ideal thumper?

[johnr]

ok, ive wondered about this for a while, so here goes.
ive always liked thumpers, theres something about the sound and feel of a single cylinder thats just got something about it. but, and this isnt really an old versus new thing, whats in your eyes the ideal capacity for a single cylinder engine? why am i asking? well, ive a few thumpers in the shed. my latest is a dr big suzuki, a 750cc 4 valve monster of an engine. it can haul my 20 stones upto a genuine 100mph, even with its breeze block aerodynamics, so what it would be l;ike in a small streamlined chassis i can only imagine. however, its not ideal. when i snap open the gas, the dr launches off like a scalded cat, but in high gear and low rev situations it clatters and bangs like someone dragging a gas cooker down a cobbled street, above 3500 rpm and its off and well nigh unstoppable. but its the low down thump thump thump of a big lazy single that it lacks. myconclusion is that at 750cc its just TOO big, my old 250 xl honda on the other hand was much nicer at low revs and sounded and felt great , but ran out of steam much above 55mph. so is there an optimum size or layout for a single to use on the road? my old hybrid of a gn400 motor in an xl125 chassis was a hoot to ride and could be pottered about at 10mph or screamed upto 80, but it want 'quite' right, my m21 beesa on the other hand is really overspecified in the gearbox dept, it only needs bottom and top, the rest inbetween are useless, but at 50mph flat out and weighing a bloody ton, its little more than an ancient toy.
so come on guys, whats your ideal engine capacity? 400? 500? sidevalve ? 2 or 4 valve? and why. part of me yearns for the tidy one owner from new xl500 i sold last year, but then another part of me looks at the dr big and wonders "would the 800 be better? "

[SteveC#222]

  my latest is a dr big suzuki, a 750cc 4 valve monster of an engine. it can haul my 20 stones upto a genuine 100mph, even with its breeze block aerodynamics, so what it would be l;ike in a small streamlined chassis i can only imagine.



Like this!

[johnr]

well yeee har! i bet that goes like the bloody clappers steve!!

[OMEGAMAN]

 
A big thumper in a streamlined chassis?

You should go watch the supermono's.

Here's a few figures from the recent Snetterton meeting-

                                                                    Best Lap                  Speed trap

Lex Van Dijk           (Over Yam)    Ned                1:13.96                   142.2  mph
Mark Lawes            (Pami BMW)   GBR                1:14.24                   138.4  mph
Nick Chadwick         (Ktm)           GBR                1:14.25                   139.2  mph
Henk From              (Pami BMW)   Ned                1:15.87                   140.4  mph
Michael Payne         (Tig Honda)   Aus                1:16.08                   134.5  mph
E. Van Der Beek       (Ducati)       Ned                1:16.22                   131.6  mph
Scott Richardson     (Suzuki)       GBR                 1:17.65                   125.2  mph
Libor Kamenicky       (Jawa)        CZE                 1:20.07                   133.9  mph

[Cold Snail]

The 800 is pretty much the same as the 750 to ride (Andy had an 800 as well), if you keep them over 4 grand they are smooth enough, but under that you start thinking of the chain.
My KLR's are more tractable lower down (from about 2500rpm) and with the 40mm Keihin CV (same as some Harleys) and a noisy pipe sound great short shifting.

The Mito's still laying around if you fancy something smaller to put the 750 motor in, but somehow I just can't picture it 

[squirrelciv]

I read somewhere (don't ask me where though) that the perfect single is a 350cc. Aparently anything bigger and the fuel can't all get burnt in time and is wasted making the motor less efficient. Don't know if this is true or not, but I do remember Jethro telling me that the 350 enfield has a sweeter motor than the 500.
I reckon my old GN400 was more flexible than my Dommie 650, but the bottom line is the Dommie does generate more power overall.

[steveD]

How about this one if you are serious about going places!!!!!!!


http://www.xt660.com/showthread.php?t=6910



I personally think that I have found my ideal!!

[steveD]

OH and by the way that is the guy that I met at the weekend!

[Cold Snail]

I read somewhere (don't ask me where though) that the perfect single is a 350cc.

I think it's pretty close to that.

If my XR400 had more road biased gearing, it would be excellent all round. As it is, I can be in top gear at under 20mph and it pulls cleanly enough all the way up to almost 80, but it's screaming it's head off at that speed, it's much happier at 55-60 and 55mpg.
Another advantage is kickstarting, the smaller bike is much easier.

[Jez F]

I'd have to agree about 350's, take Ducati singles for instance, 250 not quite fast enough, 450 like riding a jackhammer and risk your fillings coming out and the 350,smoothest and best of the bunch, all second hand opinions by the way as I've never been rich enough. I did have a '60 Enfield Bullet, which had a willing engine but handled like a pig. As for todays bikes the current crop of Supermoto 450's look very tempting.
 Jez

[guest18]

How about this one if you are serious about going places!!!!!!!


http://www.xt660.com/showthread.php?t=6910



I personally think that I have found my ideal!!

Doesn't work unless you're a board member  got a piccie?

[guest24]

I think I liked my SRX250 best.

DOHC, 4 valves, the Yamaha 1 and a half carbs, 32bhp, 122kg, and saw 155kmh (97mph) on the clock, rising rate mono shock rear, etc, oh, and a centre stand.

[themoudie]

take Ducati singles for instance, 250 not quite fast enough, 450 like riding a jackhammer and risk your fillings coming out and the 350,smoothest and best of the bunch, all second hand opinions by the way as I've never been rich enough.

Hurrumph! Money has nothing to do with it, loitering in the right place at the right time has been fruitful

Duke 250 narrow case are just magic with turbine power to 9,500 and with desmo head 10,000+, smooth. Described by racers whom have been fortunate to have a shot on a good Manx as a miniture version. 250 widecase bit heavier and not as nimble, the 239 French tax evasion model was better, with bigger carby and better cam. Duke 350 widecase with valve spring or desmo head goes well and is smooth if put together right, again, revs are not a problem. The 450 (436cc) is a softer bit of kit, yet will still go to 7,500 on occaision. Only a jack hammer if you rev the nuts off it through the gears and haven't built it right.

Main problem with all Duke singles is getting them to breath, the inlet size, shape and position in the head is s***e Only cure is to fill the head with weld and re-cut the inlet port with a steeper angle and central position, then work out where the carb goes! As for boring and stroking a 450 to ~500cc you pays your money and makes a choice Now that is expensive!

A reputable Duke single race motor with all the right bits, starts at £5K just for the labour Same again for the bits (6 speed box) and same again for the lightweight chassis + carbon fibre fairing if you are lush as an extra.

Racing never was cheap! Support those that do, if you can and enjoy. Alternatively buy a 'Condom' 350 and take it easy!

Now that crankshaft..........? Bill

[Bill Rutter]

I think Pat's comment about 350cc is about right for classic Brit bikes but the Japanese have brought technology on in leaps and bounds since the 60's & 70's what with  Honda's RVFC and Suzuki's Twin-Swirl combustion and the like. Yamaha will have done something too I'm sure. My CS has a Berlin-built Rotax with, would you believe, BMW M-Series technology. So we've come a long way from temperamental Jampots and Beezers. Bless 'em.