Bore vs. Stroke
05-04-2005, 12:16 PM
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Bore vs. Stroke Ahhhh, yes, the anchient debate amongs hot rodders since perhaps the dawn of the modern small block over 50 years ago. HotRod's June issue decided to tackle this in errnest and put the issue to bed, or so they thought. So...until I get home, browse the article again, and get more firmiliar with the principals discussed in it, might as well get the debate rolling here.
Which is better? More bore and a shorter stroke (oversquare) or longer stroke and a smaller bore (undersquare) for the amount of cubic inches that one deems appropriate. Which one will make more TQ and HP, both peak and average, and which one will have more useable RPM? Think in terms of the same cylinder head right down to the chamber and port volume and flow numbers. Same cam, same carb/throttle body, same intake manifold, effectively the same rod to stroke ratio, you name it. And in the words of Mills Lane...
"Get it on!"
Which is better? More bore and a shorter stroke (oversquare) or longer stroke and a smaller bore (undersquare) for the amount of cubic inches that one deems appropriate. Which one will make more TQ and HP, both peak and average, and which one will have more useable RPM? Think in terms of the same cylinder head right down to the chamber and port volume and flow numbers. Same cam, same carb/throttle body, same intake manifold, effectively the same rod to stroke ratio, you name it. And in the words of Mills Lane...
"Get it on!"
05-04-2005, 12:24 PM
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The longer the stroke the better leverage the pistons and rods have in turning the crank. The larger the bore the more unshrouded the valves become. I believe that a bigger bore than stroke engine will make more HP and a bigger stroke than bore engine will make more torque if c.i. of engine is the same- 382 all bore vs. 382 all stroke is an example.
05-04-2005, 12:44 PM
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Originally Posted by Bo White
The longer the stroke the better leverage the pistons and rods have in turning the crank. The larger the bore the more unshrouded the valves become. I believe that a bigger bore than stroke engine will make more HP and a bigger stroke than bore engine will make more torque if c.i. of engine is the same- 382 all bore vs. 382 all stroke is an example.
Perhaps. Article has some interesting facts in reguards to this. More discussion please.
05-04-2005, 04:31 PM
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Bo pretty much nailed it. Everything I've ever read says that the strokers are torque monsters, while all bore's make more HP and can rev a bit higher.
05-04-2005, 04:58 PM
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Yep; tractor motors have long strokes, race engines have short stroke, big bore. A short stroke also means the engine will be able to rev higher, since the piston doesn't have to travel as fast to get from one end of the bore to the other. And the larger bore will accomadate larger valves to bring in a bigger charge of fuel mixture for a given displacement.
Also consider that a large piston has more surface area, so for a given cylinder presure, it puts a higher force on the piston.
Also consider that a large piston has more surface area, so for a given cylinder presure, it puts a higher force on the piston.
05-04-2005, 05:04 PM
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Originally Posted by Cal
And the larger bore will accomadate larger valves to bring in a bigger charge of fuel mixture for a given displacement.
05-04-2005, 08:53 PM
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The following has EXCERPTS (paraphrasing) from HotRod Mag's June 2005 issue. A premedia publication. Article by David Freiburger and photography (which I don't have sorry fells) by author.
First of all...we've all heard for years the above mentioned "theories" but where have we all seen them? Various magazines and or discussing with various gear heads, ect. Stop and think for a second, have you ever challenged the idea that Bore > Stroke or vise versa? I know that I had even bought into most of these ideas, until last evening so...without further adue.
First, the motors:
Two, for all intensive purposes, equal cubic inches, big block chevy's. The big bore motor measures up @ 4.560x3.766, displacing 492. The big stroke motor measures up @ 4.280x4.250 for 489 cubes. The variance is .60%, or really negligable. Strokes for the math challenged are different by nearly half an inch (.484) and the bore is over a quater inch (@ .280). The other decently significant difference between the motors was the rod ratio. This was to keep the engines as "close to practical" for the experiment in the rod ratio. The short stroke used a rod shoter than typical that measured 6.135". The long-stroke was 6.535". This put the rod ratios at 1.63:1 and 1.54:1. Finishing out the short block were identical fasteners, head studs, Scat cranks and rods, a custom set of J&E pistons made specifically for the test. Both motors were topped off with AFR's 335 CNC-ported heads and the bottom end was buttoned up with a Milodon pan for Gen V BBC filled with Royal Purple 5w30.
When the motors were tested two different cams were used. One cam was a tame hydro roller, the other a more perf. oriented solid roller. Hydro roller was speced @ 218/224 @ .050 on 110 LDA with .510/.510 lift, which is pretty mild for a BB. The solid roller speced out at a more normal hi po cam for a big block (though still decently mild) @ 253/260 @ .050 ground on a 111 LDA with .734/.732 lift. Both the cams were run on a 1.7 rocker. The only difference was the lash seetings were moved around to alter the duration to effectively find the best AVGERAGE power in both motors. (I'm not going to get into this...read the article
).
Inconjunction with the cams...Cometic custom-mad MLS head gaskets were used to alter the comp. ratio. For the small cam, a monster thick .125" gasket was used on the big bore, to yield a 9.076:1 comp. ratio and on the big stroke a 9.039:1 comp ratio. For the big cam, a .040" gasket was used yielding 10.87/10.83:1 respecitively.
Induction was through a 975-cfm race demond carb atop an Edelbrock Performer RPM for the small cam. Same carb but a Weiand Team G single-plane was swapped in for the big cam. The set up exhaled through Hooker headers with 2" primaries and 18" extentions with Flowmasters (small cam only).
The test results
As most stated...we'd think that the big bore motor will make more HP with more RPM and the long stroke motor will make more TQ, especially down low.
Out of the box NO difference was really noticed. After some tuning, only minimal "wiggle was seen in the curves that supported the traditional theories: The long stroke made more low end and the short stroke made more top end, but not nearly the margin you've been lead to believe all these years. We're talking about differences of 7lb-ft and 12hp @ most." The author points out that even this much difference could be attributed to the very negligible difference of 4ci.
Upon further examintation of the short blocks...
The common theory is that a longer stroke acts as a longer lever arm on the crank and therefore offers better torque than a short block. Though this is not what the test discovered. What factors into this purhaps? A little thing called piston speed. Piston speed = (STROKE x RPM) / 6. So...@ 7000 RPM, the 4.250" stroke has a mean piston speed of 4,958 feet per min. The short stroke has a mean piston speed of 4,393 fpm. "That's a 11.4% reduction. Long-storkes with increased mean piston speed diossipates combustion press. more quickly than the slower piston speed of the shorter stroke, and is there fore less effective @ power production, but the concerpt of Brake Mean Effective Pressure (BMEP) conflicts with that argument. BMEP is the theoretical average pressure on the piston throughout the stroke that is required to sustain the level of horse power. BMEP = (indicated torque x 150.8) / displacement, so you can see that stroke DOES NOT equal into the equation."
The power curves
Small Cam, long stroke: Pk Tq @ 3500 and 3600 of 592, Pk HP @ 5300 of 547. Avg. of 551 TQ and 455 HP
Small cam, big bore: Pk Tq @ 3600, 3700, 3900, and 4000 of 589. Pk HP @ 5400 and 5500 of 549. Avg. of 550 TQ and 455 HP.
Big Cam, long stroke: PK TQ @ 4600 and 5500 of 643, Pk HP @ 6,600 of 717. Avg. of 612 TQ and 578 HP.
Big cam, big bore: Pk Tq @ 5400 and 5500 of 636, Pk HP @ 6,300 of 727. Avg. of 609 TQ and 576 HP.
One thing that they did discover was the ring tention may have caused a slightly more frictional loss in the big bore motor, but these are the basics of the article and I'll leave it to the rest of you gear heads to read it yourself and come to your own conclusions. Now...do keep in mind that both motors were built to be nearly identical in every aspect and they did go some what against normal conventions of engine building. One other thing of interest is that Pro Stockers and NASCAR motors tend to lean towards the Big Bore, long rod, short stroke method but this is due in part to valve size, bearing speed (assicated with piston speed for longevity) and the constraints of the rule books. Hope this was informative. Again. This is in the June 05 (titled The Great Bore vs. Stroke Showdown on page 98 and copyrighted 2005) issue of HotRod and was excerpted from author David Freiburger's article. I am in no way associated with HotRod, Primeda, or it's staff. This was for educational purposes only.
First of all...we've all heard for years the above mentioned "theories" but where have we all seen them? Various magazines and or discussing with various gear heads, ect. Stop and think for a second, have you ever challenged the idea that Bore > Stroke or vise versa? I know that I had even bought into most of these ideas, until last evening so...without further adue.
First, the motors:
Two, for all intensive purposes, equal cubic inches, big block chevy's. The big bore motor measures up @ 4.560x3.766, displacing 492. The big stroke motor measures up @ 4.280x4.250 for 489 cubes. The variance is .60%, or really negligable. Strokes for the math challenged are different by nearly half an inch (.484) and the bore is over a quater inch (@ .280). The other decently significant difference between the motors was the rod ratio. This was to keep the engines as "close to practical" for the experiment in the rod ratio. The short stroke used a rod shoter than typical that measured 6.135". The long-stroke was 6.535". This put the rod ratios at 1.63:1 and 1.54:1. Finishing out the short block were identical fasteners, head studs, Scat cranks and rods, a custom set of J&E pistons made specifically for the test. Both motors were topped off with AFR's 335 CNC-ported heads and the bottom end was buttoned up with a Milodon pan for Gen V BBC filled with Royal Purple 5w30.
When the motors were tested two different cams were used. One cam was a tame hydro roller, the other a more perf. oriented solid roller. Hydro roller was speced @ 218/224 @ .050 on 110 LDA with .510/.510 lift, which is pretty mild for a BB. The solid roller speced out at a more normal hi po cam for a big block (though still decently mild) @ 253/260 @ .050 ground on a 111 LDA with .734/.732 lift. Both the cams were run on a 1.7 rocker. The only difference was the lash seetings were moved around to alter the duration to effectively find the best AVGERAGE power in both motors. (I'm not going to get into this...read the article
).Inconjunction with the cams...Cometic custom-mad MLS head gaskets were used to alter the comp. ratio. For the small cam, a monster thick .125" gasket was used on the big bore, to yield a 9.076:1 comp. ratio and on the big stroke a 9.039:1 comp ratio. For the big cam, a .040" gasket was used yielding 10.87/10.83:1 respecitively.
Induction was through a 975-cfm race demond carb atop an Edelbrock Performer RPM for the small cam. Same carb but a Weiand Team G single-plane was swapped in for the big cam. The set up exhaled through Hooker headers with 2" primaries and 18" extentions with Flowmasters (small cam only).
The test results
As most stated...we'd think that the big bore motor will make more HP with more RPM and the long stroke motor will make more TQ, especially down low.
Out of the box NO difference was really noticed. After some tuning, only minimal "wiggle was seen in the curves that supported the traditional theories: The long stroke made more low end and the short stroke made more top end, but not nearly the margin you've been lead to believe all these years. We're talking about differences of 7lb-ft and 12hp @ most." The author points out that even this much difference could be attributed to the very negligible difference of 4ci.
Upon further examintation of the short blocks...
The common theory is that a longer stroke acts as a longer lever arm on the crank and therefore offers better torque than a short block. Though this is not what the test discovered. What factors into this purhaps? A little thing called piston speed. Piston speed = (STROKE x RPM) / 6. So...@ 7000 RPM, the 4.250" stroke has a mean piston speed of 4,958 feet per min. The short stroke has a mean piston speed of 4,393 fpm. "That's a 11.4% reduction. Long-storkes with increased mean piston speed diossipates combustion press. more quickly than the slower piston speed of the shorter stroke, and is there fore less effective @ power production, but the concerpt of Brake Mean Effective Pressure (BMEP) conflicts with that argument. BMEP is the theoretical average pressure on the piston throughout the stroke that is required to sustain the level of horse power. BMEP = (indicated torque x 150.8) / displacement, so you can see that stroke DOES NOT equal into the equation."
The power curves
Small Cam, long stroke: Pk Tq @ 3500 and 3600 of 592, Pk HP @ 5300 of 547. Avg. of 551 TQ and 455 HP
Small cam, big bore: Pk Tq @ 3600, 3700, 3900, and 4000 of 589. Pk HP @ 5400 and 5500 of 549. Avg. of 550 TQ and 455 HP.
Big Cam, long stroke: PK TQ @ 4600 and 5500 of 643, Pk HP @ 6,600 of 717. Avg. of 612 TQ and 578 HP.
Big cam, big bore: Pk Tq @ 5400 and 5500 of 636, Pk HP @ 6,300 of 727. Avg. of 609 TQ and 576 HP.
One thing that they did discover was the ring tention may have caused a slightly more frictional loss in the big bore motor, but these are the basics of the article and I'll leave it to the rest of you gear heads to read it yourself and come to your own conclusions. Now...do keep in mind that both motors were built to be nearly identical in every aspect and they did go some what against normal conventions of engine building. One other thing of interest is that Pro Stockers and NASCAR motors tend to lean towards the Big Bore, long rod, short stroke method but this is due in part to valve size, bearing speed (assicated with piston speed for longevity) and the constraints of the rule books. Hope this was informative. Again. This is in the June 05 (titled The Great Bore vs. Stroke Showdown on page 98 and copyrighted 2005) issue of HotRod and was excerpted from author David Freiburger's article. I am in no way associated with HotRod, Primeda, or it's staff. This was for educational purposes only.
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05-04-2005, 08:57 PM
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Phew, that was a lot to re-read, digest, and try and condense. I urge you to get the magazine and read the article for yourself as it was very informative and I only touched upon the meat of the article. It does go into more depth as well as quotes from the likes of Reher-Morrison headman Darin Morgan and Charles Jenckes, cheif engine builder for Dale Earnhardt Inc. Thanks again for letting me share and again...I hope it was informative.
05-05-2005, 08:24 AM
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No problem guys, and it was actually cool to contribute to the meat and potatos of the forum.
Just to say what else was in the article...a side bar on rod ratio, issues regarding timing, and the implied friction and mass of the reciprocating assembly. Also, HotRod boiled it down to end use of an engine and what's better. So I don't steal sales or subject myself to a lawsuit...I'm going to continue urging people to get the mag and read the whole article. Hopefully the powers that be over @ Primedia realise that gear heads are a close nit community and love to share info (unless you're compeeting against one another) and you won't hear about "Aaron X..." in the newspapers
.
-Aaron
Just to say what else was in the article...a side bar on rod ratio, issues regarding timing, and the implied friction and mass of the reciprocating assembly. Also, HotRod boiled it down to end use of an engine and what's better. So I don't steal sales or subject myself to a lawsuit...I'm going to continue urging people to get the mag and read the whole article. Hopefully the powers that be over @ Primedia realise that gear heads are a close nit community and love to share info (unless you're compeeting against one another) and you won't hear about "Aaron X..." in the newspapers
.-Aaron
05-05-2005, 07:45 PM
#12
There was also no big advantage of "using a longest connecting rod as possible" for a street driven motor. A short rod snaps the piston down quicker thus unshrouding the valves sooner.
Does this mean I can use the shorter OEM LS1 connecting rods with my standerd bore,4.00" stroke 383 motor project? I know I would have to have custom pistons made. Are the OEM rods up to the task?
Does this mean I can use the shorter OEM LS1 connecting rods with my standerd bore,4.00" stroke 383 motor project? I know I would have to have custom pistons made. Are the OEM rods up to the task?
05-05-2005, 09:04 PM
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Originally Posted by gollum
There was also no big advantage of "using a longest connecting rod as possible" for a street driven motor. A short rod snaps the piston down quicker thus unshrouding the valves sooner.
Does this mean I can use the shorter OEM LS1 connecting rods with my standerd bore,4.00" stroke 383 motor project? I know I would have to have custom pistons made. Are the OEM rods up to the task?
Does this mean I can use the shorter OEM LS1 connecting rods with my standerd bore,4.00" stroke 383 motor project? I know I would have to have custom pistons made. Are the OEM rods up to the task?
Stoke length wouldn't be a bad thing, but what kind of power levels are you shooting for? Definately need to step up to better rod bolts and make sure to resize properly. I've heard that they can stand up to some harsh **** for a while but eventually will give out. If there is one thing about the powdered metal rods, it's strong for cheap. Which means if you plan on making big power, do yourself a favor, protect your investment, and go forged.
05-05-2005, 09:40 PM
#14
The LS1 powder forged rods are the strongest small block production rod GM ever made. They would be safe if shifting at 6000-6250 RPM and keeping bearing crush tolorences on the tight side.
But after buying custom pistons and rebuilding my OEM rods. I do not think I would save enough money to justify going the OEM rod route.
But after buying custom pistons and rebuilding my OEM rods. I do not think I would save enough money to justify going the OEM rod route.
05-05-2005, 09:55 PM
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Not a very good test IMO
Why would someone increase the bore size of an engine and not change the cylinder head to take advantage of it?
Also I think they may have had different results with an inline valve head vrs a canted valve head JMO In other words I think the test was towards favoring the large stroked engine from the start.
Why would someone increase the bore size of an engine and not change the cylinder head to take advantage of it?
Also I think they may have had different results with an inline valve head vrs a canted valve head JMO In other words I think the test was towards favoring the large stroked engine from the start.
05-05-2005, 10:06 PM
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Originally Posted by BrentB@TEA
Not a very good test IMO
Why would someone increase the bore size of an engine and not change the cylinder head to take advantage of it?
Also I think they may have had different results with an inline valve head vrs a canted valve head JMO In other words I think the test was towards favoring the large stroked engine from the start.
Why would someone increase the bore size of an engine and not change the cylinder head to take advantage of it?
Also I think they may have had different results with an inline valve head vrs a canted valve head JMO In other words I think the test was towards favoring the large stroked engine from the start.
Did you read the full article? Can you elaborate further? How can you relate TEA's experience to what I posted? (Don't take this the wrong way as I'm seriously curious. I'm a tad skeptical [about the article] but the article was quite convincing).
Remember the test wasn't to take advantage of a different head, it was to only test the stroke vs. bore theory. From a stand point of nearly identical engines, which way to make displacement is better for making power and or tq? I do believe that I did mention they were not adhearing to a normal convention of building a big bore motor with a shorter rod than typical. They also mentioned in the article that the AFR 335s for Big Block Chevy (which last I checked are all splayed valve heads) perhaps were too good and would cover up any inconsistancies in the theories.
05-05-2005, 10:24 PM
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the reason I think it was geared to a large stroke engine are simple.
A large bore will flow more air if taken advantage of by properly porting the head for the bore size. Also with a larger bore a larger valve could be run.
Look at an LS1, it would have been a much easier engine to prove the point on. Because it is limited more by its bore to start with. Compare a 382 stroker to a 382 all bore.
Max out a set of heads for each engine.
Here is the fun part both engines have different cylinder head needs. I would use a 2.055 valve 5.3 head or ls6 head on the stroker and a 2.100 valve ls6 or 6.0l head on the all bore engine. I thinkwith some pretty good stuff on them the best a guy could hope for is flow in the 320-low 330 range for the smaller bore combinations and airflow in the 340-360 range for the all bore combination.
I think it is pretty clear that the all bore will make a good bit more power.
Even using a head that did not take advantage of the larger bore , still had the big bore engine coming out on top. Now take advantge of the bore and see the stroker engine get the beatdown of its life
A large bore will flow more air if taken advantage of by properly porting the head for the bore size. Also with a larger bore a larger valve could be run.
Look at an LS1, it would have been a much easier engine to prove the point on. Because it is limited more by its bore to start with. Compare a 382 stroker to a 382 all bore.
Max out a set of heads for each engine.
Here is the fun part both engines have different cylinder head needs. I would use a 2.055 valve 5.3 head or ls6 head on the stroker and a 2.100 valve ls6 or 6.0l head on the all bore engine. I thinkwith some pretty good stuff on them the best a guy could hope for is flow in the 320-low 330 range for the smaller bore combinations and airflow in the 340-360 range for the all bore combination.
I think it is pretty clear that the all bore will make a good bit more power.
Even using a head that did not take advantage of the larger bore , still had the big bore engine coming out on top. Now take advantge of the bore and see the stroker engine get the beatdown of its life
05-05-2005, 11:24 PM
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A bigger bore engine has better ability to "breathe" on its own from more room for bigger valves and head ports. This all means more power and torque if you build and tune it right. A longer stroke in theory will make more torque but doesn't always come through. A good example is the 4.6 and 5.4 Ford. Both have a small bore of 3.55". And despite the 5.4 having a 4.1" stroke, it still isn't as torquey as a regular Vortec 350. Only with the new 3 valve heads is the 5.4 actually more torquey than the 350. The 4.6 has always been short on torque in every form since it was introduced in 91, worse than the 302 which has a 3" stroke and 4" bore. Yet the 4.6 is also very limited in potential as is the 5.4 because the bore is so small. That is why you have to supercharge these motors to make serious power and with a 302, you can still get some serious power keeping it N/A.
Jason
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05-05-2005, 11:46 PM
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Originally Posted by BrentB@TEA
the reason I think it was geared to a large stroke engine are simple.
A large bore will flow more air if taken advantage of by properly porting the head for the bore size. Also with a larger bore a larger valve could be run.
Look at an LS1, it would have been a much easier engine to prove the point on. Because it is limited more by its bore to start with. Compare a 382 stroker to a 382 all bore.
Max out a set of heads for each engine.
Here is the fun part both engines have different cylinder head needs. I would use a 2.055 valve 5.3 head or ls6 head on the stroker and a 2.100 valve ls6 or 6.0l head on the all bore engine. I thinkwith some pretty good stuff on them the best a guy could hope for is flow in the 320-low 330 range for the smaller bore combinations and airflow in the 340-360 range for the all bore combination.
I think it is pretty clear that the all bore will make a good bit more power.
Even using a head that did not take advantage of the larger bore , still had the big bore engine coming out on top. Now take advantge of the bore and see the stroker engine get the beatdown of its life
A large bore will flow more air if taken advantage of by properly porting the head for the bore size. Also with a larger bore a larger valve could be run.
Look at an LS1, it would have been a much easier engine to prove the point on. Because it is limited more by its bore to start with. Compare a 382 stroker to a 382 all bore.
Max out a set of heads for each engine.
Here is the fun part both engines have different cylinder head needs. I would use a 2.055 valve 5.3 head or ls6 head on the stroker and a 2.100 valve ls6 or 6.0l head on the all bore engine. I thinkwith some pretty good stuff on them the best a guy could hope for is flow in the 320-low 330 range for the smaller bore combinations and airflow in the 340-360 range for the all bore combination.
I think it is pretty clear that the all bore will make a good bit more power.
Even using a head that did not take advantage of the larger bore , still had the big bore engine coming out on top. Now take advantge of the bore and see the stroker engine get the beatdown of its life
, this is the reason I used this comparison in my first post. You are going from a 3.898 bore to a 4.100 bore and right off the bat the cylinder heads gain upwards of 7% in cfm and then on top of that porting to take advantage of the larger bore and the flow increases probably that much more. The stroker gains 3/8" of inch of leverage to turn the crank for better torque production but will be rpm limited because of piston speed and worse rod/stroke ratio(if the same rods are used) much less the more shrouding of the valves previously mentioned. Its like riding a bicycle, the longer the crank- the easier it is to pedal the bike up to speed but you are able to pedal so many rpms because of the distance you have to move. The shorter the crank- the harder it is to get moving but once in motion you can pedal your *** off lol.