View Full Version : small "thunk" sound in rear end
AA-ron
06-11-2023, 05:19 PM
Hello everyone,
I finished my build last month and will be getting my VIN on Tuesday. I haven't been able to give this thing a proper shake down in go-cart mode because I live on a busy street;so I've been limited to my 60-foot driveway. Since the first time I drove it, I noticed a light "thunk" sound in the rear when I let out the clutch--especially after going from reverse to first. When I first heard it, I didn't have the body fully mounted and assumed it was something going on with either the body shifting or my trunk latch not being installed. Now that everything is buttoned up, I just realized today that I'm still hearing it (its softer sounding because carpet and sound mat is installed). It's a pretty subtle sound and I'm wondering if it's normal. I've obviously owned manual shifting cars before, but I've never owned one where you're basically sitting over the rear dif, which is why I'm asking if its normal to hear "something" back there. I have a t-5 with IRS.
I hope to get this on the road mid-week and I'm wondering if there's some likely places for me to look to make sure all is ok before I head out on the road.
Blitzboy54
06-11-2023, 06:34 PM
The issue is a little to vague at this point for me to comment. I had a clunk but it turned out to be the roll bar. I will say this, like you said you are right on top of the engine , transmission, driveshaft and differential with noting but thin aluminum between you and it. There were a lot of noises that concerned me early that turned out to be nothing.
Having said that you could very well have an issue but if it were me I would keep an eye on it. I wouldn't loose any sleep over it unless it if it got worse.
Ford & Jeep Fan
06-11-2023, 11:09 PM
I agree with post number 2 and I'd add These cars are not like your typical everyday cars. they are more like race cars in that the engine trans and suspensions are usually mounted Urethane bushings which transmit a much more noise, vibration and harshness than the rubber bushings in everyday cars.
AA-ron
06-12-2023, 07:15 AM
Yep, I figured it would be too vague of a question/description to diagnose the problem-- if it's even a problem. I will drive it and keep an eye on it.
Thanks!
Yes, it is vague, but I have an idea to share with you. If I read your signature line correctly, your differential/center section has been sitting for at least 6 months. Was it new or used? I had a thunking sound in my rear end, and I attributed it to the axle bearings not being lubricated sufficiently because they had been sitting for a while. When I asked FFR about that, I learned that frequently the locking pieces in the differential will "stick" if they don't get used enough/ broken in..
Two other alternatives: 1. your driveshaft is loose. Check the bolts from the driveshaft to the differential. 2. If you have a rebuilt unit, there could be play in the gears.
So, tighten your driveshaft bolts, and drive a lot. I bet the problem goes away.
rthomas98
06-12-2023, 09:10 AM
One last thing to check is your driveline angles. You could be feeling driveline lash. You will want to make sure that your pinion angle is equal and opposite of the back of your transmission angle. If these are not correct that is most likely your issue. This can only be checked with the cars weight on the suspension. If you do it in the air with frame supports you will get a wrong measurement.
Ted G
06-12-2023, 10:19 AM
AA-ron,
I too have IRS with that same small clunk. I especially get it when I let up after a hard acceleration. So far I have about 100 miles on my go-kart and no issues yet; I've also driven it pretty hard at times. I also need to verify my pinion angle but I'm pretty sure it's spot on. I verified it on jack stands but rthomas88 said that will affect measurements; not sure if that is the case with IRS though. Body goes back on this week!!
Ted
RRussellTx
06-12-2023, 11:48 AM
Y
1. your driveshaft is loose. Check the bolts from the driveshaft to the differential.
I had a similar issue to the one described here on my IRS driveshaft. The bolts for the adapter plate were just a smidge too long and when it was torqued down it was just bottoming out and not fully securing the adapter plate. Ground the ends off the bolts, re-torqued and all was fine after that.
I could consistently recreate the noise by hitting the gas and letting off. At low speeds it was obvious. If you can't recreate it this way then it's probably not the same issue.
Richard
AA-ron
06-12-2023, 03:08 PM
Hey thanks for all the advice. From what you all have described with the IRS exactly duplicates my situation. I did have the problem with the bolts on the adapter plate being ever so slightly long but caught that during assembly. I'll check the pinion angle but I believe it to be good. I'll spend some time driving it and just keep an eye on it.
The other piece of good advice is the fact that it's a brand new center section with about 300-feet on it (my driveway only). It could just need to be driven a bit more. I've checked fluids and all is good there.
I'll keep you all posted if I find anything as I drive it.
rich grsc
06-12-2023, 05:48 PM
Pinion angle shouldn't cause a 'clucking', if it was way off, it would be more of a vibration.
Joel Hauser
06-13-2023, 07:15 AM
I'll second Al_C's recommendation, and it is good advice even if it turns out it is not the cause of the clunk "Two other alternatives: 1. your driveshaft is loose. Check the bolts from the driveshaft to the differential.....So, tighten your driveshaft bolts, and drive a lot. I bet the problem goes away." Many builders install a drive shaft loop as a safety precaution. They are inexpensive insurance, and easy to install.
AA-ron
06-16-2023, 01:41 PM
So I'm going to try not to be one of those guys that questions EVERY little bump and thunk I hear in my car... but....
AFter a bit of driving (about 30 miles), I'm definitely hearing the thunk between shifts and even a bit during low RPM non-acceleration when the drivetrain is under no tension.
I jacked up the car and took a look to see if anything was obvious. Here's what I found.
(and I hope I'm getting my terminology correct)
Where the drive shaft goes into the center section on my IRS is nice and tight, with absolutly no play anywhere. The "thunk" relates to the slack between the input and output to the spindles. Basically, I'm able to turn the drive shaft at the input to the dif about 1/8" in either direction (rotationally) before it seems to engage the wheels. This is definitely the sound I'm hearing. Is this normal? Its a brand new IRS (everything) from Factory Five. I've checked fluid levels and it is right where it should be in the dif (right at the fill hole) so its not a fluid thing.
Am I just being too sensitive? Is this something that happens with all IRS cars, and because I'm sitting right on top of it, I'm here it here?
Any advice would be great.
FYI-- I HAVE NEVER HAD SO MUCH FUN DRIVING! This thing is an absolute blast in every way. Granted I've only had it on back roads and only up to about 45mph.... ha!
Nigel Allen
06-16-2023, 06:12 PM
The backlash on the differential seems spot on. My car has the older IRS and I get the occasional clunk changing gears, mainly when when the drive line is at running temperature. I also get some minor gear backlash noise from the TKO. Probably should have put some sound deadening on the underside of the transmission tunnel. These are all new components.
Glad you are having fun driving. It’s our winter here, and it has been really wet. Only been for one drive in last 2months. Getting itchy.
Do you still need rear wheel alignment info? I have a write-up I can send.
Great custom job on your car. I love it!
Cheers,
Nigel
AA-ron
06-17-2023, 10:11 AM
The backlash on the differential seems spot on. My car has the older IRS and I get the occasional clunk changing gears, mainly when when the drive line is at running temperature. I also get some minor gear backlash noise from the TKO. Probably should have put some sound deadening on the underside of the transmission tunnel. These are all new components.
Glad you are having fun driving. It’s our winter here, and it has been really wet. Only been for one drive in last 2months. Getting itchy.
Do you still need rear wheel alignment info? I have a write-up I can send.
Great custom job on your car. I love it!
Cheers,
Nigel
Hey thanks for the info. I'm going to drive it and just keep an eye on it to see if it gets worse. I'm all set with alignment, thanks. It's all aligned and drives like a dream-- tracks straight, turns beautifully...etc. Better than I ever imagined, to be honest.
David Williamson
06-17-2023, 01:01 PM
My coupe with IRS has the same. Between the gears in the diff and the CV joints there is a fair amount of back lash and you can hear it while driving, on off the gas and changing gears, 5000+ miles and no change.
David W
Is your center section new or rebuilt? If rebuilt, it could be play in the gears. After 5K miles, everything should be lubricated and broken in. If your driveshaft is nice and tight, I'd check the the internals. Just my 2 cents.
AA-ron
06-21-2023, 12:21 PM
I'm sorry to keep this thread alive, but I'm still trying to get to the bottom of this "thunking" sound. I've verified that the sound is coming from slop between the input to the dif and the output to the axles. Like I mentioned before, there's about 1/4" play between rotating the input (after the drive shaft, so it's not in play here) and when it seems to engage the axles. There's nothing else loose anywhere. I'm only bringing this up because it seems to be getting a bit louder after the first 150 miles. I found an older thread about going to a thicker fluid in the dif, and that helped to fix it.
I just don't want to be causing damage further down the road.
Thanks and thanks for tolerating a newbie sorting through new car sounds.
cc2Arider
06-21-2023, 04:55 PM
Funny anecdote that I can share...
I work for a company that provides Powertrain solutions to many different market segments, but one of our big Customers is RAM trucks. Anyway, many years back the Test Engineers and the Marketing types were complaining that our engine was too torquey and exacerbated the differential's driveline lash. I think maybe RAM was using a lower cost supplier? :confused:
We developed a feature to smooth out the torque transitions, and one of our more colorful Engineering Managers christened it the "Declunkinator" feature and it stuck ;)
Craig C