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Caterham A Frames - check them as part of your annual maintenance


Croc

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I ordered one  2 years ago as mine was bent when I lost a wheel bearing, and then the wheel, while on track. I mentioned viewing the A-frame as a wear item to Bruce Beachman when ordering a replacement  and he said he wasn’t aware of them giving problems. And that applied to their Caterham race cars which get much harder use.

 

In any event, having had  both rear bearing hubs fail at different times, with attendant dramatic and $$$$ consequences, followed by endless waits for replacements, I would add them to the list of items to replace on a semi-regular basis.

 

Whoops, just noticed I have made this point earlier in this thread. Oh well, maybe it bears repeating:).

Edited by Kitcat
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Thank you - I have a bunch of stuff coming from Bruce and was surprised when he told me he wasn't aware of the issue. Bruce is very helpful and knows the cars. Despite paying to ship parts back and forth across the country I like to get stuff from him. He guided me towards the stronger front hubs, axles and bearings, and I need to check the rear wheel bearings and upgrade those if I have the old ones. I am going to have all the suspension apart to replace the bushings with Power flex bushings. I hadn't considered how the A frame / lateral link worked, but it is under a twisting moment. I do not understand why they would do that. I am either going to replace it or at least have it magnafluxed. A friend of mine regularly runs against a guy who races a Caterham. I am going to speak to him as well. I am not putting urethane bushings in the A frame. The more compliant rubber will reduce the bending moment on the tube. Getting my car back from body work next week. Hope your car is fixed or close.

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Just to defend the honor of the A frame a little, the bent A frame I got last month came after a track weekend at COTA on slicks. I looked back at the video and I was getting 2g lateral force at really bumpy parts of the track, particularly some of the bad curbs. I raced a well sorted E46 a few weeks later there, the engine belt came off at the Esses the vibration is so harsh.

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I think it is lots of track time that makes the A frame a risk.  My A frame on the 420R broke at roughly 10,000 track miles.  All high G loading tracks.  I don't run with slicks but rather the Avon ZZS (it always rains in the UK so why bother with slicks).  I do use the curbs which would not help.  I also have the benefit of storing my car with a group of Caterhams which are even more heavily tracked than mine so I get to see and hear about more regular failure points on these cars under heavy track usage.  

 

This is a non-issue on road cars with mild track usage unless they were quite aged.  Carl's car was extensively tracked by its previous owner so it is worth him checking for peace of mind.  

 

 

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My interest was in understanding maintenance requirements both the intervals and the procedures. I have a race car background and want to insure reliability. These cars are about halfway between a race car and a production car, and the maintenance is about halfway as well. The more track time the more maintenance.    

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The A Frame handles latteral forces under acceleration and axle wrap, the latter more so in the live axle cars.

It is easy one of the most stressed parts of the car.  Regular bush service and corrosion prevention is necessary.   No A frame, no car.

Ive done a Y16 replacement on my A frame.   Those suckers are tight!

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It is horrifically bumpy.  Also hard to find reference points for Turns 1 and 17 - its this vast expanse of asphalt and there is a better (i.e. slightly smoother) line through there that you have to pick out.  I have only driven there in a 1965 Mustang and it was fairly lively.  

 

Great video - one of the best I have seen for driving Sebring as you can see clearly what car and driver are doing at each point.  

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Wow - Great Video! - Good car and great driver. Thank you for the video, and it does convince me to not get too excited. I have an older car and I do not believe it had the best maintenance. I think I may have implied that I was overly concerned about the A frames breaking without notice. I wasn't. I wanted to understand the failures and I hadn't looked at the load paths on the part. Between your pictures and Croc"s pictures I am very comfortable with what needs to be done. At least what I am going to do. I have never driven at Sebring, but I have been there. Old World War II runways are still bumpy after they have been paved. Great catches in the video. 1.6 /1.7 Gs will certainly put a bending moment on the tube. The maintenance is simple. Inspect the A frame for visible damage and any bending and you should be fine. I have to say I do not understand why they built the car that way. They could have put a ball joint at the attachment to the DeDion tube, I think. I have to think about why they did it the way they did. The watts links on the rear are interesting as well. I assume they went to all that trouble for roll steer, but I haven't measured it.  

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13 hours ago, Croc said:

It is horrifically bumpy.  Also hard to find reference points for Turns 1 and 17 - its this vast expanse of asphalt and there is a better (i.e. slightly smoother) line through there that you have to pick out.  I have only driven there in a 1965 Mustang and it was fairly lively.  

 

Great video - one of the best I have seen for driving Sebring as you can see clearly what car and driver are doing at each point.  

Thanks Croc!

5 hours ago, CarlB said:

Wow - Great Video! - Good car and great driver. Thank you for the video, and it does convince me to not get too excited. I have an older car and I do not believe it had the best maintenance. I think I may have implied that I was overly concerned about the A frames breaking without notice. I wasn't. I wanted to understand the failures and I hadn't looked at the load paths on the part. Between your pictures and Croc"s pictures I am very comfortable with what needs to be done. At least what I am going to do. I have never driven at Sebring, but I have been there. Old World War II runways are still bumpy after they have been paved. Great catches in the video. 1.6 /1.7 Gs will certainly put a bending moment on the tube. The maintenance is simple. Inspect the A frame for visible damage and any bending and you should be fine. I have to say I do not understand why they built the car that way. They could have put a ball joint at the attachment to the DeDion tube, I think. I have to think about why they did it the way they did. The watts links on the rear are interesting as well. I assume they went to all that trouble for roll steer, but I haven't measured it.  

Thanks Carl! Good luck with your project! 

 

Edited by sf4018
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On 1/15/2022 at 4:14 PM, CarlB said:

My interest was in understanding maintenance requirements both the intervals and the procedures. I have a race car background and want to insure reliability. These cars are about halfway between a race car and a production car, and the maintenance is about halfway as well. The more track time the more maintenance.    

I found this on my computer, I think it's from the Caterham maintenance manual which you may already have, but here it is anyways...

 

Caterham Service Schedule - Duratec (1).xlsx

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  • 1 year later...

I can see why it failed this way (I also investigate GA aircraft accidents and approve modifications). The sudden change in cross section outboard of the cross bar weld ensures flexing occurs in this small length of tube instead of the load being spread evenly along the length of the arm. Typical gussets could be used (one on each side; never down the center of the tube) but a better way and typical of aircraft tube frames would be to use .050" sheet steel cut from a mocked up kraft paper pattern (cereal box works well). This will help spread the load.

dedion reduced notch sensitivity.jpg

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  • 1 month later...

Bumping this, as I just realized that caterham makes a "race" version of the a frame that is brazed apparently. Not sure what makes it stronger.  Wondering if anything knows anything about this part and if it's worth upgrading to it for cars that are heavily abused on track. 

 

https://caterhamparts.co.uk/a-frames/7849-race-a-frame-brazed.html

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6 hours ago, KnifeySpoony said:

Bumping this, as I just realized that caterham makes a "race" version of the a frame that is brazed apparently. Not sure what makes it stronger.  Wondering if anything knows anything about this part and if it's worth upgrading to it for cars that are heavily abused on track. 

 

https://caterhamparts.co.uk/a-frames/7849-race-a-frame-brazed.html

brazing would be stronger because of the lower heat and furnace brazed more consistent.  john   ( but far more expensive and even more difficult to repair or modify).

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9 hours ago, KnifeySpoony said:

Bumping this, as I just realized that caterham makes a "race" version of the a frame that is brazed apparently. Not sure what makes it stronger.  Wondering if anything knows anything about this part and if it's worth upgrading to it for cars that are heavily abused on track. 

 

https://caterhamparts.co.uk/a-frames/7849-race-a-frame-brazed.html

I'm guessing that's an S3 size? Not sure what they mean by "metric"?

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6 minutes ago, sf4018 said:

I'm guessing that's an S3 size? Not sure what they mean by "metric"?

 

Caterham switched from imperial to metric chassis around 2006/2007.  A lot of parts are not interchangeable between the two due to small dimensional changes.

 

-John

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As I have mentioned, former Caterham engineer/mechanic, Nathan Down, participated in the refurbishment of my Caterham before it’s sale last year.
 

He told me that the A-frames were under constant development and that over a short period (6 months), there might be 7-8 different sizes. They would try one out on the track, change it, try again, repeatedly. They have probably settled on a standard size but mid 90’s, early 2000’s there is no one size.
 

So stick with what you have as long as it works. When I replaced mine with the wrong side, it bent the shocks so much they became solid and my rear suspension turned into a pogo stick.

 

And Croc: 10,000 track miles in the Caterham in  UK alone? Combine that with miles you’ve logged at NJMP and on European tracks and you must hold some kind of Caterham world record?

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Mine is a 2021 kit so I assume the current race a frame would fit. Perhaps not worth fitting as prevention, I'm not sure. They also have "race" DeDion tubes. I'm not sure how the construction differs on these. Swapping an A frame is very quick. De Dion swap not a huge deal either but have to deal with break lines so not as quick a swap. I do a fair number of days at Sonoma which is quite bumpy and I'm not afraid of curbs, so I'd rather have a small hassle now to prevent a huge off into a wall...

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