had to do with going somewhat aggressive on the front end. My CRG distributor finished P1 in SuperPro during the 2002 SuperNats and currently has a successful CRG race team; he advised me to try raising the front of my KT2 and going 3 spacers out each side. I’ve been pushing in a critical turn so I dropped my kingpin back with 2 lines of positive castor showing on my sniper plate. Anticipating a lot of jacking with this setup, I wanted to increase roll resistance in the rear . That is why I experimented with a tight bumper. Yesterday afternoon, in a reversal from last Thursday, the fastest tight bumper lap beat the loose bumper lap by .45 sec. I am not sure that the lower HP class I’ve chosen to run makes a difference; I’ve actually had a slight increase in speed around some turns due, I believe, from my over-braking in my Rotax. I’ve also had higher (a one time outlier of 2.93 g’s) lateral acceleration with my VLR 100.

Slides, they just kinda bent into the mount so it’s stuck ish

If you can do it in the shop they can be bent / hammered back into shape. They just have to slide. Might be tough to do at the track. OR replace them and keep the old set as a back up.

I rarely run with it tight but when I do the track is green or cold… basically just adds rear grip.

Maybe 4-5 sessions a year… generally never in a final but it has happened before.

1 Like

I would imagine running the rear bumper tight would be in the same handling direction that adding a rear bar would also (at least on my OTK kart). I’ve never run the rear bar and always have my rear bumper loose for TAG racing, but was of the understanding that if road racing and/or long sweeping corners to run it tighter or add the rear bar. On a side note, a tip I learned from the now defunct OGP, was to file down the brackets and make them extra loose with the benefit being less chassis bind. Now you know one of my secrets. Lol!

If you need to drastically reduce your inside rear wheel lift, then tightening the rear bumper is worth a shot. In my experience, I haven’t had a need to go that far in that direction, as most of the time if the kart is jacking too hard, reducing caster, narrowing the front, softening the front, widening the rear etc., will all accomplish what I need without tying down both frame rails.

What is the grip level of the track you’re running on?

First, thank each of you for your replies. I began to wonder about the track itself; your comments reinforced this thinking. As I understand it, our track in Tucson is considered a momentum track. There are 3 turns where I am just over 35 mph – the rest are all faster. I believe these 3 are all 40 mph + turns when the “national” level drivers show up for the Challenge of the Americas. That 3 day event is the ONLY time during the year with significant rubber on the track surface. On all other 362 days we have a green track with more or less grip depending on weather (and the use of the track by sports cars and drift cars). With the VLR 100 I do not find any braking zone that requires full pressure; I usually make a moderate stab at the brakes, get right back on throttle before turn in, and, with the low HP engine, and slight delay in response, I’m able to maintain speed around the turns without binding. Do my driving habits and track conditions explain the apparent lack of difference between tight and loose rear bumper?

I haven’t raced at Tucson in probably 7 years, but I am familiar with the track.

It’s possible with the lower grip levels (especially if the track is sandy), the tight rear bumper is giving you better traction off the corner. What did you feel in the kart’s handling when you tightened it and went .45 faster in that session?

I’m embarrassed to adm, but I couldn’t feel a real difference. Probably because I was only about a 1/10th faster going into what I call the Toilet Turn (the one at the end of the back straight where, if your brake explodes or something, sends you into the rest room). This being the last turn before the main straight (actually about a 180 degree left followed by a 90 degree right that takes you down the straight), the 2 setups were nip and tuck until I blew those final turns with the bumper loose – driver error, not bumper error.
Looking back at my notes, the first evening I experimented with the bumper was mid July. My notes and my memory reflect that I felt much more planted and aggressive with the bumper tight as I rolled out onto the track – and my times were immediately a few tenths better than the previous session with a loose bumper.
Grip varies, but I don’t think I was dealing with sand; if that stuff finds its way onto our track, it is usually at one spot or another and you get an apple surprise., i.e., ooops, WHAT IS HE DOING IN THE CACTUS?


It sounds like that .45 was a bit of a fluke than, like you just missed a corner.

It should feel more planted, as you said. Whether or not that’s a good thing or a bad thing depends on the driver and the stopwatch. If you’re faster with it tight, go for it and keep it tight. I think most people would leave it loose and use other adjustments to solve the problem but the stopwatch doesn’t lie!

Just be aware of how it performs over a longer run. There’s a chance it could tighten up pretty bad once tires get hot, if you’re restricting that inside rear wheel from unloading.

1 Like

TJ, I almost always agree but I have found if I think the track is going to tighten up which most do throughout the race day/weekend. I may set it tight for warm-ups then loosen it for later sessions. Its such an easy change to make compared to others just my opinion.

That being said, if the track is going to be slick throughout I will seek other adjustments that I wont have to worry about changing back later.

Basically, I am lazy

2 posts were split to a new topic: “I went faster. Was it the chassis adjustment or my driving?”

If you’re going to leave your bumper loose, do that by mounting it with clevis pins at one or both ends, rather than by inadequately tightening fasteners. The force of friction when clamped is what keeps nuts from coming off bolts. My CRG’s bumper has both lateral and vertical freedom and it handles beautifully…