Why do current Thumpers share the oil sump?

DougRoost

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#1
I inserted this question on the "Full synthetic oil in my Thumper" thread but it really warrants it's own thread.

Can someone please tell us why the current production 4 stroke dirt bikes share have the engine and transmission sumps combined, rather than using separate sumps with the appropriate lube in each?! Especially since they use a wet clutch setup. This makes no sense to me but there has to be a good reason to put us through these frequent oil changes. Even with them we still risk damage from clutch material clogging those small oil passages to the valvetrain.

This makes even less sense to me since their 2-smokes have gear oil in their gear boxes. I'd have thought these manufacturers would have just added a separate oil sump for the engine.

Meanwhile both Cannondale and Honda seem to have come to the same conclusion now (CR450F, not the XRs, in the case of Honda). I also understand the old BSA single cylinder bikes used separate sumps as do Harleys (of course these are street bikes but still large thumpers). I guess what surprises me most is that KTM didn't beat them to it with their 400 and 520 RFS's since they are such an engineering-oriented company.

I know it isn't weight since that was one reason Honda claims to have separated separated the sumps on the CR450F (you can use just enough of each of the proper oils rather than a bunch of one).

Do the Japanese street bikes also use just one shared sump? Was it cost due to simplicity of the engine cases? If so I have to ask again why KTM didn't do it since they are not striving for the lowest price point.

Maybe it just comes down to the fact that dirt bikes have such short warranties that the manufacturers don't care. Or maybe it's not that big a deal and I should stop worrying about it. Thanks for anyone with insight on this matter.:think
 

Magellan

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#2
Just guessing, but I think they do it because they can. With a 2S, there's no option, they require different lubes. But the 4S engine and tranny can use the same lube.

I suspect it's cheaper.

I can't remeber if my street bike had a separate sump, but I don't think so. I think I'd remember if I had to change tranny/clutch oil separately.
 

DougRoost

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#3
I kind of do, too. Yet that doesn't explain why KTM uses one sump -- and uses 2 oil filters no less! Likewise for Yamaha since the YZF price point is considerably higher than the TTR's (ala Honday CRF vs. XR), yet the YZF shares a sump. Worse, these bikes also have pricier components (magnesium, titanium, etc.) and higher revs than the TTRs.