Last visit was: Wed Dec 02, 2020 6:55 pm It is currently Wed Dec 02, 2020 6:55 pm

All times are UTC+01:00




 [ 29 posts ]  Go to page 1 2 Next
Author Message
 Post subject: A Rod to Spare?
PostPosted: Mon Nov 16, 2020 9:24 pm 
Offline
User avatar

Joined: Fri May 08, 2009 1:13 pm
Posts: 3072
Current ride: XB9SX
This is a subject that has interested me even before I bought my XB, but probably understood it even less then. After riding both the 984cc and 1200cc versions I always thought that there was another engine lurking within those Sportster XL cases.
I even wrote to Erik several months before I got my City X with my ideas. He replied eventually to wish me luck modifying my own. Buell were probably already contemplating the Helicon twin! :|

As many on here may already be aware that con rod length and the length to piston stroke ratio has some impact on how a motor makes it’s power. Briefly a longer rod to stroke ratio reduces piston acceleration at the top of the stroke and gives longer dwell time increasing efficiency of the motor at higher rpm, also reducing angular momentum of the rod and sideways loading on the piston walls. While a shorter rod to stroke ratio gives faster acceleration of the piston away from TDC and improving intake gas draw down and mixing and therefore better torque at lower revs. Obviously many other factors like cam timing, compression ratios, etc, etc can complicate the equation.

The XB12 appears to have an optimum rod/stroke ratio for its operating range of 1.84 while the XB9 has a somewhat F1 like 2.33 ratio. But spare a thought for the poor little Blast which was blessed with a ratio of 2.38 I think. F1 motors like the Ford DFV had high ratios for their day of 2.25 if memory serves, but later 19,000 F1 motors were at a heady 2.7! :shock:

According to some stories the Blast begat the XB9, and the XB9 begat the XB12, but the truth is probably more mundane and a case of convenience. The architecture of the 12 was already in existence and the Blast was designed to be made with economies in mind to be assembled on the Sportster production line with as many common parts as were available. I read that the Blast uses a con rod unlike any of the Evo twins being the same length as the iron barrelled XL61 (1000cc). Maybe HD had crates full of unused pre Evo rods gathering dust so they suggested using the front inner fork rod from that stock?

Given that the design would use a standard Sportster barrel this meant that a standard Sportster piston would reach too high at TDC. Then I read that the ‘deck’ of the Blast’s crankcases was raised 0.168 inches to allow for the longer rod. Those with more accurate knowledge may wish to comment? The Blast crank was heralded as being new and innovative in one article, going on to influence future crank design…..does that simply mean that it was to no longer be a serviceable item and needed to be replaced complete when it wears like my XB9? :roll:

The XB9 would appear to have been given its own unique rod length so that it would match the rest of the ‘X’ twin production. Although the design gave higher revs from a allegedly less stressed motor that rod ratio of 2.33 was probably made out of convenience and cost constraints as well to utilise a 1200 set of barrels and cases. Didn't stop the 9 lunching its pistons though. I am conscious of the fact that my old Honda 250RSA used a 'forged' piston back in 1982! :?

My Yamaha SRX400E ‘Mono’ has the same architecture as the bigger SRX600E (the motors are interchangeable) as the distance between the crank centre line and head gasket is identical. The 400 therefore uses a longer rod and higher ratio to stroke. If you look at the torque curves of the 4 and 6 you see the same sort of differences as between the XB9 and XB12.
The 600 motor has a fat hump or constant torque curve, while the 400 has a flatter curve with a pronounced step in the mid range just like the XB9. Long rod characteristics?

You can feel these differences in the power characteristics of the different sized XB motors and that is quite apart from one engine being 22% bigger, it fills its boots better in the midrange! 8-)

I'll be quite honest and admit that I am not a very sporty rider. In my teens I had a 12,000 rpm 125 twin and that was enough for me. A few years later I rode a friends newly acquired Combat Commando 750 and ever since I've wanted torque, but with an edge. I wanted to see a sort of hybrid motored XB with a stroke of say 3.41 inches which would reduce the rod/stroke ratio giving something of the best of both the 984 and 1200…..a 1074cc! One day maybe?

I guess my constant fettling with a venture has sort of tricked my 9 into thinking that it has a shorter rod/stroke ratio. Maybe I'll try it on the dyno when my MOT man recovers from his injuries, to see what's actually happening at the rear wheel. ;)


Top
   
 
 Post subject: Re: A Rod to Spare?
PostPosted: Tue Nov 17, 2020 5:13 pm 
Offline
User avatar

Joined: Fri May 08, 2009 1:13 pm
Posts: 3072
Current ride: XB9SX
V-E-N-T-U-R-I........damn predictive spelling! lOl


Top
   
 
 Post subject: Re: A Rod to Spare?
PostPosted: Tue Nov 17, 2020 6:15 pm 
Offline
User avatar

Joined: Fri May 08, 2009 1:13 pm
Posts: 3072
Current ride: XB9SX
http://victorylibrary.com/tech/rod-c.htm

check my maths! :?

average of '9' and '12' rod ratios (1.84 + 2.33) / 2 = 2.085

(3.41 in - 3.125 in) = 0.285 in increased stroke of 1074cc motor

0.285/2 = 0.1425 reduction in rod length

XB9 Rod = 7.027 in

7.027 - 0.1425 = 7.1275 inch rod for 1074

7.1275 / 3.41 = 2.09 rod/stroke ratio! Best of both? :? ...but would require special crowned piston to make up compression ratio to the optimum 10 to 1.


Top
   
 
 Post subject: Re: A Rod to Spare?
PostPosted: Tue Nov 17, 2020 7:26 pm 
Offline
User avatar

Joined: Wed May 06, 2009 9:29 pm
Posts: 1460
What are you doing, having a chat with yourself? :?


Top
   
 
 Post subject: Re: A Rod to Spare?
PostPosted: Tue Nov 17, 2020 8:01 pm 
Offline
User avatar

Joined: Fri May 08, 2009 1:13 pm
Posts: 3072
Current ride: XB9SX
spondon440 wrote:
What are you doing, having a chat with yourself? :?



Yes, unwinding alleviates the madness! lOl

Just adding bits I missed. Thanks for reading anyway. I expect that you could write reams of better stuff if you had the time. ;)


Top
   
 
 Post subject: Re: A Rod to Spare?
PostPosted: Tue Nov 17, 2020 8:11 pm 
Offline
User avatar

Joined: Mon Nov 07, 2011 7:10 pm
Posts: 3161
Current ride: Buell ulysses
Location: Telford
edd wrote:
spondon440 wrote:
What are you doing, having a chat with yourself? :?



Yes, unwinding alleviates the madness! lOl

Just adding bits I missed. Thanks for reading anyway. I expect that you could write reams of better stuff if you had the time. ;)

https://theconversation.com/is-talking- ... dict-77058

_________________
Buell Ulysses XB12X 06/08
CCM R30 (650cc) 02
Norton Dominators


Top
   
 
 Post subject: Re: A Rod to Spare?
PostPosted: Tue Nov 17, 2020 8:12 pm 
Offline
Milf Hunter
User avatar

Joined: Wed May 06, 2009 4:47 pm
Posts: 4615
Current ride: '98 S1
Location: Wessex
Well, that's 10 minutes of my life I won't get back..

Just spank it until it breaks ;)

_________________
I started out with nothing and still got most of it left.


Top
   
 
 Post subject: Re: A Rod to Spare?
PostPosted: Tue Nov 17, 2020 8:57 pm 
Offline
User avatar

Joined: Wed May 06, 2009 1:43 pm
Posts: 4447
Current ride: The wife
Location: Cofa's tree
mojomick wrote:
Just spank it until it breaks ;)

Not advice I would give :rotfl:

_________________
I hate being strapped for cash....but it pays the bills


Top
   
 
 Post subject: Re: A Rod to Spare?
PostPosted: Tue Nov 17, 2020 10:10 pm 
Offline
User avatar

Joined: Tue May 05, 2009 8:00 pm
Posts: 10033
Current ride: X1 2001
Location: southampton
....worst fishing thread ever :sad1:


Top
   
 
 Post subject: Re: A Rod to Spare?
PostPosted: Tue Nov 17, 2020 10:21 pm 
Offline
User avatar

Joined: Fri May 08, 2009 1:13 pm
Posts: 3072
Current ride: XB9SX
I thought we were back on corporal punishment again! :?


Top
   
 
 Post subject: Re: A Rod to Spare?
PostPosted: Wed Nov 18, 2020 12:05 am 
Offline
User avatar

Joined: Wed May 06, 2009 9:29 pm
Posts: 1460
edd wrote:
spondon440 wrote:
What are you doing, having a chat with yourself? :?



Yes, unwinding alleviates the madness! lOl

Just adding bits I missed. Thanks for reading anyway. I expect that you could write reams of better stuff if you had the time. ;)



Couldn't be arsed mate, the future is orange.

ImageIMG_1095 by ANDREW FRANCHI, on Flickr

Andy


Top
   
 
 Post subject: Re: A Rod to Spare?
PostPosted: Wed Nov 18, 2020 12:26 am 
Offline
User avatar

Joined: Tue May 05, 2009 8:00 pm
Posts: 10033
Current ride: X1 2001
Location: southampton
8-) very nice Andy :yup:


Top
   
 
 Post subject: Re: A Rod to Spare?
PostPosted: Wed Nov 18, 2020 9:11 am 
Offline
User avatar

Joined: Fri May 08, 2009 1:13 pm
Posts: 3072
Current ride: XB9SX
:yt: Technically brilliant, but it’s so ugly l’d want it totally in satin black. I’d also get busted for mono wheeling everywhere as soon as it was run in. ;)


Top
   
 
 Post subject: Re: A Rod to Spare?
PostPosted: Wed Nov 18, 2020 5:26 pm 
Offline
User avatar

Joined: Fri May 08, 2009 7:54 pm
Posts: 2486
Current ride: 1999 X1
Location: Texas, Gerrards Cross
I'd ridden a 790 Duke and yes, a brilliant machine. Now, the 1190 Duke and 1290 Super Adv I rode were just pure evil incarnate. Amazing and what power...oooooo. But as you say Edd, I can't get past the Predator-like fangs. I have yet to see a body kit by Airtech or the like to alter it. Apparently enough like it. Buy I think an opportunity is being missed for those that don't. Even KTM might make a few more sales if there were aesthetic options. I like twins and I like steel frames. And KTM has one of the best electronics suites on the planet, even compared to many true race replicas and homologation specials.

_________________
Bob Krzeszkiewicz
2012 XR1200X - 2011 Kawasaki Z1000SX - 1999 Buell X1


Top
   
 
 Post subject: Re: A Rod to Spare?
PostPosted: Wed Nov 18, 2020 5:44 pm 
Offline
User avatar

Joined: Fri May 08, 2009 7:54 pm
Posts: 2486
Current ride: 1999 X1
Location: Texas, Gerrards Cross
BTW, regarding your long post about rod ratios and different accelerations either BTDC or ATDC has more to do with cylinder offset. It doesn't matter what length con rod you have when the cylinder axis is directly over the crank axis, the piston acceleration will always be the same, you just may have undesirable thrust loads on the piston side walls.

When you offset the cylinder axis away from the crank axis, that's when you see differences in acceleration. Forward (usually 2-4 mm), the compression stroke slows down more and has more dwell just BTDC allowing for more combustion heat to accumulate and allowing retarded injection and firing timing. As such, the piston has a better angle to push down on at the big end journal, allowing for more leverage on the crank, giving more torque and it also accelerates faster on the way down.

Easy to engineer in with an inline or with offset journals on vees. Can't really do it on a twin with a shared rod journal. Move one axis how you want and the other moves in a direction you don't want. You'd have to change the angle of the faces of the barrels in relation to the crank axis to simulate an offset. Our Buells cylinder axis is pointing directly at the axis of the crank so that at TDC, the piston is always pushing directly down on that axis where there is no leverage.

_________________
Bob Krzeszkiewicz
2012 XR1200X - 2011 Kawasaki Z1000SX - 1999 Buell X1


Top
   
 
 [ 29 posts ]  Go to page 1 2 Next

All times are UTC+01:00


Who is online

Users browsing this forum: CommonCrawl [Bot] and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Jump to:  
Powered by phpBB® Forum Software © phpBB Limited